In this paper, we describe the wide-field spectroscopic survey telescope (WST) project. WST is a 12-metre wide-field spectroscopic survey telescope with simultaneous operation of a large field-of-view (3 sq. degree), high-multiplex (20,000) multi-object spectrograph (MOS), with both a low and high-resolution modes, and a giant 3×3 arcmin2 integral field spectrograph (IFS). In scientific capability, these specifications place WST far ahead of existing and planned facilities. In only 5 years of operation, the MOS would target 250 million galaxies and 25 million stars at low spectral resolution, plus 2 million stars at high resolution. Without need for pre-imaged targets, the IFS would deliver 4 billion spectra offering many serendipitous discoveries. Given the current investment in deep imaging surveys and noting the diagnostic power of spectroscopy, WST will fill a crucial gap in astronomical capability and work in synergy with future ground and space-based facilities. We show how it can address outstanding scientific questions in the areas of cosmology; galaxy assembly, evolution, and enrichment, including our own Milky Way; the origin of stars and planets; and time domain and multi-messenger astrophysics. WST’s uniquely rich dataset may yield unforeseen discoveries in many of these areas. The telescope and instruments are designed as an integrated system and will mostly use existing technology, with the aim to minimise the carbon footprint and environmental impact. We will propose WST as the next European Southern Observatory (ESO) project after completion of the 39-metre ELT.
Publisher's Note: This paper, originally published on 18 July 2024, was replaced with a corrected/revised version on 30 August 2024. If you downloaded the original PDF but are unable to access the revision, please contact SPIE Digital Library Customer Service for assistance.
We analyze aperiodic fiber Bragg gratings (FBGs) fabricated using an aperiodic phase mask, involving partial overlaps of distinct mask regions. Multichannel aperiodic FBG filter is a promising candidate for suppressing hydroxyl (OH) emission lines in ground-based near-infrared astronomical observations. However, the fabrication of such multichannel aperiodic FBGs demands high repeatability. We explore the design of phase masks with repeatable FBG inscriptions. Previously, we explored a phase mask (1st generation) designed and fabricated with partial overlapping regions using logical OR operation. Continuing this effort, we investigate three distinct phase mask designs which are capable of generating FBGs corresponding to five OH lines. The first mask features five discrete regions, each producing a specific FBG channel, while the other two masks incorporate numerically obtained overlap regions using logical OR and XOR operations. We present here the performances of the fabricated masks by comparing the Bragg wavelengths and the reflectivities of the inscribed FBGs.
Photonic Lanterns (PLs) play a crucial role in astrophotonic technologies, converting multi-mode inputs to single-mode outputs while being theoretically low loss. Despite technical advancements, the reproducibility of PLs remains unexplored. We present a study characterizing multiple PLs to address the challenges of mass production. Initial results indicate high taper rate consistency, vital for PL stability and their integration into astrophotonic instruments. Beyond taper measurements, our comprehensive evaluation includes throughput, near-field, and chromatic analysis, ensuring mass produced PLs meet stringent telescope requirements.
A major limiting factor of using photonic integrated circuits (PICs) in astronomical instruments is that they are functional only in the single-mode regime. As number of modes M in the PSF scales with diameter D of the telescope (D∕4λ2), it is impractical to use PICs based spectrographs without extreme adaptive optics (exAO) in ground- based observatories. To increase the coupling efficiency of the FoV into a PICs based spectrograph, we can employ a lower order adaptive optics (LOAO) to partially correct the PSFs. The partially corrected FoV is then sampled with an integral field unit (IFU) comprising of micro-lens fed MMF/FMFs, which feed into an array of photonic lanterns (PLs). The multiple SMF outputs of the PLs are butt-coupled across a stack of AWGs, one PL connected to the corresponding AWG or connected across the stack. CAWSMOS is first of its kind concept that exploits the ability of AWGs to spectrally disperse light from more than one fibre simultaneously. Each AWG disperses the wavelengths horizontally, and the cross-disperser disperses the orders vertically. The echellogram from each fibre on an AWG is spatially shifted vertically to occupy the space between the orders. Each AWG is assigned to individual real-estate on the detector area.
BlueMUSE is a blue-optimised, medium spectral resolution, panoramic integral eld spectrograph under development for the Very Large Telescope (VLT). With an optimised transmission down to 350 nm, spectral resolution of R~3500 on average across the wavelength range, and a large FoV (1 arcmin2), BlueMUSE will open up a new range of galactic and extragalactic science cases facilitated by its specific capabilities. The BlueMUSE consortium includes 9 institutes located in 7 countries and is led by the Centre de Recherche Astrophysique de Lyon (CRAL). The BlueMUSE project development is currently in Phase A, with an expected rst light at the VLT in 2031. We introduce here the Top Level Requirements (TLRs) derived from the main science cases, and then present an overview of the BlueMUSE system and its subsystems ful lling these TLRs. We speci cally emphasize the tradeo s that are made and the key distinctions compared to the MUSE instrument, upon which the system architecture is built.
Arrayed waveguide gratings (AWG) have gained attention as promising integrated spectrographs for ground-based telescopes, airborne applications, and spaceborne instrumentation due to their low mass, diffraction limit characteristics, thermal stability, and robustness against vibrations and misalignment. The Potsdam Arrayed Waveguide Spectrograph (PAWS) is a cross-dispersive instrument based on an integrated photonic spectrograph (IPS) that is optimized for the astronomical H-Band and was designed and developed by innoFSPEC at the Leibniz Institute for Astrophysics Potsdam (AIP). The main element is a second-generation AWG that is fibre coupled and works as a first dispersive element. To work as an IPS, the dispersed light of the AWG is sampled at the output facet and magnified by a microscope objective. The light is then fed into a free-space optical system housed in a cryostat working at 140 K. An afocal relay re-images the exit pupil of the microscope into the plane of a cross-dispersive element containing a diffractive grating. Subsequently, an objective focuses the resulting echellogram on a Teledyne 2k × 2k H2RG near-infrared array working at 80 K. To validate the functionality of the system, different light sources have been used. A tunable laser source generated an echellogram through frame stacking. Furthermore, the cross-dispersed output of a supercontinuum source and of an in-house developed frequency comb system were recorded under varying input conditions of the AWG, successfully achieving first light laboratory measurements. Throughout multiple cycles and measurements of the instrument, several parameters and characteristics were identified, providing opportunities for optimization to enhance the instrument’s performance and facilitate the miniaturization of future iterations. In this manuscript, we will provide a concise overview of the PAWS instrument, the preliminary results of laboratory measurements, and lessons learned to improve the future iterations of the next generation near-infrared cross-dispersed integrated photonic spectrograph. PAWS represents a pioneering demonstration of an astronomy optimized AWG chip, showcasing the advantageous capabilities of integrated photonic spectrograph.
Photonic Integrated Circuits (PIC) are best known for their important role in the telecommunication sector, e.g. high speed communication devices in data centers. However, PIC also hold the promise for innovation in sectors like life science, medicine, sensing, automotive etc. The past two decades have seen efforts of utilizing PIC to enhance the performance of instrumentation for astronomical telescopes, perhaps the most spectacular example being the integrated optics beam combiner for the interferometer GRAVITY at the ESO Very Large Telescope. This instrument has enabled observations of the supermassive black hole in the center of the Milky Way at unprecedented angular resolution, eventually leading to the Nobel Price for Physics in 2020. Several groups worldwide are actively engaged in the emerging field of astrophotonics research, amongst them the innoFSPEC Center in Potsdam, Germany. We present results for a number of applications developed at innoFSPEC, notably PIC for integrated photonic spectrographs on the basis of arrayed waveguide gratings and the PAWS demonstrator (Potsdam Arrayed Waveguide Spectrograph), PIC-based ring resonators in astronomical frequency combs for precision wavelength calibration, discrete beam combiners (DBC) for large astronomical interferometers, as well as aperiodic fiber Bragg gratings for complex astronomical filters and their possible derivatives in PIC.
BlueMUSE is an integral field spectrograph in an early development stage for the ESO VLT. For our design of the data reduction software for this instrument, we are first reviewing capabilities and issues of the pipeline of the existing MUSE instrument. MUSE has been in operation at the VLT since 2014 and led to discoveries published in more than 600 refereed scientific papers. While BlueMUSE and MUSE have many common properties we briefly point out a few key differences between both instruments. We outline a first version of the flowchart for the science reduction, and discuss the necessary changes due to the blue wavelength range covered by BlueMUSE. We also detail specific new features, for example, how the pipeline and subsequent analysis will benefit from improved handling of the data covariance, and a more integrated approach to the line-spread function, as well as improvements regarding the wavelength calibration which is of extra importance in the blue optical range. We finally discuss how simulations of BlueMUSE datacubes are being implemented and how they will be used to prepare the science of the instrument.
MARCOT Pathfinder is a precursor for MARCOT (Multi Array of Combined Telescopes) at Calar Alto Observatory (CAHA) in Spain. MARCOT is intended to provide CARMENES, currently fiber-fed from the CAHA 3.5m Telescope, with a 5-15m light collecting area from a battery of several tens of small telescopes that are incoherently fed into the final joint single fiber feed of the spectrograph. The modular concept, based on commercially available telescopes, results in cost estimates that are a fraction of the ones for extremely large telescopes (ELT). As a novel approach, MARCOT will employ Multi-Mode Photonic Lanterns (MM-PL) that are being developed as a variant of classical photonic lanterns, to combine the light from the individual telescopes to a single fiber feed to the instrument. This progress report presents the overall concept of MARCOT, the pathfinder telescope and enclosure that is being commissioned at CAHA, the concept of MM-PL, and the next step of installing the Potsdam Multiplex Raman Spectrograph (MRS). MARCOT Pathfinder will be used to validate the conceptual design and predicted performance of MM-PL on sky with a 7-unit telescope prototype.
BlueMUSE is a novel instrument under development for the ESO VLT, that builds on the legacy of MUSE, however with a blue wavelength range, a larger field-of-view (FoV), and higher spectral resolution. Driven by high-profile and unique science cases, the requirements present new challenges to the development of the instrument, although the fundamental layout will be based on the successful modular structure of the classical MUSE. In order to achieve the expected mean spectral resolution of R=3600 and radial velocity measurement accuracy of better than 1 km/s, as well as spectrophotometric performance, BlueMUSE must be equipped with a calibration unit to perform accurate wavelength, flat-field, and geometrical calibration. Lessons learned from MUSE show that the variation of the line-spread-function (LSF) across the FoV as a consequence of the field-splitter and image slicer layout requires a methodology to accurately measure the LSF as a function of x and y. Moreover, classical spectral line lamps that have been used traditionally for wavelength calibration present the problem of a scarce emission line coverage in the blue. BlueMUSE has entered pre-Phase-A in 2022. We report first results from conceptual design studies to address these challenges, in particular concepts of Fabry-Perot based tunable frequency combs, and as an alternative approach novel concepts with laser frequency combs or micro-ring resonator based combs in the blue.
