Differential optical transfer function (dOTF) is a model-independent image-based wavefront sensor for measuring the complex field. This sensor-less wavefront sensing technique is efficient for precise non-common path aberrations (NCPA) measurement and the phasing of segmented telescopes. In this communication, we report on a thorough exploration of the dOTF method with the SPEED facility for NCPA and cophasing optics application.
The PIAACMC is one example of a coronagraphic concept that approaches close to ideal efficiency and is well-suited to the small inner working angle (IWA) paradigm. In this context, it combines scientific opportunities with technical challenges. The small IWA and high efficiency enable access to habitable zones of late-type stars and a large number of targets. However, PIAACMC technology is challenging and more demanding than more conservative concepts. In a recent development of PIAACMC for the SPEED test bed, we had to improve the manufacturing accuracy of the PIAA unit through a new production campaign. Meanwhile, we adopted the APCMC (apodized pupil complex mask coronagraph) as the SPEED coronagraph, which is essentially our PIAACMC without a PIAA unit. Both APCMC and PIAACMC share the same multi-zone phase-shifting focal plane mask, but the PIAA unit is redesigned into a pupil amplitude apodizer mask. In this communication, we report on the design exploration and trades when transforming a PIAA unit into a conventional apodizer to achieve high-contrast performance similar to the original PIAACMC design.
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.
MOSAIC is the Multi-Object Spectrograph (MOS) for the 39m Extremely Large Telescope (ELT) of the European Southern Observatory (ESO), with unique capabilities in terms of multiplex, wavelength coverage and spectral resolution. It is a versatile multi-object spectrograph working in both the Visible and NIR domains, designed to cover the largest possible area (∼40 arcmin2) on the focal plane, and optimized to achieve the best possible signal-to-noise ratio on the faintest sources, from stars in our Galaxy to galaxies at the epoch of the reionization. In this paper we describe the main characteristics of the instrument, including its expected performance in the different observing modes. The status of the project will be briefly presented, together with the positioning of the instrument in the landscape of the ELT instrumentation. We also review the main expected scientific contributions of MOSAIC, focusing on the synergies between this instrument and other major ground-based and space facilities.
RISTRETTO is the evolution of the original idea of coupling the VLT instruments SPHERE and ESPRESSO,1 aiming at High Dispersion Coronagraphy. RISTRETTO is a visitor instrument that should enable the characterization of the atmospheres of nearby exoplanets in reflected light, by using the technique of high-contrast, high-resolution spectroscopy. Its goal is to observe Prox Cen b and other planets placed at about 35mas from their star, i.e. 2λ/D at λ=750nm. The instrument is composed of an extreme adaptive optics, a coronagraphic Integral Field Unit, and a diffraction-limited spectrograph (R=140.000, λ =620-840 nm).
We present the status of our studies regarding the coronagraphic IFU and the XAO system. The first in particular is based on a modified version of the PIAA apodizer, allowing nulling on the first diffraction ring. Our proposed design has the potential to reach ≥ 50% coupling and ≤ 10−4 contrast at 2λ/D in median seeing conditions.
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.
The SPEED test-bed is completed and operational at Lagrange Laboratory in Nice. The bench optical design allows a wide range of applications in high-contrast imaging: cophasing optics, coronagraphy, and wavefront shaping for large segmented on-axis telescopes. SPEED offers an ad hoc and representative lab environment for studying the effect of pupil fragmentation in a high-correction contrast regime to assess quasi-static speckle control and stability. The SPEED prime goal is to demonstrate high contrast at short angular separations with an unfriendly telescope aperture. After years of developments, from early design to first lights, we thoroughly present the SPEED facility by discussing the principal elements that have driven the design. The main characteristics and exploitation modes of this unique facility are presented.
