Characterisation of Herschel-SPIRE flight model optical performances

The Spectral and Photometric Imaging Receiver (SPIRE) is one of three scientific instruments on ESA's Herschel Space Observatory. This long wavelength instrument covers 200 to 670μm with a three band photometric camera and a two band imaging Fourier Transform Spectrometer (IFTS). Following first results reported in a previous paper, we discuss the in-band optical performances of the flight model as measured extensively during several dedicated test campaigns. Complementary to the experimentally probed spectral characteristics of the instrument detailed in an accompanying paper (see L.D. Spencer et al., in these proceedings), attention is focused here on a set of standard but key tests aimed at measuring the spatial response of the Photometer and Spectrometer end-to-end optical chain, including detector. Effects of defocus as well as source size extent, in-band wavelength, and polarization are also investigated over respective Photometer and Spectrometer field-of-views. Comparison with optical modelling, based on instrument design knowledge and some of the internal component measured characteristics, is performed. Beyond the specific characterisation of each effect, this allows estimating in each band where optical behaviour and detector behaviour respectively dominates and also reconstructing some of the contributors to the instrument throughput. Based on this analysis, retrieved optical performances are finally assessed against the related science-driven instrument requirements

Apodized Lyot Coronagraph for VLT-SPHERE: Laboratory tests and performances of a first prototype in the visible

We present some of the High Dynamic Range Imaging activities developed around the coronagraphic test-bench of the Laboratoire A. H. Fizeau (Nice). They concern research and development of an Apodized Lyot Coronagraph (ALC) for the VLT-SPHERE instrument and experimental results from our testbed working in the visible domain. We determined by numerical simulations the specifications of the apodizing filter and searched the best technological process to manufacture it. We present the results of the experimental tests on the first apodizer prototype in the visible and the resulting ALC nulling performances. The tests concern particularly the apodizer characterization (average transmission radial profile, global reflectivity and transmittivity in the visible), ALC nulling performances compared with expectations, sensitivity of the ALC performances to misalignments of its components

Sensing and control of segmented mirrors with a pyramid wavefront sensor in the presence of spiders

The segmentation of the telescope pupil (by spiders & the segmented M4) create areas of phase isolated by the width of the spiders on the wavefront sensor (WFS), breaking the spatial continuity of the wavefront. The poor sensitivity of the Pyramid WFS (PWFS) to differential piston leads to badly seen and therefore uncontrollable differential pistons. In close loop operation, differential pistons between segments will settle around integer values of the average sensing wavelength. The differential pistons typically range from one to ten times the sensing wavelength and vary rapidly over time, leading to extremely poor performance. In addition, aberrations created by atmospheric turbulence will contain large amounts of differential piston between the segments. Removing piston contribution over each of the DM segments leads to poor performance. In an attempt to reduce the impact of unwanted differential pistons that are injected by the AO correction, we compare three different approaches. We first limit ourselves to only use the information measured by the PWFS, in particular by reducing the modulation. We show that using this information sensibly is important but will not be sufficient. We discuss possible ways of improvement by using prior information. A second approach is based on phase closure of the DM commands and assumes the continuity of the correction wavefront over the entire unsegmented pupil. The last approach is based on the pair-wise slaving of edge actuators and shows the best results. We compare the performance of these methods using realistic end-to-end simulations. We find that pair-wise slaving leads to a small increase of the total wavefront error, only adding between 20-45 nm RMS in quadrature for seeing conditions between 0.45-0.85 arcsec. Finally, we discuss the possibility of combining the different proposed solutions to increase robustness.Comment: 12 pages, 15 figures, AO4ELT5 Proceedings, Adaptive Optics for Extremely Large Telescopes 5, Conference Proceeding, Tenerife, Canary Islands, Spain, June 25-30, 201

Exoplanet imaging with ELTs: exploring a second-stage AO with a Zernike wavefront sensor on the ESO/GHOST testbed