The Potsdam Arrayed Waveguide Spectrograph (PAWS) is built upon an integrated photonic spectrograph designed for astronomy. Similar to integrated optic beam combiners for interferometry, PAWS is intended to demonstrate on sky how a traditional bulk optics spectrograph with R = 15.000 in the H band can be miniaturized to fit on a chip. The integrated photonic spectrograph is based on second-generation Arrayed Waveguide Gratings (AWG) with unprecedented performance in terms of spectral resolution and throughput. The fibre-coupled AWG serves as a first dispersive element. The pre-dispersed light is fed into a free-space optical system located in a cryostat. Here the overlapping spectral orders are separated by cross-dispersion. The resulting echellogram is recorded by a Teledyne 2kx2k H2RG near-infrared array. Locally controlled constant cryogenic temperatures are required for the operation of the cryostat. This was achieved by fine-tuning and optimizing the original cryostat design using experimental data from multiple cryogenic cycles. These steps included the optimization of thermal interfaces, gold coating of the radiation shield, and an appropriate cooling sequence using the constraints of the allowed cooling rate for H2RG focal plane arrays. Using the readout electronics and GEIRS software provided by the Max Planck Institute for Astronomy (MPIA), frames of the H2RG were obtained, allowing performance calculations and dark pre-characterization of the system. For the optimum alignment of the optical system, the coefficient of thermal expansion (CTE) was measured with an interferometric set-up that recorded the spatial displacements of two reflecting optical elements within the cryostat during a cryogenic cycle. An appropriate strategy was developed to adjust the cryogenic cross dispersion optics inside the vacuum chamber to the AWG coupling optics outside the chamber.
We recently performed tests of the discrete beam combiner (DBC) through an on-sky experiment using a 4-input pupil remappers-based integrated optics device. Here, we report on the lessons learned, as well as visibilities and closure phase results for our stellar target, Vega. Through complementary simulations, we analyze how the residual phase errors, input power imbalance at the waveguides, slow environmental changes, and different photon levels affect the performance of the DBC. This is an important aspect to improve future on-sky calibration strategies for this type of beam combiner, in particular when combining a large number of apertures.
We report the ultrafast laser inscription (ULI) of a 2-telescope integrated optic (IO) beam combiner for K-band interferometry in commercial Infrasil glass. The ULI setup used for this work is based on a 1030 nm femtosecond laser which is paired with a spatial-light-modulator (SLM). The SLM controls the numerical aperture of the focused beam used to write waveguides in the substrate. The optimum ULI parameters were found to inscribe straight single-mode waveguides exhibiting an insertion loss of 1.1 ± 0.1 dB for a 17 mm long chip over the entire K-band. To develop optimal directional couplers, we focused our efforts on investigating the effect of varying the core-to-core separation and the effect of detuning the waveguide parameters in the coupler. By doing so, we have identified fabrication parameters that are suitable for the fabrication of a beam combiner integrating an achromatic 3 dB directional coupler and two photometric taps with a splitting ratio of 80:20. These results demonstrate the capability of the ULI fabrication technique to inscribe efficient achromatic directional couplers in the K-band range. A final fabrication step will involve simple assembly of the beam combiner with input/output fibers in preparation for on-sky testing at the CHARA array planned for July 2022.
We present the optomechanical design of the Potsdam Arrayed Waveguide Spectrograph (PAWS), which is the first on-sky demonstrator of an integrated photonic spectrograph specifically designed and optimized for astronomy. The instrument is based on an arrayed waveguide grating (AWG) that was designed by and custom fabricated for the innovation center innoFSPEC Potsdam. The commissioning of the instrument is planned at the Calar Alto 2:2m Telescope in southern Spain. The core of the instrument is the AWG-chip as the primary dispersive element. The AWG device is coupled to the telescope module via a single-mode fibre (SMF). The spectral image on the output facet of the AWG is a superposition of multiple spectral orders due to the cyclic dispersive behavior of the waveguide array. The output of the AWG is fed into a free-space optical system housed inside a cryostat via an infinity-corrected microscope objective. The overlapping spectral orders are separated by a second dispersion stage using a ruled grating as a cross-dispersive element, and the resulting echellogram is projected onto a Teledyne 2k x 2k H2RG near-infrared array. The requirement of sub-micron accuracy of the fibre-chip alignment has led to an advanced photonic packaging method. In order to avoid on-site alignment procedures during the on-sky testing, the AWG mount, fibre-support, and microscope objective were integrated into a single monolithic module. Optical and thermal simulations and the design of the cryostat were realized by Andes Scientific. The read-out electronics and the compatible operating software for the detector was provided by the Max Planck Institute for Astronomy (MPIA). Data analysis is performed using the open-source data reduction software P3D, which provides functionality for the removal of the instrument signature, extraction of the spectra, correction for the blaze function, wavelength calibration, and processed data file export.
We assembled a testbed to study coupling of starlight through atmospheric turbulence via astronomical telescopes into astrophotonic devices. The setup allows for varying the turbulence strength and investigating the effects of different levels of adaptive optics correction on the efficiency of integrated optics. In addition to recording optical powers and wavefront errors, focal plane images are captured from which spots sizes and Strehl ratios are also measured. Novel astrophotonic components proposed as alternatives to conventional optical instruments can therefore be qualified in terms of coupling efficiency and throughput on the testbed before they are tested on the sky.
We present a method of assembling a fiber-optic pseudo-slit, inside a custom FC connector. 19 SMFs with 80 μm cladding diameters are arranged in a 1,511 μm pseudoslit, held in the center of a connector ferrule. The SMFs in the pseudo-slit are well positioned and well ordered, having an average core separation in the ‘long’ direction of 79.5 μm and an StDev in the ‘narrow’ direction of 2.68 μm. The nearfield output distribution of the pseudo-slit was measured under 615-730 nm light, finding an FWHM intensity distribution ratio between the two directions of 1 : 21.9. This method could be used with other types of optical connector, allowing pseudo-slits to be used conveniently with existing optical instruments.
The project NAIR "Novel Astronomical Instrumentation based on photonic light Reformating" is a DFG-funded collaboration to exploit the recognized potential of photonics solutions for a radically new approach to astronomical instrumentation for optical/infrared high precision spectroscopy and high angular resolution imaging. We present a project update, with the developments in our ULI waveguides and 3D printed structures for astronomical instrumentation and on sky testing results obtained at the WHT, Subaru (SCExAO) and LBT. This shows the NAIR project is helping to lead to important technological breakthroughs facilitating uniquely functionality and technical solutions for the next generation of instrumentation.
We report the ultrafast laser inscription (ULI) and characterization of 3 dB directional achromatic couplers for K-band between 2 and 2.4 μm. The couplers were fabricated in commercial Infrasil glass using 1030 nm femtosecond laser pulses. Straight waveguides inscribed using optimal fabrication parameters exhibit an average propagation loss of ∼1.21 dB over full range of K-band with a single-mode behavior for a length of 17 mm. Directional couplers with different interaction lengths and waveguide widths were fabricated and characterized. We demonstrate that 3 dB achromatic directional couplers for K-band can be fabricated using ULI. These results show that ULI can fabricate highquality couplers for future applications in astronomical interferometry. Our eventual aim is to develop a two-telescope K-band integrated optical beam combiner to replace JouFLU at CHARA.
In long-baseline interferometry, over the last few decades integrated optics beam combiners have become at- tractive technological solutions for new-generation instruments operating at infrared wavelengths. We have investigated different architectures of discrete beam combiners (DBC), which are 2D lattice arrangement of channel waveguides that can be fabricated by exploiting the 3D capability of the ultrafast laser inscription (ULI) fabrication techniques. Here, we present the first interferometric on-sky results of an integrated optics beam combiner based on a coherent pupil remapper and 4 input/23 output zig-zag based DBC, both written monolith- ically in a single borosilicate glass. We show the preliminary results of visibility amplitudes and closure phases obtained from the Vega star by using the previously calibrated transfer matrix of the device.
The generation of dissipative Kerr solitons is experimentally investigated in ring resonators with optical feedback. This new double-resonator geometry allows generating frequency combs with smooth solitonic spectral shape over much broader spectral bandwidths if compared with the standard ring resonator architecture. By using an amplitude modulated pump, the repetition rate of the generated frequency comb is locked to the external modulation and exhibits a stability comparable to the modulating radio frequency signal, i.e. the repetition rate linewidth is very narrow (20 Hz). Furthermore, the energy conversion efficiency (pump-to-frequency comb) can be up to 60%, being a record for microresonators.
The problem of atmospheric emission from OH molecules is a long standing problem for near-infrared astronomy. We are now close to solving this problem for the first time with the PRAXIS instrument. PRAXIS is a unique spectrograph which is fed by fibres that remove the OH background, and is optimised specifically to benefit from OH-Suppression. The OH suppression is achieved with fibre Bragg gratings, which were tested successfully on the GNOSIS instrument. The OH lines are suppressed by a factor of ~1000, leading to a reduction of the integrated background of a factor ≈9. A future upgrade to multicore fibre Bragg gratings will further increase this reduction. PRAXIS has had two commissioning runs, with a third commissioning run planned for July 2019, which will be presented at the conference. PRAXIS has a measured throughput of ≈20 %, demonstrating high efficiency in an OH suppression instrument for the first time. Science verification observations of Seyfert galaxies demonstrate the potential of OH suppression.
Frequency combs in a Silicon-Nitride-Microring resonator with an ultra-stable repetition frequency of 28.55 GHz were generated by means of an amplitude modulated pump laser at 1568.8 nm and compared to numerical calculation based on a modified Lugiato-Lefever-Equation. The comb spectrum at a power level of -40 dB with respect to the pump line spans a wavelength range of 70 nm.
In pursuit of miniaturization of spectrograph systems, various wavelength-dispersive technologies such as arrayed waveguide gratings (AWGs) [1] and stationary-wave integrated Fourier transform spectrographs (SWIFTS) [2] have been studied as possible candidates for practical implementations of compact, lightweight integrated spectrographs. Integrated echélle-grating (EG) based wavelength demultiplexers have been proposed as an alternative to AWGs for use as the main diffractive element in such a compact spectrograph [3]. Apart from the simple Rowland mount type, more sophisticated geometries, such as the perfect chirped grating (PCG) [4] and two-stigmatic-point gratings (SEG) [5] exist. In this work, we present the first planar integrated echélle grating based on SEG geometry and specifically designed for astronomical spectroscopy in the NIR range ~1500 nm to 1600 nm.
We will show the first results for a pupil remapping device with an integrated optics discrete beam combiner. Our expected monochromatic visibility functions are in good agreement with simulation and experiment. The device will be used for our upcoming on-sky tests at 4-m Willian-Herschel Telescope (WHT) in canary islands.
Droplet based microfluidic technology is a miniaturized platform for microbial analysis on picoliter scale. With its costefficiency, high-throughput and feasibility of complex handling protocols, droplet microfluidics is a favorable platform for applications such as microorganism screening or synthetic biology. Scattered-light-based microbial detection, in comparison to the widely used fluorescent-label-based approach, provides a contact-free and label-free, yet sensitive measuring solution. The angular dependency of scattered light delivers an elaborate information about the morphology and the physical properties, e.g. size and refractive index, of microbial samples. Due to the complexity and ambiguity of the droplet contents, an angle resolved scattered light detection system could provide powerful method for a label-free identification and quantification of the microbes in droplets. In this paper, a novel approach of light scattering measurement in Polydimethylsiloxane (PDMS) microfluidic chips is presented, engaging optical fibers for a light-scattering-based on-chip microbial detection. Optical fibers, with their fast readout and compact size, are very suitable for easier system integration towards flexible and versatile lab-on-a-chip applications.