The segmented pupil experiment for exoplanet detection (SPEED) facility aims to improve knowledge and insight into various areas required for gearing up high-contrast imaging instruments adapted to the unprecedented high angular resolution and complexity of the forthcoming extremely large telescopes (ELTs). SPEED combines an ELT simulator, cophasing optics, wavefront control and shaping with a multi-deformable mirror (DM) system, and optimized small inner-working angle (IWA) coronagraphy. The fundamental objective of the SPEED setup is to demonstrate deep contrast into a dark hole optimized for small field of view and very small IWA, adapted to the hunt of exoplanets in the habitable zone around late-type stars. SPEED is designed to implement an optimized small IWA coronagraph: the phase-induced amplitude apodization complex mask coronagraph (PIAACMC). The PIAACMC consists in a multi-zone phase-shifting focal plane mask (FPM) and two apodization mirrors (PIAA-M1 and PIAA-M2), with strong manufacturing specifications. Recently, a first-generation prototype of a PIAACMC optimized for the SPEED facility has been designed and manufactured. The manufacturing components exhibit high optical quality that meets specifications. In this paper, we present how these components have been characterized by a metrological instrument, an interferential microscope, and then we show what is yielded from this characterization for the FPM and the mirrors. Eventually, we discuss the results and the perspectives of the implementation of the PIAACMC components on the SPEED setup.
The SPEED project aims at developing and testing key recipes for high-contrast imaging at small angular separations with unfriendly telescope apertures. SPEED combines optimized segmented aperture coronagraphy, dual-deformable mirrors wavefront control and shaping architecture for creating a dark hole in the scientific image by deformable mirror (DM) actuation. The challenge is to overcome the various fundamental limitations for quasi-static speckle calibration at very small angular separations. We report on the progress made in elaborating an accurate simulated model of our instrument in preparation for the wavefront control and wavefront shaping strategy with a multi-DM setup.
SPEED – the segmented pupil experiment for exoplanet detection – currently in final integration phase, is designed to test strategies and technologies for high-contrast instrumentation with segmented telescopes by offering an ideal cocoon to progress in these domains with complex telescope apertures. SPEED combines precision segment phasing architectures, optimised small inner-working angle (IWA) coronagraphy, and wavefront shaping to create a small IWA and small field of view (FoV) dark hole in the science detector. Over the years SPEED has made significant hardware and software progress to start the exploitation of the bench. We have completed several key hardware including the common-path wavefront sensor for cophasing optics based on the self-coherent camera (SCC) concept. In this paper, we report on the wavefront sensing strategy designed for SPEED, from the adaptation of the SCC concept to cophasing optics towards an alternative implementation of the conventional SCC, called the fast-modulated SCC, for both wavefront control and shaping applications. We present a progress overview on this wavefront sensor for (i) cophasing control and monitoring from the scientific image, as well as (ii) its interest for the wavefront shaping unit of the bench.
KEYWORDS: Coronagraphy, Space telescopes, Signal to noise ratio, Planets, Point spread functions, Stars, Wavefronts, Telescopes, Optical instrument design, Adaptive optics
The Optimal Optical Coronagraph (OOC) Workshop at the Lorentz Center in September 2017 in Leiden, the Netherlands gathered a diverse group of 25 researchers working on exoplanet instrumentation to stimulate the emergence and sharing of new ideas. In this first installment of a series of three papers summarizing the outcomes of the OOC workshop, we present an overview of design methods and optical performance metrics developed for coronagraph instruments. The design and optimization of coronagraphs for future telescopes has progressed rapidly over the past several years in the context of space mission studies for Exo-C, WFIRST, HabEx, and LUVOIR as well as ground-based telescopes. Design tools have been developed at several institutions to optimize a variety of coronagraph mask types. We aim to give a broad overview of the approaches used, examples of their utility, and provide the optimization tools to the community. Though it is clear that the basic function of coronagraphs is to suppress starlight while maintaining light from off-axis sources, our community lacks a general set of standard performance metrics that apply to both detecting and characterizing exoplanets. The attendees of the OOC workshop agreed that it would benefit our community to clearly define quantities for comparing the performance of coronagraph designs and systems. Therefore, we also present a set of metrics that may be applied to theoretical designs, testbeds, and deployed instruments. We show how these quantities may be used to easily relate the basic properties of the optical instrument to the detection significance of the given point source in the presence of realistic noise.