We propose to explore a cascade extreme Adaptive optics (ExAO) approach with a second stage based on a Zernike wavefront sensor (ZWFS) for exoplanet imaging and spectroscopy. Most exoplanet imagers currently use a single-stage ExAO to correct for the effects of atmospheric turbulence and produce high-Strehl images of observed stars in the near-infrared. While such systems enable the observation of warm gaseous companions around nearby stars, adding a second-stage AO enables to push the wavefront correction further and possibly observe colder or smaller planets. This approach is currently investigated in different exoplanet imagers (VLT/SPHERE, Mag-AOX, Subaru/SCExAO) by considering a Pyramid wavefront sensor (PWFS) in the second arm to measure the residual atmospheric turbulence left from the first stage. Since these aberrations are expected to be very small (a few tens of nm in the near-infrared domain), we propose to investigate an alternative approach based on the ZWFS. This sensor is a promising concept with a small capture range to estimate residual wavefront errors thanks to its large sensitivity, simple phase reconstruction and easiness of implementation. In this contribution, we perform preliminary tests on the GHOST testbed at ESO to validate this approach experimentally. Additional experiments with petalling effects are also showed, giving promising wavefront correction results. Finally, we briefly discuss a first comparison between PWFS-based and ZWFS-based second-stage AO to draw preliminary conclusions on the interests of both schemes for exoplanet imaging and spectroscopy with the upgrade of the current exoplanet imagers and the envisioned ExAO instruments for ELTs.Comment: 17 pages, 10 figures, pre-print of the proceeding of the AO4ELT7 conference held in June 2023 in Avignon, Franc

Characterisation of Herschel-SPIRE flight model optical performances

The Spectral and Photometric Imaging Receiver (SPIRE) is one of three scientific instruments on ESA's Herschel Space Observatory. This long wavelength instrument covers 200 to 670μm with a three band photometric camera and a two band imaging Fourier Transform Spectrometer (IFTS). Following first results reported in a previous paper, we discuss the in-band optical performances of the flight model as measured extensively during several dedicated test campaigns. Complementary to the experimentally probed spectral characteristics of the instrument detailed in an accompanying paper (see L.D. Spencer et al., in these proceedings), attention is focused here on a set of standard but key tests aimed at measuring the spatial response of the Photometer and Spectrometer end-to-end optical chain, including detector. Effects of defocus as well as source size extent, in-band wavelength, and polarization are also investigated over respective Photometer and Spectrometer field-of-views. Comparison with optical modelling, based on instrument design knowledge and some of the internal component measured characteristics, is performed. Beyond the specific characterisation of each effect, this allows estimating in each band where optical behaviour and detector behaviour respectively dominates and also reconstructing some of the contributors to the instrument throughput. Based on this analysis, retrieved optical performances are finally assessed against the related science-driven instrument requirements