Imaging Raman spectroscopy can be used to identify cancerous tissue. Traditionally, a step-by-step scanning of the sample is applied to generate a Raman image, which, however, is too slow for routine examination of patients. By transferring the technique of integral field spectroscopy (IFS) from astronomy to Raman imaging, it becomes possible to record entire Raman images quickly within a single exposure, without the need for a tedious scanning procedure. An IFS-based Raman imaging setup is presented, which is capable of measuring skin ex vivo or in vivo. It is demonstrated how Raman images of healthy and cancerous skin biopsies were recorded and analyzed.
KEYWORDS: Fiber Bragg gratings, Near infrared, Diffraction, Optical alignment, Control systems, Modulation, Algorithm development, Process control, Space telescopes, Telescopes
The power of the next generation of telescopes that will rely largely on the combination of light-collecting area with excellent (ideally: diffraction limited) image quality. Therefore, the focus will heavily lean on adaptive optics and the near infrared wavelength regime. A severe limiting factor is the presence and strength of atmospheric OH emission lines in the NIR. OH suppression techniques involving fiber Bragg gratings (FBG) have been proposed, however as yet not fully demonstrated on sky. We are involved in the first generation FBG prototype development with partners in Australia, including the GNOSIS and PRAXIS on-sky experiments.
Since the supply of suitable multi-notch filters is no longer available from industry, we have made an effort at innoFSPEC Potsdam to build a specialized laboratory for the development and manufacture of 2nd generation FBGs for OH suppression.
Suppression of the strong NIR OH emission lines requires a single grating that reflects multiple wavelengths, spaced at non-periodic intervals, with flat-top profile and high suppression ratio. It has been shown that aperiodic fiber Bragg gratings (AFBGs) can provide such functions. However, the fabrication technology requires accurate optical alignment of several degrees of freedom as well as complex control of modulated beams to form a varying interference pattern. In our work, an algorithm is developed from the index profile of a multi-notch AFBG to the design of a complex phase-mask that can generate a matching UV diffraction pattern, which will in turn inscribe an single-mode fiber into the chosen AFBG. With such a phase mask, the fabrication of the AFBGs will be reduced to a simple UV-exposure process, i.e., the complex alignment and control processes of the interference pattern from modulated beams are avoided altogether. The resulting reliable and reproducible fabrication process will dramatically reduce of the cost of such filters. Packaging aspects for a complete sky emission filter system will also be discussed.
Compact yet highly functional optical components are desired in modern astronomical instruments targeted at low system cost and reduced maintenance complexity. Integrated photonic spectrometers based on planar lightwave circuits are attractive as the planar miniature device can provide high spectral resolution but also great robustness and flexibility in the design of spectrograph systems. Arrayed waveguide gratings (AWGs) have the potential to be adapted and optimized to function as compact spectrometers in astronomical spectrographs. In this work high-resolution AWGs based on low-loss silica waveguides have been designed, fabricated and characterized. The measured spectral resolution exceeds 104 with Δλ = 150 pm at 1548 nm. The insertion loss (including two times fiber-chip coupling) is merely 2.07 dB, amounting to a peak throughput of 62%. Adiabatic fiber taper is developed to bring down the mode field diameter of a standard single mode fiber to match the mode size of the designed waveguide, resulting in almost lossless coupling from the fiber to the waveguide. The free-spectrum range is 48 nm and the side-band suppression is 22 dB. The AWG is also polarization-insensitive. Rotating the linearly polarized input light by 180° results in a slight shift of the central wavelength ~ 30 pm. The excellent overall performance makes this AWG an ideal candidate as the key building block for the development of an integrated astronomical spectrograph module.
Fibre fed spectroscopy requires that the output distribution of the optical fibre is as stable as possible. Effects like scrambling and FRD play an important role in any fibre fed instrument design, since they affect directly the output distribution of multi-mode fibres. These effects depend, among other factors, on the excited propagation modes. The propagation modes of different fibre geometries have different spatial distributions, therefore could show different scrambling and FRD characteristics. A model is being developed at the Leibniz-Institute for Astrophysics Potsdam (AIP) that shows the intrinsic effect of scrambling and FRD in optical fibres. The model is based on the Eigenmode Expansion Method (EEM). With this theoretical frame work should be possible to compare the results of mode excitation in different fibre geometries. This work is part of a PhD Thesis involved in the fibre system of MOSAIC, a multi-object spectrograph for the E-ELT.
The problem of atmospheric emission from OH molecules is a long standing problem for near-infrared astronomy. PRAXIS is a unique spectrograph which is fed by fibres that remove the OH background and is optimised specifically to benefit from OH-Suppression. The OH suppression is achieved with fibre Bragg gratings, which were tested successfully on the GNOSIS instrument. PRAXIS uses the same fibre Bragg gratings as GNOSIS in its first implementation, and will exploit new, cheaper and more efficient, multicore fibre Bragg gratings in the second implementation. The OH lines are suppressed by a factor of ∼ 1000, and the expected increase in the
signal-to-noise in the interline regions compared to GNOSIS is a factor of ∼ 9 with the GNOSIS gratings and a
factor of ∼ 17 with the new gratings.
PRAXIS will enable the full exploitation of OH suppression for the first time, which was not achieved by GNOSIS (a retrofit to an existing instrument that was not OH-Suppression optimised) due to high thermal emission, low spectrograph transmission and detector noise. PRAXIS has extremely low thermal emission, through the cooling of all significantly emitting parts, including the fore-optics, the fibre Bragg gratings, a long length of fibre, and the fibre slit, and an optical design that minimises leaks of thermal emission from outside the spectrograph. PRAXIS has low detector noise through the use of a Hawaii-2RG detector, and a high throughput through a efficient VPH based spectrograph. PRAXIS will determine the absolute level of the interline continuum and enable observations of individual objects via an IFU. In this paper we give a status update and report on acceptance tests.
When combined with the huge collecting area of the ELT, MOSAIC will be the most effective and flexible Multi-Object Spectrograph (MOS) facility in the world, having both a high multiplex and a multi-Integral Field Unit (Multi-IFU) capability. It will be the fastest way to spectroscopically follow-up the faintest sources, probing the reionisation epoch, as well as evaluating the evolution of the dwarf mass function over most of the age of the Universe. MOSAIC will be world-leading in generating an inventory of both the dark matter (from realistic rotation curves with MOAO fed NIR IFUs) and the cool to warm-hot gas phases in z=3.5 galactic haloes (with visible wavelenth IFUs). Galactic archaeology and the first massive black holes are additional targets for which MOSAIC will also be revolutionary. MOAO and accurate sky subtraction with fibres have now been demonstrated on sky, removing all low Technical Readiness Level (TRL) items from the instrument. A prompt implementation of MOSAIC is feasible, and indeed could increase the robustness and reduce risk on the ELT, since it does not require diffraction limited adaptive optics performance. Science programmes and survey strategies are currently being investigated by the Consortium, which is also hoping to welcome a few new partners in the next two years.
The Visible Integral-field Replicable Unit Spectrograph (VIRUS) consists of 156 identical spectrographs (arrayed as 78 pairs) fed by 35,000 fibers, each 1.5 arcsec diameter, at the focus of the upgraded 10 m Hobby-Eberly Telescope (HET). VIRUS has a fixed bandpass of 350-550 nm and resolving power R~700. VIRUS is the first example of industrial-scale replication applied to optical astronomy and is capable of surveying large areas of sky, spectrally. The VIRUS concept offers significant savings of engineering effort, cost, and schedule when compared to traditional instruments. The main motivator for VIRUS is to map the evolution of dark energy for the Hobby-Eberly Telescope Dark Energy Experiment (HETDEX‡), using 0.8M Lyman-alpha emitting galaxies as tracers. The VIRUS array is undergoing staged deployment during 2016 and 2017. It will provide a powerful new facility instrument for the HET, well suited to the survey niche of the telescope, and will open up large spectroscopic surveys of the emission line universe for the first time. We will review the production, lessons learned in reaching volume production, characterization, and first deployment of this massive instrument.
KEYWORDS: Adaptive optics, Spectrographs, Telescopes, James Webb Space Telescope, Adaptive optics, Galactic astronomy, Molybdenum, K band, Space telescopes, Near infrared, Spectral resolution
There are 8000 galaxies, including 1600 at z ≥ 1.6, which could be simultaneously observed in an E-ELT field of view of 40 arcmin2. A considerable fraction of astrophysical discoveries require large statistical samples, which can only be obtained with multi-object spectrographs (MOS). MOSAIC will provide a vast discovery space, enabled by a multiplex of 200 and spectral resolving powers of R=5000 and 20000. MOSAIC will also offer the unique capability of more than 10 `high-definition' (multi-object adaptive optics, MOAO) integral-field units, optimised to investigate the physics of the sources of reionization. The combination of these modes will make MOSAIC the world-leading MOS facility, contributing to all fields of contemporary astronomy, from extra-solar planets, to the study of the halo of the Milky Way and its satellites, and from resolved stellar populations in nearby galaxies out to observations of the earliest ‘first-light’ structures in the Universe. It will also study the distribution of the dark and ordinary matter at all scales and epochs of the Universe. Recent studies of critical technical issues such as sky-background subtraction and MOAO have demonstrated that such a MOS is feasible with state-of-the-art technology and techniques. Current studies of the MOSAIC team include further trade-offs on the wavelength coverage, a solution for compensating for the non-telecentric new design of the telescope, and tests of the saturation of skylines especially in the near-IR bands. In the 2020s the E-ELT will become the world's largest optical/IR telescope, and we argue that it has to be equipped as soon as possible with a MOS to provide the most efficient, and likely the best way to follow-up on James Webb Space Telescope (JWST) observations.
Atmospheric emission from OH molecules is a long standing problem for near-infrared astronomy. PRAXIS is a unique spectrograph, currently in the build-phase, which is fed by a fibre array that removes the OH background. The OH suppression is achieved with fibre Bragg gratings, which were tested successfully on the GNOSIS instrument. PRAXIS will use the same fibre Bragg gratings as GNOSIS in the first implementation, and new, less expensive and more efficient, multicore fibre Bragg gratings in the second implementation. The OH lines are suppressed by a factor of ~1000, and the expected increase in the signal-to-noise in the interline regions compared to GNOSIS is a factor of ~ 9 with the GNOSIS gratings and a factor of ~ 17 with the new gratings. PRAXIS will enable the full exploitation of OH suppression for the first time, which was not achieved by GNOSIS due to high thermal emission, low spectrograph transmission, and detector noise. PRAXIS will have extremely low thermal emission, through the cooling of all significantly emitting parts, including the fore-optics, the fibre Bragg gratings, a long length of fibre, and a fibre slit, and an optical design that minimises leaks of thermal emission from outside the spectrograph. PRAXIS will achieve low detector noise through the use of a Hawaii-2RG detector, and a high throughput through an efficient VPH based spectrograph. The scientific aims of the instrument are to determine the absolute level of the interline continuum and to enable observations of individual objects via an IFU. PRAXIS will first be installed on the AAT, then later on an 8m class telescope.