SPEED (Segmented Pupil Experiment for Exoplanet Detection) is an instrumental testbed designed to offer an ideal cocoon to provide relevant solutions in both cophasing and high-contrast imaging with segmented telescopes. The next generation of observatories will be made of a primary mirror with excessive complexity (mirror segmentation, central obscuration, and spider vanes) undoubtedly known to be unfavorable for the direct detection of exoplanets. Exoplanets detection around late-type stars (M-dwarfs) constitutes an outstanding reservoir of candidates, and SPEED integrates all the recipes to pave the road for this science case (cophasing sensors, multi-DM wavefront control and shaping architecture as well as advanced coronagraphy). In this paper, we provide a progress overview of the project and report on the first light with segments cophasing control and monitoring from a coronagraphic image.
The Optimal Optical Coronagraph (OOC) Workshop held at the Lorentz Center in September 2017 in Leiden, the Netherlands, gathered a diverse group of 25 researchers working on exoplanet instrumentation to stimulate the emergence and sharing of new ideas. In this second installment of a series of three papers summarizing the outcomes of the OOC workshop, we present an overview of common path wavefront sensing/control and Coherent Differential Imaging techniques, highlight the latest results, and expose their relative strengths and weaknesses. We layout critical milestones for the field with the aim of enhancing future ground/space based high contrast imaging platforms. Techniques like these will help to bridge the daunting contrast gap required to image a terrestrial planet in the zone where it can retain liquid water, in reflected light around a G type star from space.
The Optimal Optical CoronagraphWorkshop at the Lorentz Center in September 2017 in Leiden, the Netherlands gathered a diverse group of 30 researchers working on exoplanet instrumentation to stimulate the emergence and sharing of new ideas. This contribution is the final part of a series of three papers summarizing the outcomes of the workshop, and presents an overview of novel optical technologies and systems that are implemented or considered for high-contrast imaging instruments on both ground-based and space telescopes. The overall objective of high contrast instruments is to provide direct observations and characterizations of exoplanets at contrast levels as extreme as 10-10. We list shortcomings of current technologies, and identify opportunities and development paths for new technologies that enable quantum leaps in performance. Specifically, we discuss the design and manufacturing of key components like advanced deformable mirrors and coronagraphic optics, and their amalgamation in "adaptive coronagraph" systems. Moreover, we discuss highly integrated system designs that combine contrast-enhancing techniques and characterization techniques (like high-resolution spectroscopy) while minimizing the overall complexity. Finally, we explore extreme implementations using all-photonics solutions for ground-based telescopes and dedicated huge apertures for space telescopes.
Future extremely large telescopes, equipped with high-contrast instruments targeting very small Inner Working Angle, will provide the requisite resolution for detecting exoplanets in the habitable zone around M-stars. However, the ELT segmented pupil shape is unfavourable to high-contrast imaging. In this context, the SPEED project aims to develop and test solutions for high contrast with unfriendly apertures. SPEED will combine a PIAACMC coronagraph and two deformable mirrors for the wavefront shaping. In this paper, we describe an end-to-end model of SPEED, including the Fresnel wavefront propagation, the PIAACMC implementation and the dark hole algorithm, and present a statistical analysis of the predicted performance.
The Phase-Induced Amplitude Apodization Complex Mask Coronagraph (PIAACMC) is a promising corona- graphic device for direct detection of exoplanets with complex segmented telescope apertures. This concept features the bright idea of generating a pupil apodization by reflection on two mirrors whose wavefront maps are specifically optimized, and a complex focal plane mask. In this paper, we report on the design, specifications, and manufacturing of such a coronagraph for the SPEED facility (Segmented Pupil Experiment for Exoplanet Detection) struggled for deep contrast at small angular separation with complex telescope aperture.
