Spectroastrometry, which measures wavelength-dependent shifts in the center of light, is well-suited for studying objects whose morphology changes with wavelength at very high angular resolutions. Photonic lantern (PL)-fed spectrometers have the potential to enable the measurement of spectroastrometric signals because the relative intensities between the PL output SMFs contain spatial information on the input scene. To use PL output spectra for spectroastrometric measurements, it is important to understand the wavelength-dependent behaviors of PL outputs and develop methods to calibrate the effects of time-varying wavefront errors in ground-based observations. We present experimental characterizations of the three-port PL on the SCExAO testbed at the Subaru Telescope. We develop spectral response models of the PL and verify the behaviors with lab experiments. We find the sinusoidal behavior of astrometric sensitivity of the three-port PL as a function of wavelength, as expected from numerical simulations. Furthermore, we compare experimental and numerically simulated coupling maps and discuss their potential use for offsetting pointing errors. We then present a method of building PL spectral response models (solving for the transfer matrices as a function of wavelength) using coupling maps, which can be used for further calibration strategies.
We investigate the potential of photonic lantern (PL) fiber-fed spectrometers for two-dimensional spectroastrometry. Spectroastrometry, a technique for studying small angular scales by measuring centroid shifts as a function of wavelength, is typically conducted using long-slit spectrographs. However, slit-based spectroastrometry requires observations with multiple position angles to measure two-dimensional spectroastrometric signals. In a typical configuration of PL-fed spectrometers, light from the focal plane is coupled into the few-moded PL, which is then split into several single-mode outputs, with the relative intensities containing astrometric information. The single-moded beams can be fed into a high-resolution spectrometer to measure wavelength-dependent centroid shifts. We perform numerical simulations of a standard six-port PL and demonstrate its capability of measuring spectroastrometric signals. The effects of photon noise, wavefront errors, and chromaticity are investigated. When the PL is designed to have large linear responses to tip tilts at the wavelengths of interest, the centroid shifts can be efficiently measured. Furthermore, we provide mock observations of detecting accreting protoplanets. PL spectroastrometry is potentially a simple and efficient technique for detecting spectroastrometric signals.
KEYWORDS: Planets, Stars, Monte Carlo methods, Diffraction limit, Protactinium, Adaptive optics, Observational astronomy, Telescopes, Electric fields, Solar system
Innovation in high angular resolution imaging is essential to identifying planet formation on solar-system scales (∼5−10AU) in active star forming regions beyond 150pc. The photonic lantern is a novel fiber-optic device that can be used to overcome the observational challenges associated with imaging such close-in protoplanets. Photonic lanterns spatially filter out modal noise with high throughput and low power loss, making them appealing for a wide variety of applications including wavefront-sensing, nulling, and spectro-astrometry. Spectro-astrometry, a technique that identifies wavelength-dependent centroid shifts in spectrally-dispersed datasets, could enable the resolution of circumstellar structures within the diffraction limit when conducted with photonic lanterns. Here, we present simulations of spectro-astrometric observations of embedded protoplanets using photonic lanterns. We generate mock, 6-port photonic lantern observations of young stars with gapped circumstellar disks containing accreting protoplanets with emission at the Paschen β hydrogen line. The simulations assume a 10-m class telescope and realistic sources of both photon noise and residual adaptive optics errors. We demonstrate the detection of protoplanets with photonic lantern spectro-astrometry in the presence of circumstellar material by constraining planetary accretion characteristics such as planet separation, position angle, and stellar contrast, and we explore the biases introduced by the presence of the circumstellar material.