The accurate characterization of the field at the output of the optical fibres is of relevance for precision spectroscopy in astronomy. The modal effects of the fibre translate to the illumination of the pupil in the spectrograph and impact on the resulting point spread function (PSF). A Model is presented that is based on the Eigenmode Expansion Method (EEM) that calculates the output field from a given fibre for different manipulations of the input field. The fibre design and modes calculation are done via the commercially available Rsoft-FemSIM software. We developed a Python script to apply the EEM. Results are shown for different configuration parameters, such as spatial and angular displacements of the input field, spot size and propagation length variations, different transverse fibre geometries and different wavelengths. This work is part of the phase A study of the fibre system for MOSAIC, a proposed multi-object spectrograph for the European Extremely Large Telescope (ELT-MOS).
VIRUS is a massively replicated spectrograph built for HETDEX, the Hobby Eberly Telescope Dark Energy Experiment. It consists of 156 channels within 78 units fed by 34944 fibers over the 22 arcminute field of the upgraded HET. VIRUS covers a relatively narrow bandpass (350-550nm) at low resolution (R ~ 700) to target the emission of Lyman-alpha emitters (LAEs) for HETDEX. VIRUS is a first demonstration of industrial style assembly line replication in optical astronomy. Installation and testing of VIRUS units began in November of 2015. This winter we celebrated the first on sky instrument activity of the upgraded HET, using a VIRUS unit and LRS2-R (the upgraded facility Low Resolution Spectrograph for the HET). Here we describe progress in VIRUS installation and commissioning through June 2016. We include early sky data obtained to characterize spectrograph performance and on sky performance of the newly upgraded HET. As part of the instrumentation for first science light at the HET, the IFU fed spectrographs were used to test a full range of telescope system functionality including the field calibration unit (FCU).We also use placement of strategic IFUs to map the new HET field to the fiber placement, and demonstrate actuation of the dithering mechanism key to HETDEX observations.
We review the potential of Astrophotonics, a relatively young field at the interface between photonics and astronomical instrumentation, for spectro-interferometry. We review some fundamental aspects of photonic science that drove the emergence of astrophotonics, and highlight the achievements in observational astrophysics. We analyze the prospects for further technological development also considering the potential synergies with other fields of physics (e.g. non-linear optics in condensed matter physics). We also stress the central role of fiber optics in routing and transporting light, delivering complex filters, or interfacing instruments and telescopes, more specifically in the context of a growing usage of adaptive optics.
After having demonstrated that an IFU, attached to a microscope rather than to a telescope, is capable of differentiating complex organic tissue with spatially resolved Raman spectroscopy, we have launched a clinical validation program that utilizes a novel optimized fiber-coupled multi-channel spectrograph whose layout is based on the modular MUSE spectrograph concept. The new design features a telecentric input and has an extended blue performance, but otherwise maintains the properties of high throughput and excellent image quality over an octave of wavelength coverage with modest spectral resolution. We present the opto-mechanical layout and details of its optical performance.
For the past forty years, optical fibres have found widespread use in ground-based and space-based instruments. In most applications, these fibres are used in conjunction with conventional optics to transport light. But photonics offers a huge range of optical manipulations beyond light transport that were rarely exploited before 2001. The fundamental obstacle to the broader use of photonics is the difficulty of achieving photonic action in a multimode fibre. The first step towards a general solution was the invention of the photonic lantern1 in 2004 and the delivery of high-efficiency devices (< 1 dB loss) five years on2. Multicore fibres (MCF), used in conjunction with lanterns, are now enabling an even bigger leap towards multimode photonics. Until recently, the single-moded cores in MCFs were not sufficiently uniform to achieve telecom (SMF-28) performance. Now that high-quality MCFs have been realized, we turn our attention to printing complex functions (e.g. Bragg gratings for OH suppression) into their N cores. Our first work in this direction used a Mach-Zehnder interferometer (near-field phase mask) but this approach was only adequate for N=7 MCFs as measured by the grating uniformity3. We have now built a Sagnac interferometer that gives a three-fold increase in the depth of field sufficient to print across N ≥ 127 cores. We achieved first light this year with our 500mW Sabre FRED laser. These are sophisticated and complex interferometers. We report on our progress to date and summarize our first-year goals which include multimode OH suppression fibres for the Anglo-Australian Telescope/PRAXIS instrument and the Discovery Channel Telescope/MOHSIS instrument under development at the University of Maryland.
We present results of comprehensive re-design of an arrayed waveguide grating (AWG)-based integrated photonic spectrograph (IPS), using Silica-on-Silicon (SOS) technology, to tailor specific performance parameters of interest to high-resolution (resolving power, R = λ/Δλ= 60,000) exoplanet astronomy and stellar seismology. The compactness, modularity, stability, replicability and small-lightweight-payload of the IPS are a few promising and innovative features in the design of high-resolution spectrographs for astronomy or other areas of sciences. The IPS is designed to resolve up to 646 spectral lines per spectral order, with a wavelength spacing of 25 pm, at a central wavelength of 1630 nm (Hband). The fabricated test waveguides have been stress engineered in order to compensate the inherent birefringence of SOS waveguides. The birefringence values of fabricated test structures were quantified, to be on the order 10-6 (theoretical value required to avoid the formation of ghost-images), through inscription of Bragg-gratings on straight waveguides and subsequent measurement of Bragg-reflection spectra. An interferometer system has been integrated with the SOS-IPS (in the same chip) for the characterization of phase errors of the waveguide array. Moreover, promising results of first fabricated key photonics components to form other complex integrated photonic circuits (IPCs), such as astro-interferometers, using silicon nitride-on-insulator (SNOI) technology are also presented. The fabricated IPCs include multimode interference based devices (power splitter/combiners, optical cross/bar-switches), directional-couplers with varying power ratios, Mach-Zehnder interferometers and an AWG. The first results of annealed, low-hydrogen SNOI based devices are promising and comparable to SOI and commercial devices, with device excess-loss less than 2 dB and under 1 dB/cm waveguide-loss in the IR-wavelength.
We here report on recent progress on astronomical optical frequency comb generation at innoFSPEC-Potsdam and
present preliminary test results using the fiber-fed Multi Unit Spectroscopic Explorer (MUSE) spectrograph. The
frequency comb is generated by propagating two free-running lasers at 1554.3 and 1558.9 nm through two dispersionoptimized
nonlinear fibers. The generated comb is centered at 1590 nm and comprises more than one hundred lines with
an optical-signal-to-noise ratio larger than 30 dB. A nonlinear crystal is used to frequency double the whole comb
spectrum, which is efficiently converted into the 800 nm spectral band. We evaluate first the wavelength stability using
an optical spectrum analyzer with 0.02 nm resolution and wavelength grid of 0.01 nm. After confirming the stability
within 0.01 nm, we compare the spectra of the astro-comb and the Ne and Hg calibration lamps: the astro-comb exhibits
a much larger number of lines than lamp calibration sources. A series of preliminary tests using a fiber-fed MUSE
spectrograph are subsequently carried out with the main goal of assessing the equidistancy of the comb lines. Using a
P3d data reduction software we determine the centroid and the width of each comb line (for each of the 400 fibers
feeding the spectrograph): equidistancy is confirmed with an absolute accuracy of 0.4 pm.
The use of deployable fibre-bundles plays an increasing role in the design of future Multi-Object-Spectrographs (MOS).
Within a research and development project for "Enabling Technologies for the E-ELT", various miniaturized, fibrebundles
were designed, built and tested for their suitability for a proposed ELT-MOS instrument.
The paper describes the opto-mechanical designs of the bundles and the different manufacture approaches, using glued,
stacked and fused optical fibre bundles. The fibre bundles are characterized for performance, using dedicated testbenches
in the laboratory and at a telescope simulator. Their performance is measured with respect to geometric
accuracy, throughput, FRD behavior and cross-talk between channels.
C. Evans, M. Puech, B. Barbuy, P. Bonifacio, J.-G. Cuby, E. Guenther, F. Hammer, P. Jagourel, L. Kaper, S. Morris, J. Afonso, P. Amram, H. Aussel, A. Basden, N. Bastian, G. Battaglia, B. Biller, N. Bouché, E. Caffau, S. Charlot, Y. Clénet, F. Combes, C. Conselice, T. Contini, G. Dalton, B. Davies, K. Disseau, J. Dunlop, F. Fiore, H. Flores, T. Fusco, D. Gadotti, A. Gallazzi, E. Giallongo, T. Gonçalves, D. Gratadour, V. Hill, M. Huertas-Company, R. Ibata, S. Larsen, O. Le Fèvre, B. Lemasle, C. Maraston, S. Mei, Y. Mellier, G. Östlin, T. Paumard, R. Pello, L. Pentericci, P. Petitjean, M. Roth, D. Rouan, D. Schaerer, E. Telles, S. Trager, N. Welikala, S. Zibetti, B. Ziegler
Over the past 18 months we have revisited the science requirements for a multi-object spectrograph (MOS) for the
European Extremely Large Telescope (E-ELT). These efforts span the full range of E-ELT science and include input
from a broad cross-section of astronomers across the ESO partner countries. In this contribution we summarise the key
cases relating to studies of high-redshift galaxies, galaxy evolution, and stellar populations, with a more expansive
presentation of a new case relating to detection of exoplanets in stellar clusters. A general requirement is the need for
two observational modes to best exploit the large (≥40 arcmin2) patrol field of the E-ELT. The first mode (‘high
multiplex’) requires integrated-light (or coarsely resolved) optical/near-IR spectroscopy of >100 objects simultaneously.
The second (‘high definition’), enabled by wide-field adaptive optics, requires spatially-resolved, near-IR of >10
objects/sub-fields. Within the context of the conceptual study for an ELT-MOS called MOSAIC, we summarise the toplevel
requirements from each case and introduce the next steps in the design process.
The innovation of optical frequency combs (OFCs) generated in passive mode-locked lasers has provided astronomy
with unprecedented accuracy for wavelength calibration in high-resolution spectroscopy in research areas such as the
discovery of exoplanets or the measurement of fundamental constants. The unique properties of OCFs, namely a highly
dense spectrum of uniformly spaced emission lines of nearly equal intensity over the nominal wavelength range, is not
only beneficial for high-resolution spectroscopy. Also in the low- to medium-resolution domain, the OFCs hold the
promise to revolutionise the calibration techniques. Here, we present a novel method for generation of OFCs. As
opposed to the mode-locked laser-based approach that can be complex, costly, and difficult to stabilise, we propose an
all optical fibre-based system that is simple, compact, stable, and low-cost. Our system consists of three optical fibres
where the first one is a conventional single-mode fibre, the second one is an erbium-doped fibre and the third one is a
highly nonlinear low-dispersion fibre. The system is pumped by two equally intense continuous-wave (CW) lasers. To be
able to control the quality and the bandwidth of the OFCs, it is crucial to understand how optical solitons arise out of the
initial modulated CW field in the first fibre. Here, we numerically investigate the pulse evolution in the first fibre using
the technique of the solitons radiation beat analysis. Having applied this technique, we realised that formation of higherorder
solitons is supported in the low-energy region, whereas, in the high-energy region, Kuznetsov-Ma solitons appear.
The Visible Integral-field Replicable Unit Spectrograph (VIRUS) consists of a baseline build of 150 identical
spectrographs (arrayed as 75 unit pairs) fed by 33,600 fibers, each 1.5 arcsec diameter, at the focus of the upgraded 10
m Hobby-Eberly Telescope (HET). VIRUS has a fixed bandpass of 350-550 nm and resolving power R~700. VIRUS is
the first example of industrial-scale replication applied to optical astronomy and is capable of surveying large areas of
sky, spectrally. The VIRUS concept offers significant savings of engineering effort, cost, and schedule when compared
to traditional instruments.