Future extremely large telescopes will open a niche for exoplanet direct imaging at the expense of using a primary segmented mirror which is known to hamper high-contrast imaging capabilities. The focal plane diffraction pattern is dominated by bright structures and the way to reduce them is not straightforward since one has to deal with strong amplitude discontinuities in this kind of unfriendly pupil (segment gaps and secondary support). The SPEED experiment developed at Lagrange laboratory is designed to address this specific topic along with high-contrast at very small separation. The baseline design of SPEED will combine a coronagraph and two deformable mirrors to create dark zones at the focal plane. A first step in this project was to identify under which circumstances the deep contrast at small separation is achievable. In particular, the DMs location is among the critical aspect to consider and is the topic covered by this paper.
The SPEED project - the Segmented Pupil Experiment for Exoplanet Detection - in development at the Lagrange laboratory, aims at gearing up strategies and technologies for high-contrast instrumentation with segmented telescopes. This new instrumental platform offers an ideal environment in which to make progress in the domain of ELTs and/or space-based missions with complex apertures. It combines all the required recipes (phasing optics, wavefront control/shaping, and advanced coronagraphy) to get to very close angular separation imaging. In this paper, we report on the optical design and subsystems advances and we provide a progress overview.
One of the most challenging fields of astronomical instrumentation is probably high-contrast imaging since it ultimately
combines ultra-high sensitivity at low flux and the ability to cope with photon flux contrasts of several hundreds of
millions or even more. These two aspects implicitly require that high-contrast instruments should be highly stable in the
sense of the reproducibility of their measurements at different times, but also, continuously stable over time. In most
high contrast instruments or experiments, their sensitivity is broken after at most tens of minutes of operation due to
uncontrolled and unknown behaviour of the whole experiment regarding the environmental conditions. In this paper, we
introduce a general approach of an exhaustive stability study for high-contrast imaging that has been initiated at
Lagrange Laboratory, Observatoire de la Côte d'Azur (OCA). On a practical ground, one of the fundamental issues of
this study is the metrology, which is the basis of all reproducible measurements. We describe a small experiment
designed to understand the behaviour of one of our ultra-precise metrology tools (a commercial sub-nanometric 3-way
interferometer) and derive the conditions under which its operation delivers reliable results. The approach will apply to
the high-contrast imaging test-bench SPEED, under development at OCA.
Searching for nearby exoplanets with direct imaging is one of the major scientific drivers for both space and groundbased programs. While the second generation of dedicated high-contrast instruments on 8-m class telescopes is about to greatly expand the sample of directly imaged planets, exploring the planetary parameter space to hitherto-unseen regions ideally down to Terrestrial planets is a major technological challenge for the forthcoming decades. This requires increasing spatial resolution and significantly improving high contrast imaging capabilities at close angular separations. Segmented telescopes offer a practical path toward dramatically enlarging telescope diameter from the ground (ELTs), or achieving optimal diameter in space. However, translating current technological advances in the domain of highcontrast imaging for monolithic apertures to the case of segmented apertures is far from trivial. SPEED – the segmented pupil experiment for exoplanet detection – is a new instrumental facility in development at the Lagrange laboratory for enabling strategies and technologies for high-contrast instrumentation with segmented telescopes. SPEED combines wavefront control including precision segment phasing architectures, wavefront shaping using two sequential high order deformable mirrors for both phase and amplitude control, and advanced coronagraphy struggled to very close angular separations (PIAACMC). SPEED represents significant investments and technology developments towards the ELT area and future spatial missions, and will offer an ideal cocoon to pave the road of technological progress in both phasing and high-contrast domains with complex/irregular apertures. In this paper, we describe the overall design and philosophy of the SPEED bench.
The Gemini Planet Imager (GPI) has as its science instrument an infrared integral field spectrograph/polarimeter (IFS). Integral field spectrographs are scientificially powerful but require sophisticated data reduction systems. For GPI to achieve its scientific goals of exoplanet and disk characterization, IFS data must be reconstructed into high quality astrometrically and photometrically accurate datacubes in both spectral and polarization modes, via flexible software that is usable by the broad Gemini community. The data reduction pipeline developed by the GPI instrument team to meet these needs is now publicly available following GPI’s commissioning.