Spectroastrometry, which measures wavelength-dependent shifts in the center of light, is well-suited for studying objects whose morphology changes with wavelength at very high angular resolutions. Photonic lantern (PL)-fed spectrometers have potential to enable measurement of spectroastrometric signals because the relative intensities between the PL output SMFs contain spatial information on the input scene. In order to use PL output spectra for spectroastrometric measurements, it is important to understand the wavelength-dependent behaviors of PL outputs and develop methods to calibrate the effects of time-varying wavefront errors in ground-based observations. We present experimental characterizations of the 3-port PL on the SCExAO testbed at the Subaru Telescope. We develop spectral response models of the PL and verify the behaviors with lab experiments. We find sinusoidal behavior of astrometric sensitivity of the 3-port PL as a function of wavelength, as expected from numerical simulations. Furthermore, we compare experimental and numerically simulated coupling maps and discuss their potential use for offsetting pointing errors and for building PL spectral response models that could be used for further calibration strategies.
The Gemini Planet Imager 2.0 (GPI 2.0) is an upgrade to the original GPI, an instrument for directly imaging exoplanet systems, which is being moved to the Gemini North telescope atop Mauna Kea, Hawaii. Major changes involve improved coronagraph designs and upgrading the adaptive optics (AO) system with a new pyramid wavefront sensor (PWFS). The addition of these new components require revised models for evaluating the performance and understanding the limitations of the system. This in turn helps us inform the broader GPI 2.0 science goals. We run end-to-end AO simulations, to assess the performance of GPI 2.0 AO under typical atmospheric conditions on Mauna Kea. We use these simulations to help us determine operating parameters such as the limiting stellar magnitude, maximum Strehl ratio, and the contrast achieved by the joint AO-coronagraph system before speckle-suppression. This information will be used to predict the science performance on a range of targets and design observing strategies.
Current pupil-plane adaptive optics (AO) systems face two challenges: non-common-path aberrations (NCPAs), caused by path differences between the sensing and science arms of an instrument; and petaling, discontinuous aberrations which arise for systems with large, fragmented pupils. One solution is to add a dedicated wavefront sensor (WFS) which senses aberrations in the final focal plane. Previous work has demonstrated real-time wavefront control from the final focal plane using the intensity pattern of a photonic lantern (PL): a waveguide that can couple an aberrated telescope beam into multiple single-mode fibers. Here, we consider the next logical extension, where PL outputs are additionally spectrally dispersed. The additional phase information provided by spectral dispersion can potentially expand both the number of corrected modes and the dynamic range of the PL WFS. Simultaneously, a dispersed PL can enable powerful techniques such as high-resolution spectroscopy and spectroastrometry. To this end, we present an analysis of the dispersed PLWFS, in the process developing implementation strategies and culminating in an experimental demonstration on the SCExAO testbed.
The Earth’s atmosphere is comprised of turbulent layers that result in speckled and blurry images from ground-based visible and infrared observations. Adaptive Optics (AO) systems are employed to measure the perturbed wavefront with a wavefront sensor (WFS) and correct for these distortions with a deformable mirror. Therefore, understanding and characterising the atmosphere is crucial for the design and functionality of AO systems. One parameter for characterizing the atmosphere is the atmospheric coherence time, which is a function of the effective wind velocity of the atmosphere. This parameter dictates how fast the AO system needs to correct for the atmosphere. If not fast enough, phenomena such as the wind butterfly effect can occur, hindering high-contrast coronographic imaging. This effect is a result of fast, strong, high-altitude turbulent layers. This paper presents two methods for estimating the effective wind velocity, using pseudo-open loop WFS slopes. The first method uses a spatial-temporal covariance map and the second uses the power spectral density of the defocus term. We show both simulated results and preliminary results from the Gemini Planet Imager AO telemetry.
The use of a photonic lantern as focal plane wavefront sensor has seen recent widespread interest – it can remove non-common-path aberrations, accurately sense low-wind-effect and petal modes, and provide wavelength resolution. It encodes both the PSFs phase and amplitude into the intensities of its single-mode-fibre outputs, from which the wavefront is reconstructed (by neural network or other algorithm). It also offers exciting potential as an imager to resolve structure at and beyond the telescope diffraction limit, filling in a coronagraphs IWA blind spot. This can utilise interferometric techniques, or an oversampled photonic lantern, having sufficient measurement dimensions that the amplitude, phase and spatial coherence of the science field can be entirely constrained by the output fluxes, and so the wavefront-error-induced components can be disambiguated from the source spatial structure. Other applications such as fibre nulling, optimal single-mode fibre injection, spectroastrometry, and others are also in development. Here, a brief overview of the photonic lantern sensor and these various applications will be given, along with key references.