The main motivator for VIRUS is to map the evolution of dark energy for the Hobby-Eberly Telescope Dark Energy
Experiment (HETDEX), using 0.8M Lyman-α emitting galaxies as tracers. The full VIRUS array is due to be deployed starting at the end of 2014 and will provide a powerful new facility instrument for the HET, well suited to the
survey niche of the telescope, and will open up large area surveys of the emission line universe for the first time.
VIRUS is in full production, and we are about half way through. We review the production design, lessons learned in
reaching volume production, and preparation for deployment of this massive instrument. We also discuss the application
of the replicated spectrograph concept to next generation instrumentation on ELTs.
Dispersion engineering in integrated silicon nitride waveguides is numerically and experimentally investigated. We show that by modifying the transversal dimensions of the silicon nitride core, it is possible to have a good control of the chromatic dispersion. The inaccuracies due to typical fabrication process in PECD-SiXNY films shows that the dispersion uncertainty is in the order of 20 ps/nm-km at 1550 nm. Silicon nitride waveguides were then fabricated using the same PECVD process and the chromatic dispersion was measured using a low-coherence frequency domain interferometry technique. A comparison between measurements and simulations shows good agreement.
Silicon nitride ring resonators with diameter of 250 and 500 μm are fabricated and their spectral characteristics
investigated with the ultimate goal of optical frequency comb generation for astronomical spectrograph calibration. A
continuously tunable laser was used to evaluate the spectral characteristics (propagation losses and transmission
properties) of PECVD silicon nitride waveguides and ring-resonators. Losses were measured to be smaller than 0.75
dB/cm over the range between 1500 nm and 1620 nm. The transmission properties of the fabricated ring resonators were
assessed for the TE and TM modes, showing promise for the ultimate goal of astronomical optical frequency comb
generation.
The engineering of the propagation constant in integrated silicon nitride waveguides is numerically investigated. We
compare several geometrical designs and show that fairly large chromatic dispersion control is obtained when the
transversal dimensions are modified.
We numerically investigated the possibility of generating high-quality ultra-short optical pulses with broad frequencycombs spectra in a system consisting of three optical fibres. In this system, the first fibre is a conventional single-mode fibre, the second one is erbium-doped, and the last one is a low-dispersion fibre. The system is pumped with a modulated sine-wave generated by two equally intense lasers with the wavelengths λ1and λ2 such that their central wavelength is at λc = (λ1 + λ2)/2 = 1531 nm. The modelling was performed using the generalised nonlinear Schrödinger equation which includes the Kerr and Raman effects, as well as the higher-order dispersion and gain. We took a close look at the pulse evolution in the first two stages and studied the pulse behaviour depending on the group-velocity dispersion and the nonlinear parameter of first fibre, as well as the initial laser frequency separation. For these parameters, the optimum lengths of fibre 1 and 2 were found that provide low-noise pulses. To characterise the pulse energy content, we introduced a figure of merit that was dependent on the group-velocity dispersion, the nonlinearity of fibre 1, and the laser separation.
The Multi-Unit Spectroscopic Explorer (MUSE), an integral-field spectrograph for the ESO Very Large Telescope, has
been built and integrated by a consortium of 7 European institutes. MUSE can simultaneously record spectra across a
field of view of 1 square arcminute in the wavelength range from 465nm to 930nm. The calibration unit (CU) for MUSE
was developed to provide accurate flat fielding, spectral, geometrical, image quality and efficiency calibration for both
the wide-field and AO-assisted narrow-field modes. This paper describes the performance of the CU and electronics,
from the subsystem validation to the integration, alignment and use in the MUSE instrument.
The Visible Integral-field Replicable Unit Spectrograph (VIRUS) consists of a baseline build of 150 identical
spectrographs (arrayed as 75 units, each with a pair of spectrographs) fed by 33,600 fibers, each 1.5 arcsec diameter,
deployed over the 22 arcminute field of the upgraded 10 m Hobby-Eberly Telescope (HET). The goal is to deploy 82
units. VIRUS has a fixed bandpass of 350-550 nm and resolving power R~700. VIRUS is the first example of
industrial-scale replication applied to optical astronomy and is capable of spectral surveys of large areas of sky. This
approach, in which a relatively simple, inexpensive, unit spectrograph is copied in large numbers, offers significant
savings of engineering effort, cost, and schedule when compared to traditional instruments.
The main motivator for VIRUS is to map the evolution of dark energy for the Hobby-Eberly Telescope Dark Energy
Experiment (HETDEX) using 0.8M Lyman-α emitting galaxies as tracers. The full VIRUS array is due to be deployed
by early 2014 and will provide a powerful new facility instrument for the HET, well suited to the survey niche of the
telescope. VIRUS and HET will open up wide-field surveys of the emission-line universe for the first time. We present
the production design and current status of VIRUS.
GNOSIS has provided the first on-telescope demonstration of a concept to utilize complex aperioidc fiber Bragg
gratings to suppress the 103 brightest atmospheric hydroxyl emission doublets between 1.47-1.7 μm. The unit is
designed to be used at the 3.9-meter Anglo-Australian Telescope (AAT) feeding the IRIS2 spectrograph. Unlike
previous atmospheric suppression techniques GNOSIS suppresses the lines before dispersion. We present the
results of laboratory and on-sky tests from instrument commissioning. These tests reveal excellent suppression
performance by the gratings and high inter-notch throughput, which combine to produce high fidelity OH-free
spectra.
We here discuss recent progress on astronomical optical frequency comb generation at innoFSPEC-Potsdam. Two
different platforms (and approaches) are numerically and experimentally investigated targeting medium and low
resolution spectrographs at astronomical facilities in which innoFSPEC is currently involved. In the first approach, a
frequency comb is generated by propagating two lasers through three nonlinear stages – the first two stages serve for the
generation of low-noise ultra-short pulses, while the final stage is a low-dispersion highly-nonlinear fibre where the
pulses undergo strong spectral broadening. In our approach, the wavelength of one of the lasers can be tuned allowing
the comb line spacing being continuously varied during the calibration procedure – this tuning capability is expected to
improve the calibration accuracy since the CCD detector response can be fully scanned. The input power, the dispersion,
the nonlinear coefficient, and fibre lengths in the nonlinear stages are defined and optimized by solving the Generalized
Nonlinear Schrodinger Equation. Experimentally, we generate the 250GHz line-spacing frequency comb using two
narrow linewidth lasers that are adiabatically compressed in a standard fibre first and then in a double-clad Er/Yb doped
fibre. The spectral broadening finally takes place in a highly nonlinear fibre resulting in an astro-comb with 250
calibration lines (covering a bandwidth of 500 nm) with good spectral equalization.
In the second approach, we aim to generate optical frequency combs in dispersion-optimized silicon nitride ring
resonators. A technique for lowering and flattening the chromatic dispersion in silicon nitride waveguides with silica
cladding is proposed and demonstrated. By minimizing the waveguide dispersion in the resonator two goals are targeted:
enhancing the phase matching for non-linear interactions and producing equally spaced resonances. For this purpose,
instead of one cladding layer our design incorporates two layers with appropriate thicknesses. We demonstrate a nearly
zero dispersion (with +/- 4 ps/nm-km variation) over the spectral region from 1.4 to 2.3 microns.
The techniques reported here should open new avenues for the generation of compact astronomical frequency comb
sources on a chip or in nonlinear fibres.
The power losses introduced by bending multimode optical fibres have been studied since the last forty years, when the
efficient transmission of those fibres was regarded as very useful for devices that require the transmission of spatially
incoherent light (white light), e.g. Integral Field Units (IFU) for Astrophysics. In the literature, the influence of the fibre
coating on transmission properties is rarely taken in account, i.e. the fibres under test are frequently stripped, however, in
practical applications the fibres are used with their coating. We present the results of an experimental study of
attenuation due to bending stress on several large-core multimode coated optical fibres. In this experiment the
attenuation is studied as a function of applied stress in kilo pounds squared inches [kpsi]. The fibres under test are similar
to the type of optical fibre used in astronomy for fibre based spectroscopic applications, or used as probes for chemical
sensing applications. We investigate a range of different core diameters for both all-silica and hard cladding step-index
fibres. Optical-fibres manufacturers are offering a variety of coating materials and, the tested fibres are coated with the
following: silicone, polyimide, two types of fluorine doped acrylate, and acrylate.
We show that for a given coating material, applying the same bending stress on fibers introduces the same amount of
attenuation, regardless of the fibre bending radius or fibre core diameter. We also show differences in attenuation due to
the use of different coating material.
A conventional Arrayed Waveguide Grating (AWG) has been modified, without output receiver waveguides, for nonconventional
applications such as Astrophotonics and spectroscopy sensing where the input signal can have information
over the entire band and a continuum of light/spectrum. The material system chosen for the AWG design is siliconnitride/
SiO2/Si (Si3N4-SiO2-Si) for its relatively high refractive index, which for a given channel spacing allowing a
more compact device than Silicon-on-Silica. Further, CMOS compatibility and the presence of high non-liner optical
coefficient would be an added advantage to design and fabricate densely integrated photonic sub-systems, such as
calibration source and AWG, for astrophotonics and spectroscopy. The proposed AWG utilizes a flat image plane
optimized for minimal aberration. An analytical calculation, based on Gaussian beam approximation, was used to
determine the optimal flat plane position where the non-uniformity in 1/e electric field widths is minimal. This plane can
be used as the dicing plane to re-image the entire output of the AWG onto a detector array to sample the entire spectrum.
Tailored AWG, with flat image-plane, designed to resolve 48 spectral channels with 0.4nm (50GHz) resolution and
adjacent channel cross-talk level within a 0.2nm window (ITU-grid) ~ -28dB. Calculated insertion loss non-uniformity is
close to 3dB. The foot-print of high index contrast (Δn=23%) IPS is ~ 12x8.5 mm2. The modelled mean spectral
resolving power, R, at the flat image-plane is ~ 7,600. The design principle could be utilised for devices using other
material systems with different parameters.
Martin Roth, Karl Zenichowski, Nicolae Tarcea, Jürgen Popp, Silvia Adelhelm, Marvin Stolz, Andreas Kelz, Christer Sandin, Svend-Marian Bauer, Thomas Fechner, Thomas Jahn, Emil Popow, Bernhard Roth, Paul Singh, Mudit Srivastava, Dieter Wolter
Astronomical instrumentation is most of the time faced with challenging requirements in terms of sensitivity, stability,
complexity, etc., and therefore leads to high performance developments that at first sight appear to be suitable only for
the specific design application at the telescope. However, their usefulness in other disciplines and for other applications
is not excluded. The ERA2 facility is a lab demonstrator, based on a high-performance astronomical spectrograph, which
is intended to explore the innovation potential of fiber-coupled multi-channel spectroscopy for spatially resolved
spectroscopy in life science, material sciences, and other areas of research.