This paper, the first of a series, provides a broad overview of GPI data reduction, summarizes key steps, and presents the overall software framework and implementation. Subsequent papers describe in more detail the algorithms necessary for calibrating GPI data. The GPI data reduction pipeline is open source, available from planetimager.org, and will continue to be enhanced throughout the life of the instrument. It implements an extensive suite of task primitives that can be assembled into reduction recipes to produce calibrated datasets ready for scientific analysis. Angular, spectral, and polarimetric differential imaging are supported. Graphical tools automate the production and editing of recipes, an integrated calibration database manages reference files, and an interactive data viewer customized for high contrast imaging allows for exploration and manipulation of data.
The Fine Guidance Sensor (FGS) is one of the four science instruments on board the James Webb Space Telescope (JWST). FGS features two modules: an infrared camera dedicated to fine guiding of the observatory and a science camera module, the Near-Infrared Imager and Slitless Spectrograph (NIRISS) covering the wavelength range between 0.7 and 5.0 μm with a field of view of 2.2' X 2.2'. NIRISS has four observing modes: 1) broadband imaging featuring seven of the eight NIRCam broadband filters, 2) wide-field slitless spectroscopy at a resolving power of rv150 between 1 and 2.5 μm, 3) single-object cross-dispersed slitless spectroscopy enabling simultaneous wavelength coverage between 0. 7 and 2.5 μm at Rrv660, a mode optimized for transit spectroscopy of relatively
bright (J > 7) stars and, 4) sparse aperture interferometric imaging between 3.8 and 4.8 μm enabling high
contrast ("' 10-4) imaging of M < 8 point sources at angular separations between 70 and 500 milliarcsec. This
paper presents an overview of the FGS/NIRISS design with a focus on the scientific capabilities and performance offered by NIRISS.
The Aperture Masked Interferometry (AMI) mode on JWST-NIRISS is implemented as a 7-hole, 15% throughput, non-redundant mask (NRM) that operates with 5-8% bandwidth filters at 3.8, 4.3, and 4.8 microns. We present refined estimates of AMI's expected point-source contrast, using realizations of noise matched to JWST pointing requirements, NIRISS detector noise, and Rev-V JWST wavefront error models for the telescope and instrument. We describe our point-source binary data reduction algorithm, which we use as a standardized method to compare different observational strategies. For a 7.5 magnitude star we report a 10-a detection at between
8.7 and 9.2 magnitudes of contrast between 100 mas to 400 mas respectively, using closure phases and squared visibilities in the absence of bad pixels, but with various other noise sources. With 3% of the pixels unusable, the expected contrast drops by about 0.5 magnitudes. AMI should be able to reach targets as bright as M=5. There will be significant overlap between Gemini-GPI and ESO-SPHERE targets and AMI's search space, and a complementarity with NIRCam's coronagraph. We also illustrate synthesis imaging with AMI, demonstrating an imaging dynamic range of 25 at 100 mas scales. We tailor existing radio interferometric methods to retrieve a faint bar across a bright nucleus, and explain the similarities to synthesis imaging at radio wavelengths. Modest contrast observations of dusty accretion flows around AGNs will be feasible for NIRISS AMI. We show our early results of image-plane deconvolution as well. Finally, we report progress on an NRM-inspired approach to mitigate mission-level risk associated with JWST's specialized wavefront sensing hardware. By combining narrow band and medium band Nyquist-sampled images taken with a science camera we can sense JWST primary mirror segment tip-tilt to lOmas, and piston to a few nm. We can sense inter-segment piston errors of up to 5 coherence lengths of the broadest bandpass filter used ( 250-500 0m depending on the filters). Our approach scales well with an increasing number of segments, which makes it relevant for future segmented-primary space missions.