The Gemini Planet Imager (GPI) is a dedicated high-contrast imaging facility instrument. After six years, GPI has helped establish that the occurrence rate of Jovian planets peaks near the snow. GPI 2.0 is expected to achieve deeper contrasts, especially at small inner working angles, to extend GPI’s operating range to fainter stars, and to broaden its scientific capabilities. GPI shipped from Gemini South in 2022 and is undergoing an upgrade as part of a relocation to Gemini North. We present the status of the upgrades including replacing the current wavefront sensor with an EMCCD-based pyramid wavefront sensor, adding a broadband low spectral resolution prism, new apodized-pupil Lyot coronagraph designs, upgrades of the calibration wavefront sensor and increased queue operability. Further we discuss the progress of reintegrating these components into the new system and the expected performance improvements in the context of GPI 2.0’s enhanced science capabilities.
Astrophysical research into exoplanets has delivered thousands of confirmed planets orbiting distant stars. These planets span a wide range of size and composition, with diversity also being the hallmark of system configurations, the great majority of which do not resemble our own solar system. Unfortunately, only a handful of the known planets have been characterized spectroscopically thus far, leaving a gaping void in our understanding of planetary formation processes and planetary types. To make progress, astronomers studying exoplanets will need new and innovative technical solutions. Astrophotonics – an emerging field focused on the application of photonic technologies to observational astronomy – provides one promising avenue forward. In this paper we discuss various astrophotonic technologies that could aid in the detection and subsequent characterization of planets and in particular themes leading towards the detection of extraterrestrial life.
The Gemini Planet Imager (GPI) is a high-contrast imaging instrument designed to directly detect and characterize young, Jupiter-mass exoplanets. After six years of operation at Gemini South in Chile, the instrument is being upgraded and relocated to Gemini North in Hawaii as GPI 2.0. GPI helped establish that Jovian-mass planets have a higher occurrence rate at smaller separations, motivating several sub-system upgrades to obtain deeper contrasts (up to 20 times improvement to the current limit), particularly at small inner working angles. This enables access to additional science areas for GPI 2.0, including low-mass stars, young nearby stars, solar system objects, planet formation in disks, and planet variability. The necessary instrumental changes required toenable these new scientific goals are to (i) the adaptive optics system, by replacing the current Shack-Hartmann Wavefront Sensor (WFS) with a pyramid WFS and a custom EMCCD, (ii) the integral field spectrograph, by employing a new set of prisms to enable an additional broadband (Y-K band) low spectral resolution mode, as well as replacing the pupil viewer camera with a faster, lower noise C-RED2 camera (iii) the calibration interferometer, by upgrading the low-order WFS used for internal alignment and on-sky target tracking with a C-RED2 camera and replacing the calibration high-order WFS used for measuring and correcting non-common path aberrations with a self coherent camera, (iv) the apodized-pupil Lyot coronagraph designs and (v) the software, to enable high-efficiency queue operations at Gemini North. GPI 2.0 is expected to go on-sky in early 2024. Here I will present the new scientific goals, the key upgrades, the current status and the latest timeline for operations.