We discuss the development of multi-core fiber Bragg gratings (FBGs) to be applied to astrophotonics, more specifically
to near-infrared spectroscopy for ground-based instruments. The multi-core FBGs require over 100 notches to reject the
OH lines in a broad wavelength range (160 nm). The number of cores of the fiber should correspond to the mode number
in the multi-mode fibers and should be large enough to be able to capture a sufficient amount of light from the telescope.
A phase-mask based technique is used to fabricate the multi-core FBGs.
The generation of a broadband optical frequency comb with 80 GHz spacing by propagation of a sinusoidal wave
through three dispersion-optimized nonlinear stages is numerically investigated. The input power, the dispersion, the
nonlinear coefficient, and lengths are optimized for the first two stages for the generation of low-noise ultra-short pulses.
The final stage is a low-dispersion highly-nonlinear fibre where the ultra-short pulses undergo self-phase modulation for
strong spectral broadening. The modeling is performed using a Generalized Nonlinear Schrodinger Equation
incorporating Kerr and Raman nonlinearities, self-steepening, high-order dispersion and gain.
In the proposed approach the sinusoidal input field is pre-compressed in the first fibre section. This is shown to be
necessary to keep the soliton order below ten to minimize the noise build-up during adiabatic pulse compression, when
the pulses are subsequently amplified in the next fibre section (rare-earth-doped-fibre with anomalous dispersion). We
demonstrate that there is an optimum balance between dispersion, input power and nonlinearities, in order to have
adiabatic pulse compression. It is shown that the intensity noise grows exponentially as the pulses start to be compressed
in the amplifying fibre. Eventually, the noise decreases and reaches a minimum when the pulses are maximally
compressed. A train of 70 fs pulses with up to 3.45 kW peak power and negligible noise is generated in our simulations,
which can be spectrally broadened in a highly-nonlinear fibre. The main drawback of this compression technique is the
small fibre length tolerance where noise is negligible (smaller than 10 cm for erbium-doped fibre length of 15 m). We
finally investigate how the frequency comb characteristics are modified by incorporating an optical feedback. We show
that frequency combs appropriate for calibration of astronomical spectrographs can be improved by using this technique.
A conventional Arrayed Waveguide Grating (AWG) has been tailored for non-conventional applications such as Astro-Photonics, Life-science and spectroscopy where the input signal can have information over the full continuum of
light/spectrum, compared to discrete optical channels in optical communication systems. The material system chosen for
the AWG design is silicon-nitride/SiO2/Si (Si3N4-SiO2-Si) for it's relatively high refractive index, which for a given
channel spacing allowing a more compact device than Silicon-on-Silica. While existing conventional AWGs cannot be
utilized in spectroscopy when the input is a continuum, due to the fixed output waveguides where the centre wavelength
λc and therefore rest of the wavelength channels have been assigned to predetermined output waveguides, the device
under development has no output waveguides permitting to utilize the entire-image plane of the output star-coupler. The
output of the AWG can then be re-imaged onto a detector array to sample the entire output spectrum, such as the 2-D
infrared arrays used in astronomy. The designed AWG can resolve up to 40 spectral channels with wavelength spacing
0.4nm (50GHz), adjacent channel cross-talk level < -25dB at the ITU grid (25GHz) and non-uniformity ~ 2.5dB. The
modeled mean spectral resolving power, R, at the flat image-plane is ~ 12,200.
A technique for flattening the chromatic dispersion in silicon nitride waveguides with silica cladding is proposed and
numerically investigated. By modifying the transversal dimensions of the silicon nitride core and by adding several
cladding layers with appropriate refractive indices and thicknesses, we demonstrate dispersion flattening over large
spectral bandwidths in the near infrared. We analyze several cladding refractive index profiles that could be realistically
fabricated by using existing materials and doping procedures.
We show that cladding engineering allows for much more dispersion control (and flattening) in comparison with
optimizing only the core transversal dimensions. For the latter case it is demonstrated that while the zero dispersion
wavelength can be shifted to a great extent, the effect of the cross-section adjustment in the flatness is very limited. In
sharp contrast, by adding two cladding layers and decreased refractive index values, the dispersion ripple can be strongly
reduced. By further adding one more layer and by adjusting their refractive indices it is possible to obtain nearly constant
chromatic dispersion (only +/- 3 ps/nm-km variation) over the spectral region from 1.8 to 2.4 microns. In our
calculations, the analyzed change in the silica or silicon nitride refractive index is up to +/-3%. Our technique should
open new avenues for the demonstration of high-performance nonlinear devices on a chip. Furthermore highly dispersive
integrated photonic components can be envisaged for slow light applications and integrated photonics spectrographs.
GNOSIS is an OH suppression unit to be used in conjunction with existing spectrographs. The OH suppression
is achieved using fibre Bragg gratings (FBGs), and will deliver the darkest near-infrared background of any
ground-based instrument. Laboratory and on-sky tests demonstrate that FBGs can suppress OH lines by 30dB
whilst maintaing > 90% throughput between the lines, resulting in a 4 mag decrease in the background.
In the first implementation GNOSIS will feed IRIS2 on the AAT. It will consist of a seven element lenslet
array, covering 1.4" on the sky, and will suppress the 103 brightest OH lines between 1.47 and 1.70 μm. Future
upgrades will include J-band suppression and implementation on an 8m telescope.
MUSE (Multi Unit Spectroscopic Explorer) is a second generation instrument developed for ESO (European Southern
Observatory) to be installed on the VLT (Very Large Telescope) in year 2012. The MUSE project is supported by a
European consortium of 7 institutes. After a successful Final Design Review the project is now facing a turning point
which consist in shifting from design to manufacturing, from calculation to test, ... from dream to reality.
At the start, many technical and management challenges were there as well as unknowns. They could all be derived of
the same simple question: How to deal with complexity? The complexity of the instrument, of the work to de done, of
the organization, of the interfaces, of financial and procurement rules, etc.
This particular moment in the project life cycle is the opportunity to look back and evaluate the management methods
implemented during the design phase regarding this original question. What are the lessons learn? What has been
successful? What could have been done differently? Finally, we will look forward and review the main challenges of the
MAIT (Manufacturing Assembly Integration and Test) phase which has just started as well as the associated new
processes and evolutions needed.
MUSE (Multi Unit Spectroscopic Explorer) is a second generation instrument developed for ESO (European Southern Observatory) and will be assembled to the VLT (Very Large Telescope) in 2012. The MUSE instrument can simultaneously record 90.000 spectra in the visible wavelength range (465-930nm), across a 1*1arcmin2 field of view, thanks to 24 identical Integral Field Units (IFU). A collaboration of 7 institutes has successfully passed the Final Design Review and is currently working on the first sub-assemblies. The sharing of performances has been based on 5 main functional sub-systems. The Fore Optics sub-system derotates and anamorphoses the VLT Nasmyth focal plane image, the Splitting and Relay Optics associated with the Main Structure are feeding each IFU with 1/24th of the field of view.
Each IFU is composed of a 3D function insured by an image slicer system and a spectrograph, and a detection function
by a 4k*4k CCD cooled down to 163°K. The 5th function is the calibration and data reduction of the instrument. This
article depicts the breakdown of performances between these sub-systems (throughput, image quality...), and underlines
the constraining parameters of the interfaces either internal or with the VLT. The validation of all these requirements is a
critical task started a few months ago which requires a clear traceability and performances analysis.
Supercontinuum white light sources (SCLS) are intense, spatially coherent laser sources with a very broad and flat
spectral energy distribution which have very quickly found ubiquitous use in optical laboratories. As photonics is now
providing more and more applications for astronomical instrumentation, the possible use of SCLS as a calibration light
source for spectroscopy has been tested. A standard industrial SCLS was coupled to the calibration unit of the PMAS
integral field spectrophotometer and compared directly to the PMAS standard tungsten filament lamp that is normally
used for calibration exposures. We report on comparative measurements concerning flux, spectral energy distribution,
and temporal stability.
Astrophotonics offers a solution to some of the problems of building instruments for the next generation of telescopes
through the use of photonic devices to miniaturise and simplify instruments. It has already proved its worth in
interferometry over the last decade and is now being applied to nightsky background suppression. Astrophotonics offers
a radically different approach to highly-multiplexed spectroscopy to the benefit of galaxy surveys such as are required to
determine the evolution of the cosmic equation of state. The Astrophotonica Europa partnership funded by the EU via
OPTICON is undertaking a wide-ranging survey of the technological opportunities and their applicability to high-priority
astrophysical goals of the next generation of observatories. Here we summarise some of the conclusions.
The Multi-Unit Spectroscopic Explorer (MUSE) is an integral-field spectrograph for the ESO Very Large Telescope.
After completion of the Final Design Review in 2009, MUSE is now in its manufacture and assembly phase. To achieve
a relative large field-of-view with fine spatial sampling, MUSE features 24 identical spectrograph-detector units. The
acceptance tests of the detector sub-systems, the design and manufacture of the calibration unit and the development of
the Data Reduction Software for MUSE are under the responsibility of the AIP. The optical design of the spectrograph
implies strict tolerances on the alignment of the detector systems to minimize aberrations. As part of the acceptance
testing, all 24 detector systems, developed by ESO, are mounted to a MUSE reference spectrograph, which is illuminated
by a set of precision pinholes. Thus the best focus is determined and the image quality of the spectrograph-detector
subsystem across wavelength and field angle is measured.
The quantity and length of optical fibers required for the Hobby-Eberly Telescope* Dark Energy eXperiment
(HETDEX) create unique fiber handling challenges. For HETDEX‡, at least 33,600 fibers will transmit light from the
focal surface of the telescope to an array of spectrographs making up the Visible Integral-Field Replicable Unit
Spectrograph (VIRUS). Up to 96 Integral Field Unit (IFU) bundles, each containing 448 fibers, hang suspended from the
telescope's moving tracker located more than 15 meters above the VIRUS instruments. A specialized mechanical system
is being developed to support fiber optic assemblies onboard the telescope. The discrete behavior of 448 fibers within a
conduit is also of primary concern. A life cycle test must be conducted to study fiber behavior and measure Focal Ratio
Degradation (FRD) as a function of time. This paper focuses on the technical requirements and design of the HETDEX
fiber optic support system, the electro-mechanical test apparatus for accelerated life testing of optical fiber assemblies.
Results generated from the test will be of great interest to designers of robotic fiber handling systems for major
telescopes. There is concern that friction, localized contact, entanglement, and excessive tension will be present within
each IFU conduit and contribute to FRD. The test apparatus design utilizes six linear actuators to replicate the movement
of the telescope over 65,000 accelerated cycles, simulating five years of actual operation.
ERASMUS-F is a pathfinder study for a possible E-ELT 3D-instrumentation, funded by the German Ministry for
Education and Research (BMBF). The study investigates the feasibility to combine a broadband optical spectrograph
with a new generation of multi-object deployable fibre bundles. The baseline approach is to modify the spectrograph of
the Multi-Unit Spectroscopic Explorer (MUSE), which is a VLT integral-field instrument using slicers, with a fibre-fed
input. Taking advantage of recent developments in astrophotonics, it is planed to equip such an instrument with fused
fibre bundles (hexabundles) that offer larger filling factors than dense-packed classical fibres.
The overall project involves an optical and mechanical design study, the specifications of a software package for 3Dspectrophotometry,
based upon the experiences with the P3d Data Reduction Software and an investigation of the
science case for such an instrument. As a proof-of-concept, the study also involves a pathfinder instrument for the VLT,
called the FIREBALL project.