KEYWORDS: James Webb Space Telescope, Stars, Space telescopes, Visibility, Sensors, Planets, Telescopes, Calibration, Point spread functions, Wavefronts
Non-redundant masking (NRM) is a high contrast high resolution technique that is relevant for future space
missions dedicated to either general astrophysics or extrasolar planetary astronomy. On the ground NRM
has opened a rich target space between 0.5 to 4 resolution elements from bright stars. It enabled moderate
contrast very high angular resolution observations that have provided dynamical masses for targets beyond the
resolution of the Hubble Space Telescope. Such observations challenge the best models of ultra-cool dwarf stars'
atmospheres and interiors. The technique succeeds because it sidesteps the effects of speckle noise that plagues
direct imaging at moderate Strehl ratios. On a space telescope NRM mitigates instrument-induced speckle
noise, thus enabling high contrast even when images are barely diffraction-limited. The non-redundant mask in
the Fine Guidance Sensor Tunable Filter Imager (FGS-TFI) on the James Webb Space Telescope (JWST) will
open up a search space between 50 and 400 mas at wavelengths longer than 3.8μm. We present simulations that
estimate achievable contrast on JWST, and report preliminary results of a testbed experiment using a mask with
the same geometry as JWST's. We expect contrast of the order of 104 will be achievable in a 10 ks exposure
of an M = 7 star, with observing, target acquisition, and data calibration methods common to the three other
imaging instruments on board JWST. As an example of the potential science possible with NRM, we show that
if a planet were responsible for clearing the inner 5 AU of the disk around HR8799, it would likely be detectable
using JWST FGS-TFI's NRM at 4.6 microns. Stars as bright as M = 3 will also be observable with JWST's
NRM, meshing well with next-generation ground-based extreme adaptive optics coronagraphs. JWST NRM's
parameter space is inaccessible to both JWST coronagraphs and future 30-m class ground-based telescopes,
especially in the mid-IR.
KEYWORDS: Fabry–Perot interferometers, Speckle, Calibration, James Webb Space Telescope, Point spread functions, Tunable filters, Prototyping, Sensors, Capacitance, Space telescopes
One of the four science instruments aboard the James Webb Space Telescope (JWST) is the Tunable Filter
Imager (TFI) provided as part of the Canadian contribution of the JWST Fine Guidance Sensor. The TFI
features a low-order Fabry-Perot etalon which enables imaging spectroscopy at an average resolving power of
100. TFI also includes a coronagraph for high-contrast imaging applications such as exoplanet imaging. In
this paper we demonstrate experimentally a TFI prototype etalon's performance of speckle suppression through
multi-wavelength imaging, a technique widely used by existent and future ground-based high contrast imaging
instruments. The improvement in contrast ranges from a factor of ~10 at large working angles increasing to a
factor of ~60 in the inner regions with very high signal. This result is consistent with our theoretical model.
The Fine Guidance Sensor (FGS) of the James Webb Space Telescope (JWST) features a tunable filter imager (TFI)
module covering the wavelength range from 1.5 to 5.0 μm at a resolving power of ~100 over a field of view of
2.2'×2.2'. TFI also features a set of occulting spots and a non-redundant mask for high-contrast imaging. This paper
presents the current status of the TFI development. The instrument is currently under its final integration and test phase.
KEYWORDS: James Webb Space Telescope, Space telescopes, Stars, Wavefronts, Point spread functions, Coronagraphy, Telescopes, Sensors, Calibration, Mirrors
Non-redundant masking (NRM) is a high contrast high resolution technique that is relevant for future space
missions dedicated to either general astrophysics or extrasolar planetary astronomy. NRM mitigates not only
atmospheric but instrument-induced speckle noise as well. The recently added mask in the Fine Guidance
Sensor Tunable Filter Imager (FGS-TFI) on the James Webb Space Telescope (JWST) will open up a search
space between 50 and 400 mas at wavelengths longer than 3.8μm. Contrast of 104 will be achievable in a 10
ks exposure of an M = 7 star, with routine observing, target acquisition, and data calibration methods. NRM
places protoplanets in Taurus as well as Jovians younger than 300Myr and more massive than 2MJ orbiting
solar type stars within JWST's reach. Stars as bright as M = 3 will also be observable, thus meshing well
with next-generation ground-based extreme adaptive optics coronagraphs. This parameter space is inaccessible
to both JWST coronagraphs and future 30-m class ground-based telescopes, especially in the mid-IR. We show
that NRM used on future space telescopes can deliver unsurpassed image contrast in key niches, while reducing
mission risk associated with active primary mirrors.