The Photonic Lantern (PL) is a novel optical technology consisting of a multi-mode fiber adiabatically merged to several single-mode fibers. PLs efficiently split light into its individual modes, revealing both phase and amplitude information. This makes them attractive for use in focal plane wavefront sensing and spectroscopy. Spectro-astrometry, a technique that involves searching for wavelength-dependent centroid shifts in spectrally-dispersed datasets, can be conducted with PLs to resolve circumstellar structures with extremely small angular separations that are not accessible with traditional imaging techniques. Here, we investigate the application of PLs for spectro-astrometry of young stars hosting protoplanetary disks with embedded accreting planets. Although spectro-astrometry of point-source accreting companions with PLs has been numerically explored in the past, those simulations did not include the effects of scattered light by the protoplanetary disk. We carry out numerical simulations of accretion signatures inside protoplanetary disks to understand the feasibility of using PLs to detect accreting planets under realistic conditions. We simulate the response of a 6 port PL to young stars with a circumstellar disk containing an accretion hotspot centered on the Paschen beta hydrogen line. We discuss the lower limit of the hotspot-to-star contrast detectable by a PL in the context of contamination by disk signals after introducing both random and systematic noise sources. The simulations also demonstrate the effects of scattered light by the circumstellar disk on the PL response to an embedded accreting protoplanet with a fixed planet-to-star contrast.
The Earth’s turbulent atmosphere results in speckled and blurred images of astronomical objects when observed by ground based visible and near-infrared telescopes. Adaptive optics (AO) systems are employed to reduce these atmospheric effects by using wavefront sensors (WFS) and deformable mirrors. Some AO systems are not fast enough to correct for strong, fast, high turbulence wind layers leading to the wind butterfly effect, or wind-driven halo, reducing contrast capabilities in coronagraphic images. Estimating the effective wind speed of the atmosphere allows us to calculate the atmospheric coherence time. This is not only an important parameter to understand for site characterization but could be used to help remove the wind butterfly in post processing. Here we present a method for estimating the atmospheric effective wind speed from spatio-temporal covariance maps generated from pseudo open-loop (POL) WFS data. POL WFS data is used as it aims to reconstruct the full wavefront information when operating in closed-loop. The covariance maps show how different atmospheric turbulent layers traverse the telescope. Our method successfully recovered the effective wind speed from simulated WFS data generated with the soapy python library. The simulated atmospheric turbulence profiles consist of two turbulent layers of ranging strengths and velocities. The method has also been applied to Gemini Planet Imager (GPI) AO WFS data. This gives insight into how the effective wind speed can affect the wind-driven halo seen in the AO image point spread function. In this paper, we will present results from simulated and GPI WFS data.
The Gemini Planet Imager (GPI) is a high contrast imaging instrument designed to directly detect and characterize young Jupiter-mass exoplanets. After six years of operation at Gemini South in Chile, the instrument is being upgraded and moved to Gemini North in Hawaii as GPI 2.0. As part of this upgrade, several improvements will be made to the adaptive optics (AO) system. This includes replacing the current Shack-Hartmann wavefront sensor (WFS) with a pyramid wavefront sensor (PWFS) and a custom EMCCD. These changes are expected to increase GPI’s sky coverage by accessing fainter targets, improving corrections on fainter stars and allowing faster and ultra-low latency operations on brighter targets. The PWFS subsystem is being independently built and tested to verify its performance before its integration into the GPI 2.0 instrument. In this paper, we will present the design and pre-integration test plan of the PWFS.
GPI is a facility instrument designed for the direct detection and characterization of young Jupiter mass exoplanets. GPI has helped establish that the occurrence rate of Jovian planets peaks near the snow line (~3 AU), and falls off toward larger separations. This motivates an upgrade of GPI to achieve deeper contrasts, especially at small inner working angles, to extend GPI’s operating range to fainter stars, and to broaden its scientific capabilities, all while leveraging its historical success. GPI was packed and shipped in 2022, and is undergoing a major science-driven upgrade. We present the status and purpose of the upgrades including an EMCCD-based pyramid wavefront sensor, broadband low spectral resolution prisms, new apodized-pupil Lyot coronagraph designs, upgrades of the calibration wavefront sensor and increased queue operability. We discuss the expected performance improvements and enhanced science capabilities to be made available in 2024.
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