The PMAS integral field spectrophotometer, operated at the Calar Alto Observatory 3.5m Telescope, is one of the most
demanded instruments of its kind. The optical system was designed for a camera field of view to accommodate a 4K×4K
detector with 15μm pixels. However, due to a failure of one of the initially foreseen 2K×4K CCDs in a mosaic
configuration, only half of the available field of view could be covered to date. Owing to the high demand from the user
community, an upgrade to the full complement of 4K×4K pixels was envisaged, based on the availability of the new e2v
CCD231 device. We describe the specification, implementation, test, and commissioning of this new detector for PMAS.
The Visible Integral-field Replicable Unit Spectrograph (VIRUS) consists of a baseline build of 150 identical
spectrographs (arrayed as 75 units, each with a pair of spectrographs) fed by 33,600 fibers, each 1.5 arcsec diameter,
deployed over the 22 arcminute field of the upgraded 10 m Hobby-Eberly Telescope (HET). The goal is to deploy 96
units. VIRUS has a fixed bandpass of 350-550 nm and resolving power R~700. VIRUS is the first example of
industrial-scale replication applied to optical astronomy and is capable of spectral surveys of large areas of sky. The
method of industrial replication, in which a relatively simple, inexpensive, unit spectrograph is copied in large numbers,
offers significant savings of engineering effort, cost, and schedule when compared to traditional instruments.
The main motivator for VIRUS is to map the evolution of dark energy for the Hobby-Eberly Telescope Dark Energy
Experiment (HETDEX+) using 0.8M Lyman-α emitting galaxies as tracers. The full VIRUS array is due to be deployed
in late 2011 and will provide a powerful new facility instrument for the HET, well suited to the survey niche of the
telescope. VIRUS and HET will open up wide field surveys of the emission-line universe for the first time. We present
the design, cost, and current status of VIRUS as it enters production, and review performance results from the VIRUS
prototype. We also present lessons learned from our experience designing for volume production and look forward to
the application of the VIRUS concept on future extremely large telescopes (ELTs).
Summary: The Multi Unit Spectroscopic Explorer (MUSE) is a second-generation VLT panoramic integral-field
spectrograph currently in manufacturing, assembly and integration phase. MUSE has a field of 1x1 arcmin2 sampled at
0.2x0.2 arcsec2 and is assisted by the VLT ground layer adaptive optics ESO facility using four laser guide stars. The
instrument is a large assembly of 24 identical high performance integral field units, each one composed of an advanced
image slicer, a spectrograph and a 4kx4k detector. In this paper we review the progress of the manufacturing and report
the performance achieved with the first integral field unit.
KEYWORDS: Calibration, Visualization, Data processing, James Webb Space Telescope, Algorithm development, Sensors, Data analysis, Cryogenics, Spectrographs, Molybdenum
The future James Webb Space Telescope (JWST), developed jointly by the American, European and Canadian
space agencies (NASA, ESA and CSA), is scheduled for launch in 2013. The near-infrared spectrograph NIRSpec
will be a major element of its instrument suite and is built by EADS Astrium for ESA. NIRSpec is a multiobject
spectrograph allowing astronomers to obtain the spectra of more than one hundred objects in a single
exposure. NIRSpec is currently under construction and, when finished, will be subjected to a stringent onground
test campaign to verify its performance. These tests are conducted in collaboration with ESA. A rapid
and reliable system to handle and analyse the data is crucial in this phase as the time available to run the
cryogenic tests is limited. To facilitate this process we are developing a toolbox of dedicated algorithms and
interactive visualisation modules. These standalone modules form the basis of the Instrument Quick Look
Analysis and Calibration (IQLAC) software. Individual workflows, optimized for specific tests, can then be
generated efficiently using this toolbox. Furthermore, this set of dedicated algorithms will provide a reference
frame for the development of the operational data processing software by ESA.
We have conducted extensive tests of both transmission and focal ratio degradation (FRD) on two integral field
units currently in use on the VIRUS-P integral field spectrograph. VIRUS-P is a prototype for the VIRUS
instrument proposed for the Hobby-Eberly Telescope at McDonald Observatory. All tests have been conducted
at an input f-ratio of F/3.65 and with an 18% central obscuration in order to simulate optical conditions on the
HET. Transmission measurements were conducted with narrow-band interference filters (FWHM: 10 nm) at 10
discrete wavelengths (337 to 600 nm), while FRD tests were made at 365 nm, 400 nm and 600 nm. The influence
of wavelength, end immersion, fiber type and length on both FRD and transmission is explored. Most notably,
we find no wavelength dependence on FRD down to 365 nm. All fibers tested are within the VIRUS instrument
specifications for both FRD and transmission. We present the details of our differential FRD testing method and
explain a simple and robust technique of aligning the test bench and optical fiber axes to within ±0.1 degrees.
innoFSPEC Potsdam is presently being established as in interdisciplinary innovation center for fiber-optical
spectroscopy and sensing, hosted by Astrophysikalisches Institut Potsdam and the Physical Chemistry group of Potsdam
University, Germany. The center focuses on fundamental research in the two fields of fiber-coupled multi-channel
spectroscopy and optical fiber-based sensing. Thanks to its interdisciplinary approach, the complementary methodologies
of astrophysics on the one hand, and physical chemistry on the other hand, are expected to spawn synergies that
otherwise would not normally become available in more standard research programmes. innoFSPEC targets future
innovations for next generation astrophysical instrumentation, environmental analysis, manufacturing control and
process monitoring, medical diagnostics, non-invasive imaging spectroscopy, biopsy, genomics/proteomics, high-throughput
screening, and related applications.
During 2007, a new polarimetric observing mode was added to the existing integral-field spectrograph PMAS. Initially,
this instrumental upgrade is aimed to measure the linear polarization states and to determine the three Stokes parameters
I, Q and U. The PMAS instrument offers an integral-field of view of up to 256 square arcseconds, while the spectrograph
covers a wavelength region from 340 to 900 nm. The paper presents the opto-mechanical design of the polarimetric unit,
summarizes calibration and test results and describes the first data taken during commissioning at the Calar Alto
observatory. Given the range of applications and the large parameter space (two spatial coordinates, one wavelength
dimension, plus polarimetric information), the realization of the PMAS 2D-Spectro-Polarimeter provides a unique
capability for night-time astrophysical observations, such as the study of scattering processes or magnetic fields for a
range of astronomical targets.
The Multi-Unit Spectroscopic Explorer (MUSE) is an integral-field spectrograph for the VLT for the next decade. Using
an innovative field-splitting and slicing design, combined with an assembly of 24 spectrographs, MUSE will provide
some 90,000 spectra in one exposure, which cover a simultaneous spectral range from 465 to 930nm. The design and
manufacture of the Calibration Unit, the alignment tests of the Spectrograph and Detector sub-systems, and the
development of the Data Reduction Software for MUSE are work-packages under the responsibility of the AIP, who is a
partner in a European-wide consortium of 6 institutes and ESO, that is led by the Centre de Recherche Astronomique de
Lyon. MUSE will be operated and therefore has to be calibrated in a variety of modes, which include seeing-limited and
AO-assisted operations, providing a wide and narrow-field-of-view. MUSE aims to obtain unprecedented ultra-deep 3D-spectroscopic
exposures, involving integration times of the order of 80 hours at the VLT. To achieve the corresponding
science goals, instrumental stability, accurate calibration and adequate data reduction tools are needed. The paper
describes the status at PDR of the AIP related work-packages, in particular with respect to the spatial, spectral, image
quality, and geometrical calibration and related data reduction aspects.
The Hobby-Eberly Telescope Dark Energy eXperiment [HETDEX] will employ over 43,000 optical fibers to feed light
to 192 Visible Integral-Field Replicable Unit Spectrographs [VIRUS]. Each VIRUS instrument is fed by 224 fibers. To
reduce cost, the spectrographs are combined into pairs; thus, two bundles of 224 fibers are combined into a single
Integral Field Unit [IFU] of 448 fibers. On the input end the fibers are arranged in a square 'dense-pack' array at the
HET focal surface. At the output end the IFU terminates in two separate linear arrays which provide entry slits for each
spectrometer unit. The IFU lengths must be kept to an absolute minimum to mitigate losses; however, consideration of
overall project cost and duration of the science mission have resulted in the generation of two competing concepts.
Multiple axes of motion are imposed on the IFUs as they span the shortest distance from the focal surface to each
VIRUS unit. Arranging and supporting 96 IFUs, that have a total mass over 450 kg, in a manner that is compatible with
these complex translations, together with the management of accompanying forces on the tracking mechanism of the
HET, presents a significant technical challenge, which is further compounded by wind buffeting. The longer IFU
concept is favored due to overall project cost, but requires tests to assure that the fibers can withstand forces associated
with a height differential of 16.25 meters without FRD losses or breakage.
We describe the design, construction, and performance of VIRUS-P (Visible Integral-field Replicable Unit
Spectrograph - Prototype), the prototype for 150+ identical fiber-fed integral field spectrographs for the Hobby-Eberly
Telescope Dark Energy Experiment (HETDEX). VIRUS-P was commissioned in 2007, is in regular service on the
McDonald Observatory 2.7 m Smith telescope, and offers the largest field of any integral field spectrograph. The 246-fiber IFU uses a densepak-type fiber bundle with a 1/3 fill factor. It is fed at f/3.65 through a telecentric, two-group
dioptric focal reducer. The spectrograph's double-Schmidt optical design uses a volume phase holographic grating at
the pupil between the articulating f/3.32 folded collimator and the f/1.33 cryogenic prime focus camera. High on-sky
throughput is achieved with this catadioptric system by the use of high reflectivity dielectric coatings, which set the
340-670 nm bandwidth. VIRUS-P is gimbal-mounted on the telescope to allow short fibers for high UV throughput,
while maintaining high mechanical stability. The instrument software and the 18 square arcmin field, fixed-offset guider
provide rapid acquisition, guiding, and precision dithering to fill in the IFU field. Custom software yields Poisson noise
limited, sky subtracted spectra. The design characteristics are described that achieved uniformly high image quality with
low scattered light and fiber-to-fiber cross talk. System throughput exceeds requirements and peaks at 40%. The
observing procedures are described, and example observations are given.
VIRUS is a planned integral-field instrument for the Hobby-Eberly Telescope (HET). In order to achieve a large field-of-view and high grasp at reasonable costs, the approach is to replicate integral-field units (IFU) and medium sized spectrographs many times. The Astrophysical Institute Potsdam (AIP) contributes to VIRUS with the development and testing of the IFU prototype. While the overall project is presented by Hill et al.1, this paper describes the opto-mechanical design and the manufacture of the fiber-based IFU subsystem. The initial VIRUS development aims to produce a prototype and to measure its performance. Additionally, techniques will be investigated to allow industrial replication of the highly specific fiber-bundle layout. This will be necessary if this technique is to be applied to the next generation of even larger astronomical instrumentation.