The Fine Guidance Sensor (FGS) of the James Webb Space Telescope (JWST) features a tunable filter imager (TFI)
module covering the wavelength range from 1.6 to 4.9 μm at a resolving power of ~100 over a field of view of
2.2'x2.2'. TFI also features a set of 4 occulting spots for coronagraphy. A review of the current design and development
status of TFI is presented along with two key TFI science programs: the detection of first light, high-redshift Lyα
emitters and the detection/characterization of exoplanets.
We present the prototyping results and laboratory characterization of a narrow band Fabry-Perot etalon flight model
which is one of the wavelength selecting elements of the Tunable Filter Imager. The latter is a part of the Fine Guidance
Sensor which represents the Canadian contribution to NASA's James Webb Space Telescope. The unique design of this
etalon provides the JWST observatory with the ability to image at 30 Kelvin, a 2.2'x2.2' portion of its field of view in a
narrow spectral bandwidth of R~100 at any wavelength ranging between 1.6 and 4.9 μm (with a gap in coverage
between 2.5 and 3.2 μm). Extensive testing has resulted in better understanding of the thermal properties of the
piezoelectric transducers used as an actuation system for the etalon gap tuning. Good throughput, spectral resolution and
contrast have been demonstrated for the full wavelength range.
KEYWORDS: Point spread functions, Coronagraphy, Halftones, Imaging systems, Prototyping, Signal attenuation, James Webb Space Telescope, Sensors, Mirrors, Solids
The JWST Fine Guidance Sensor (FGS) Tunable Filter Imager (TFI) will feature a coronagraph for high contrast
imaging applications. TFI will provide unique narrow-band imaging and coronagraphic capabilities impacting the
detection of "First Light", i.e. stellar systems formed just after the Big Bang, and the detection and characterization of
exoplanets. The TFI coronagraph is made of a set of four occulting spots and four Lyot stops. The TFI focal-plane masks
under consideration are apodized (Gaussian profile) or hard masks. The masks are operating in reflection and
implemented as small cones directly engraved on the pick-off mirror located at the telescope focus. This paper presents
laboratory performance of two prototype masks. The hard mask results are in good agreement with the expected
performance of a standard Lyot coronagraph. On the other hand, the halftone mask shows significant departure from
theoretical expectation; these results could be due to diffraction effects within the halftone mask.
Direct exoplanet detections are limited by the speckle noise of the point spread function (PSF). This noise can
be reduced by subtracting PSF images obtained simultaneously in adjacent narrow spectral bands using a multichannel
camera (MCC). Experiments have shown that speckle attenuation performances are severely degraded
by differential optical aberrations between channels that decorrelate the PSFs of the different spectral bands.
We present a new technique which can greatly alleviate this problem: the introduction of a holographic diffuser
at the focal plane of the MCC. The holographic diffuser converts the PSF image into an incoherent illumination
scene that is then re-imaged with the MCC. This imaging process is equivalent to a convolution of the scene
with the PSF of each channel of the MCC. The optical aberrations in the MCC then affect only the convolution
kernel of each channel and not the PSF globally, resulting in more correlated images. We report laboratory
measurements with a dual channel prototype (1.575 μm and 1.625 μm) to validate this approach. We achieved
a speckle noise suppression factor of 12-14, which is ~4-6 times better than what has been achieved by existing
MCCs.
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