The first generation of ELT instruments includes an optical-infrared high resolution spectrograph, indicated as ELT-HIRES and recently christened ANDES (ArmazoNes high Dispersion Echelle Spectrograph). ANDES consists of three fibre-fed spectrographs ([U]BV, RIZ, YJH) providing a spectral resolution of ∼100,000 with a minimum simultaneous wavelength coverage of 0.4-1.8 μm with the goal of extending it to 0.35-2.4 μm with the addition of an U arm to the BV spectrograph and a separate K band spectrograph. It operates both in seeing- and diffraction-limited conditions and the fibre-feeding allows several, interchangeable observing modes including a single conjugated adaptive optics module and a small diffraction-limited integral field unit in the NIR. Modularity and fibre-feeding allows ANDES to be placed partly on the ELT Nasmyth platform and partly in the Coudé room. ANDES has a wide range of groundbreaking science cases spanning nearly all areas of research in astrophysics and even fundamental physics. Among the top science cases there are the detection of biosignatures from exoplanet atmospheres, finding the fingerprints of the first generation of stars, tests on the stability of Nature’s fundamental couplings, and the direct detection of the cosmic acceleration. The ANDES project is carried forward by a large international consortium, composed of 35 Institutes from 13 countries, forming a team of almost 300 scientists and engineers which include the majority of the scientific and technical expertise in the field that can be found in ESO member states.
The first generation of ELT instruments includes an optical-infrared high resolution spectrograph, indicated as ELT-HIRES and recently christened ANDES (ArmazoNes high Dispersion Echelle Spectrograph). ANDES consists of three fibre-fed spectrographs (UBV, RIZ, YJH) providing a spectral resolution of ∼100,000 with a minimum simultaneous wavelength coverage of 0.4-1.8 µm with the goal of extending it to 0.35-2.4 µm with the addition of a K band spectrograph. It operates both in seeing- and diffraction-limited conditions and the fibre-feeding allows several, interchangeable observing modes including a single conjugated adaptive optics module and a small diffraction-limited integral field unit in the NIR. Its modularity will ensure that ANDES can be placed entirely on the ELT Nasmyth platform, if enough mass and volume is available, or partly in the Coudé room. ANDES has a wide range of groundbreaking science cases spanning nearly all areas of research in astrophysics and even fundamental physics. Among the top science cases there are the detection of biosignatures from exoplanet atmospheres, finding the fingerprints of the first generation of stars, tests on the stability of Nature’s fundamental couplings, and the direct detection of the cosmic acceleration. The ANDES project is carried forward by a large international consortium, composed of 35 Institutes from 13 countries, forming a team of more than 200 scientists and engineers which represent the majority of the scientific and technical expertise in the field among ESO member states.
High resolution spectroscopy enables the detection of atmospheres of exoplanets. To reach the required radial velocity precision of about 1 m/s, calibration with even more precise sources is mandatory. HIRES will employ several calibration sources, the most important ones are an Laser Frequency Comb (LFC) and Fabry-P´erots (FP). The LFC needs to be filtered with a set of FP. One possible solution is to illuminate this set of FP with a broadband light source and use them as calibrators, when they are not used for filtering the LFC. It has been demonstrated that passively-stabilized FP can perform better than 10 cm/s per night. We give an overview of the currently used FP in different surveys and compare their individual features. For the FP which may be used in HIRES we discuss different configuration. We show that the Finesse and FSR of the FP needs to be optimized with regard to the resolution of the spectrograph and we outline how we aim to fulfill the requirements of HIRES.
Astrophysical calibration sources which can be used for high-precision radial-velocity spectroscopy require a calibration with even higher precision and accuracy. Calibration of these sources can be achieved with a high-resolution Fourier-Transform-Spectrograph (FTS). The precision (~ 20 m/s) of the FTS is mainly driven by its reference, often a stabilized HeNe-laser. To reach an acceptable precision, either averaging over a large number of measurements or a better reference is needed.
We developed a setup including a Laser-Frequency-Comb (LFC) for referencing a high-resolution FTS. Due to the pulsed source specific evaluation methods have to be used to retrieve the spectrum properly. We extend the sub-nominal method used in absorption spectroscopy by showing an algorithm to determine the interferogram cut points from the interferogram itself, rather than calculating them from the repetition rate and reference laser wavelength. Furthermore, we show that line position errors measured from comb spectra are associated with amplitude variability and phase noise. We give an estimate of the measured line position stability for different evaluation methods (truncation, shifting, apodization, zerofilling) on scales not dominated by these errors.
Precise astronomical spectroscopy with the forthcoming E-ELT and its high resolution spectrograph HIRES will address a number of important science cases,1 e.g. detection of atmospheres of exoplanets. Challenging technical requirements have been identified to achieve these cases, principal among which is the goal to achieve a radial velocity precision on the order of 10 cms-1. HIRES will experience systematic errors like intrapixel variations and random variations like fiber noise, caused by the non-uniform illumination of the coupling fibers, with these and other systematic errors affecting the performance of the spectrograph. Here, we describe the requirements for the calibration sources which may be used for mitigating such systematic errors in HIRES. Precise wavelength calibration with wide-mode-spacing laser frequency combs (LFCs), so called astrocombs, has been demonstrated with different astronomical spectrographs. Here we present a comparison of currently used astrocombs and outline a possible solution to meet the requirements of HIRES with a single broadband astrocomb.
The instrumentation plan for the E-ELT foresees a High Resolution Spectrograph (HIRES). Among its main goals are the detection of atmospheres of exoplanets and the determination of fundamental physical constants. For this, high radial velocity precision and accuracy are required. HIRES will be designed for maximum intrinsic stability. Systematic errors from effects like intrapixel variations or random errors like fiber noise need to be calibrated. Based on the main requirements for the calibration of HIRES, we discuss different potential calibration sources and how they can be applied. We outline the frequency calibration concept for HIRES using these sources.
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