Luciola Hypertelescope Space Observatory

Luciola is a large (one kilometer) "multi-aperture densified-pupil imaging interferometer", or "hypertelescope" employing many small apertures, rather than a few large ones, for obtaining direct snapshot images with a high information content. A diluted collector mirror, deployed in space as a flotilla of small mirrors, focuses a sky image which is exploited by several beam-combiner spaceships. Each contains a pupil densifier micro-lens array to avoid the diffractive spread and image attenuation caused by the small sub-apertures. The elucidation of hypertelescope imaging properties during the last decade has shown that many small apertures tend to be far more efficient, regarding the science yield, than a few large ones providing a comparable collecting area. For similar underlying physical reasons, radio-astronomy has also evolved in the direction of many-antenna systems such as the proposed Low Frequency Array having hundreds of thousands of individual receivers . With its high limiting magnitude, reaching the mv=30 limit of HST when 100 collectors of 25cm will match its collecting area, high-resolution direct imaging in multiple channels, broad spectral coverage from the 1200 Angstrom ultra-violet to the 20 micron infra-red, apodization, coronagraphic and spectroscopic capabilities, the proposed hypertelescope observatory addresses very broad and innovative science covering different areas of ESA s Cosmic Vision program. In the initial phase, a focal spacecraft covering the UV to near IR spectral range of EMCCD photon-counting cameras ( currently 200 to 1000nm), will image details on the surface of many stars, as well as their environment, including multiple stars and clusters. Spectra will be obtained for each resel. It will also image neutron star, black-hole and micro-quasar candidates, as well as active galactic nuclei, quasars, gravitational lenses, and other Cosmic Vision targets observable with the initial modest crowding limit. With subsequent upgrade missions, the spectral coverage can be extended from 120nm to 20 microns, using four detectors carried by two to four focal spacecraft. The number of collector mirrors in the flotilla can also be increased from 12 to 100 and possibly 1,000. The imaging and spectroscopy of habitable exoplanets in the mid infra-red then becomes feasible once the collecting area reaches 6m2 , using a specialized mid infra-red focal spacecraft. Calculations ( Boccaletti et al., 2000) have shown that hypertelescope coronagraphy has unequalled sensitivity for detecting, at mid infra-red wavelengths, faint exoplanets within the exo-zodiacal glare. Later upgrades will enable the more difficult imaging and spectroscopy of these faint objects at visible wavelengths, using refined techniques of adaptive coronagraphy (Labeyrie. & Le Coroller, 2004). Together, the infra-red and visible spectral data carry rich information on the possible presence of life. The close environment of the central black-hole in the Milky Way will be imageable with unprecedented detail in the near infra-red . Cosmological imaging of remote galaxies at the limit of the known universe is also expected, from the ultra-violet to the near infra-red, following the first upgrade, and with greatly increasing sensitivity through successive upgrades. These areas will indeed greatly benefit from the upgrades, in terms of dynamic range, limiting complexity of the objects to be imaged, size of the elementary Direct Imaging Field , and limiting magnitude, approaching that of an 8-meter space telescope when 1000 apertures of 25cm are installed. Similar gains will occur for addressing fundamental problems in physics and cosmology, particularly when observing neutron stars and black holes, single or binary, including the giant black holes, with accretion disks and jets, in active galactic nuclei beyond the Milky Way. Gravitational lensing and micro-lensing patterns, including time-variable patterns and perhaps millisecond lensing flasheshich may be beamed by diffraction from sub-stellar masses at sub-parsec distances (Labeyrie, 1994) , will also be observable initially in the favourable cases, and upgrades will greatly improve the number of observable objects. The observability of gravitational waves emitted by binary lensing masses, in the form of modulated lensing patterns, is a debated issue ( Ragazzoni et al., 2003) but will also become addressable observationally. The technology readiness of Luciola approaches levels where low-orbit testing and stepwise implementation will become feasible in the 2015-2025 time frame. For the following decades beyond 2020, once accurate formation flying techniques will be mastered, much larger hypertelescopes such as the proposed 100km Exo-Earth Imager and the 100,000 km Neutron Star Imager should also become feasible. Luciola is therefore also seen as a precursor toward such very powerful instruments

Prototyping coronagraphs for exoplanet characterization with SPHERE

The detection and characterization of extrasolar planets with SPHERE (Spectro Polarimetric High contrast Exoplanet REsearch) is challenging and in particular relies on the ability of a coronagraph to attenuate the diffracted starlight. SPHERE includes 3 instruments, 2 of which can be operated simultaneously in the near IR from 0.95 to 1.8 microns. This requirements is extremely critical for coronagraphy. This paper briefly introduces the concepts of 2 coronagraphs, the Half-Wave Plate Four Quadrant Phase Masks and the Apodized Pupil Lyot Coronagraph, prototyped within the SPHERE consortium by LESIA (Observatory of Paris) and FIZEAU (University of Nice) respectively. Then, we present the measurements of contrast and sensitivity analysis. The comparison with technical specifications allows to validate the technology for manufacturing these coronagraphs.Comment: 10 pages, will be published in the proceeding of the SPIE conference Volume 7015 "Adaptive Optics", held in Marseille from 23 to 28 june 200

BIGRE: a low cross-talk integral field unit tailored for extrasolar planets imaging spectroscopy

Integral field spectroscopy (IFS) represents a powerful technique for the detection and characterization of extrasolar planets through high contrast imaging, since it allows to obtain simultaneously a large number of monochromatic images. These can be used to calibrate and then to reduce the impact of speckles, once their chromatic dependence is taken into account. The main concern in designing integral field spectrographs for high contrast imaging is the impact of the diffraction effects and the non-common path aberrations together with an efficient use of the detector pixels. We focus our attention on integral field spectrographs based on lenslet-arrays, discussing the main features of these designs: the conditions of appropriate spatial and spectral sampling of the resulting spectrograph's slit functions and their related cross-talk terms when the system works at the diffraction limit. We present a new scheme for the integral field unit (IFU) based on a dual-lenslet device (BIGRE), that solves some of the problems related to the classical TIGER design when used for such applications. We show that BIGRE provides much lower cross-talk signals than TIGER, allowing a more efficient use of the detector pixels and a considerable saving of the overall cost of a lenslet-based integral field spectrograph.Comment: 17 pages, 18 figures, accepted for publication in Ap