We present the design of, and the science drivers for, the Visible Integral-field Replicable Unit Spectrograph (VIRUS). This instrument is made up of 145 individually small and simple spectrographs, each fed by a fiber integral field unit. The total VIRUS-145 instrument covers ~30 sq. arcminutes per observation, providing integral field spectroscopy from 340 to 570 nm, simultaneously, of 35,670 spatial elements, each 1 sq. arcsecond on the sky. This corresponds to 15 million resolution elements per exposure. VIRUS-145 will be mounted on the Hobby-Eberly Telescope and fed by a new wide-field corrector with 22 arcminutes diameter field of view. VIRUS represents a new approach to spectrograph design, offering the science multiplex advantage of huge sky coverage for an integral field spectrograph, coupled with the engineering multiplex advantage of >100 spectrographs making up a whole. VIRUS is designed for the Hobby-Eberly Telescope Dark Energy Experiment (HETDEX) which will use baryonic acoustic oscillations imprinted on the large-scale distribution of Lyman-α emitting galaxies to provide unique constraints on the expansion history of the universe that can constrain the properties of dark energy.
The Multi Unit Spectroscopic Explorer (MUSE) is a second-generation VLT panoramic integral-field spectrograph under preliminary design study. MUSE has a field of 1x1 arcmin2 sampled at 0.2x0.2 arcsec2 and is assisted by the VLT ground layer adaptive optics ESO facility using four laser guide stars. The simultaneous spectral range is 0.465-0.93 μm, at a resolution of R~3000. MUSE couples the discovery potential of a large imaging device to the measuring capabilities of a high-quality spectrograph, while taking advantage of the increased spatial resolution provided by adaptive optics. This makes MUSE a unique and tremendously powerful instrument for discovering and characterizing objects that lie beyond the reach of even the deepest imaging surveys. MUSE has also a high spatial resolution mode with 7.5x7.5 arcsec2 field of view sampled at 25 milli-arcsec. In this mode MUSE should be able to obtain diffraction limited data-cubes in the 0.6-0.93 μm wavelength range. Although the MUSE design has been optimized for the study of galaxy formation and evolution, it has a wide range of possible applications; e.g. monitoring of outer planets atmosphere, environment of young stellar objects, super massive black holes and active nuclei in nearby galaxies or massive spectroscopic surveys of stellar fields in the Milky Way and nearby galaxies.
Unlike some integral field units (IFUs) in front of conventional slit spectrographs, PMAS is a dedicated fiber-optical integral field spectrograph, featuring two different types of IFUs to address both high spatial resolution and wide field-of-view (FoV) in a single instrument. The instrument was designed, built, and tested completely in-house at the Astrophysical Institute Potsdam from 1996 to 2000. It was commissioned at the Calar Alto 3.5m Telescope in May 2001. PMAS employs an all-refractive fiber spectrograph, built with CaF2 optics, to provide good transmission and high image quality over the entire nominal wavelength range. A set of user-selectable reflective gratings provides low to medium spectral resolution in first order of approx. 1.5, 3.2, and 7 Å, depending on the groove density (1200, 600, 300 gr/mm). The standard IFU uses a 16×16 element lens array, which provides seeing-limited sampling in a relatively small field-of-view (FOV) in one of three magnifications (8×8, 12×12, or 16×16 arcsec2, respectively). The additional fiber bundle IFU (PPak) expands the FOV to a hexagonal area with a footprint of 65×74 arcsec2.
PPak is a new fiber-bundle, developed at the Astrophysical Institute Potsdam for the existing PMAS 3D-instrument. The intention of PPak is to provide a large integral field-of-view in combination with a large collecting area per fiber for the study of extended low-surface brightness objects. The PPak system consists of a focal reducer lens and a fiber bundle, featuring an innovative design with object, sky and calibration fibers. With a field-of-view of 74 x 65 arcseconds, PPak currently is the world's widest integral field unit that provides a semi-contiguous regular sampling of extended astronomical objects. Its pre-optics and fiber-diameter, combined with the versatility and efficiency of the PMAS spectrograph, allows PPak to make a unique trade-off between total light-collecting power and spectral resolution.
PMAS is a fiber-coupled lens array type of integral field spectrograph, which was commissioned at the Calar Alto 3.5m Telescope in May 2001. The optical layout of the instrument was chosen such as to provide a large wavelength coverage, and good transmission from 0.35 to 1 μm. One of the major objectives of the PMAS development has been to perform 3D spectrophotometry,
taking advantage of the contiguous array of spatial elements over
the 2-dimensional field-of-view of the integral field unit. With science results obtained during the first two years of operation, we illustrate that 3D spectroscopy is an ideal tool for faint object spectrophotometry.
The MUSE (Multi Unit Spectroscopic Explorer) instrument is a second-generation integral-field spectrograph candidate for the VLT, operating in the visible and near IR wavelength range (0.465 - 0.93 μm). It is combining a large 1' x 1' Field of View with a spectral resolution of 3000 and a spatial resolution of 0.2" coupled to a sophisticated ground-layer Adaptive Optics (AO) system. After a brief summary of the major instrumental requirements, we will focus on the opto-mechanical design of MUSE, including core subsystems such as the Fore-Optics, the Image Slicers and the Spectrographs, the Structure and the Calibration Unit. The most creative trends of the instrument will be underlined, such as the specific choices adopted to reduce the costs, weight and volume of the Slicer and Spectrograph units, that need to be manufactured and installed on the VLT Nasmyth platform into twenty-four replicas. Finally, a realistic estimate of the expected performance (in both throughput and image quality), and the future development program for the forthcoming detailed design phase will be presented.
The Multi Unit spectroscopic Explorer (MUSE) is a second generation VLT panoramic integral-field spectrograph operating in the visible wavelength range. MUSE has a field of 1 x 1 arcmin2 sampled at 0.2x0.2 arcsec2 and is assisted by a ground layer adaptive optics system using four laser guide stars. The simultaneous spectral range is 0.465-0.93 μm, at a resolution of R~3000. MUSE couples the discovery potential of a large imaging device to the measuring capabilities of a high-quality spectrograph, while taking advantage of the increased spatial resolution provided by adaptive optics. This makes MUSE a unique and tremendously powerful instrument for discovering and characterizing objects that lie beyond the reach of even the deepest imaging surveys. MUSE has also a high spatial resolution mode with 7.5 x 7.5 arcse2 field of view sampled at 25 milli-arcsec. In this mode MUSE should be able to get diffraction limited data-cube in the 0.6-1 μm wavelength range. Although MUSE design has been optimized for the study of galaxy formation and evolution, it has a wide range of possible applications; e.g. monitoring of outer planets atmosphere, young stellar objects environment, supermassive black holes and active nuclei in nearby galaxies or massive spectroscopic survey of stellar fields.
PMAS is a versatile integral field spectrograph based on the principle of a fiber-coupled lens array type of IFU. The instrument was commissioned at the Calar Alto 3.5m Telescope in May 2001. PMAS is offered as a common user instrument at Calar Alto since 2002. However, it has remained flexible enough to be used as a testbed for new observing techniques. Since the instrument is sensitive in the wavelength range from 0.35 to 1 μm, it is being used to experiment with faint object 3D spectroscopy for a variety of objects in stellar and extragalactic astronomy. Among these experiments, we have implemented a nod-shuffle mode of operation, which is a beam switching technique to achieve a high degree of sky subtraction accuracy. We describe the technical details of the special solution found for PMAS and first results obtained in test observations of faint haloes of planetary nebulae.
PMAS, the Potsdam Multi-Aperture Spectrophotometer, was successfully
commissioned at the Calar Alto 3.5m telescope during 2001. PMAS is a medium-resolution, lensarray/fiber based integral field spectrograph,
covering the whole optical wavelength range from 350 to 900 nm with optimized high efficiency in the blue. We review the commissioning activities and present the current status of this new instrument.
PMAS, the Potsdam Multi-Aperture Spectrophotometer, has a modular layout which was intended to provide for flexible operation as a travelling instrument and to accomodate different telescopes. The Telescope Module is the part of the instrument which serves the purpose of mechanical and optical interfacing to the telescope. It contains optical systems to re-image the telescope focal plane onto the lens array, to illuminate the lens array from an internal calibration light source, and to observe an area around the 3D spectroscopy field-of-view with a cryogenic CCD system for acquisition, guiding, and for the simultaneous determination of point-spread-function templates for 3D deconvolution. We discuss the opto-mechanical design and manufacture of these subsystems.
PMAS, the Potsdam Multi-Aperture Spectrophotometer, is a new integral field (IF or 3D) instrument. It features a lenslet/optical fiber type integral field module and a dedicated fiber
spectrograph. As the instrumental emphasis is on photometric stability and high efficiency, good flat field characteristic across the integral field is needed. The PMAS fiber module is unique in the sense that the design allows the replacement of individual fibers. This property, together with the fact that the fibers are index-matched at both ends, makes it possible to achieve and maintain a high efficiency. We present the opto-mechanical design for this fiber-module and, using various data sets from previous observing runs, demonstrate the increase of performance as a result of the optimization of the fiber-components.
The Potsdam Multi-Aperture Spectrophotometer (PMAS) is a flexible UV-visual integral field spectrograph designed for operation at different telescopes. It is based on a dedicated fiber spectrograph with a novel, fully dioptic collimator-camera system. The optical system was specifically optimized in terms of efficiency and stability for operation with a fiber input. The final optical design is described with remarks concerning the manufacture and acceptance test results.
Martin Roth, Svend-Marian Bauer, Frank Dionies, Thomas Fechner, Thomas Hahn, Andreas Kelz, Jens Paschke, Emil Popow, Juergen Schmoll, Dieter Wolter, Uwe Laux, Werner Altmann
PMAS has been designed and is currently being integrated as a traveling instrument of the Astrophysical Institute Potsdam. It is a UV-visual integral field spectrograph, with optimized efficiency and stability for use as a 3D spectrophotometer. PMAS is prototyped for first light at the Calar Alto 3.5m telescope with an option to go to other telescopes. We present the final design layout, details of the mechanics, optics, detector systems, and instrument control. We report on the current status of the integration.
PMAS, the Potsdam Multiaperture Spectrophotometer, is a new integral field spectrograph currently under development at the Astrophysical Institute Potsdam (AIP). The design is optimized for linear and stable behavior in order to allow for 2D spectrophotometry which is expected to become an important new observing technique at 8 - 10 m class telescopes.
The Astrophysical Institute Potsdam (AIP) has decided to develop an integral field spectrograph/spectrophotometer making the best possible use of 8 - 10 m telescopes with good image quality. The scientific motivation and driving requirements are outlined, along with a predesign study for the fiber-coupled spectrograph as part of the instrument. The fully-dioptric, high throughput fiber spectrograph is a self- contained module which may be of interest for other applications as well, e.g. multi-object fiber spectrographs.
This study investigates the feasibility of a fast, wide-field imaging telescope using a mosaic CCD detector system. A 3- mirror design is proposed to obtain good image quality over a 2 degree field-of-view at an aperture of 2.5 m. The layout is shown to be practical with current technologies, leading to a modern telescope with potentially very good seeing properties.
Access to the requested content is limited to institutions that have purchased or subscribe to SPIE eBooks.
You are receiving this notice because your organization may not have SPIE eBooks access.*
*Shibboleth/Open Athens users─please
sign in
to access your institution's subscriptions.
To obtain this item, you may purchase the complete book in print or electronic format on
SPIE.org.
INSTITUTIONAL Select your institution to access the SPIE Digital Library.
PERSONAL Sign in with your SPIE account to access your personal subscriptions or to use specific features such as save to my library, sign up for alerts, save searches, etc.