In the near-infrared wavelength regime, atmospheric turbulence fluctuates at a scale of a few milliseconds, and its precise control requires the use of extreme adaptive optics (XAO) systems equipped with fast and sensitive detectors operating at kHz speeds. The C-RED One cameras developed by First Light Imaging (FLI), based on SAPHIRA detectors made of HgCdTe e-APD array sensitive to 0.8 to 2.5-μm light, featuring a 320×256 pixels with 24-μm pitch, offering sub-electron readout noise and the ability to read subarrays, at frame-rates of up to few 10-kHz, are state-of-the-art for XAO wavefront sensing. The Observatory of Geneva purchased two C-RED One cameras identified as necessary for RISTRETTO (a proposed high-contrast high-resolution spectrograph for the VLT) and SAXO+ (an upgrade of the VLT/SPHERE XAO system) projects. We present a comprehensive characterization and comparative analysis of both the cameras. We present test results examining key noise contributors, including readout noise, detector bias, etc. And we also study their temporal variability. Additionally, we assess the conversion gain and the avalanche gain calibration of the detector. We also study the evolution some of these parameters over time.
RISTRETTO is the evolution of the original idea of coupling the VLT instruments SPHERE and ESPRESSO, 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=620-840nm). We present the RISTRETTO XAO architecture that reach the specification, providing contrasts down to 5× 10−5 at 2λ/D from the star in the visible, in the presence of atmosphere and low wind effect. This performance is allowed by a new two-sensors-one-dm architecture, some variations to the already known concepts of unmodulated pyWFS and zWFS, and exploiting to the maximum of their capabilities the state-of-the-art high speed, low noise cameras and fast DM. We present the result of end-to-end simulations, that demonstrate stable closed loop operation of an unmodulated pyramid and a zernike WFS (together), and in presence of low wind effect.
The RISTRETTO instrument, a proposed visible high-contrast, high-resolution spectrograph for the VLT, has the primary science goal of detecting reflected light from nearby exoplanets and characterizing their atmospheres. Specifically, it aims to atmospherically characterize Proxima b, our closest temperate rocky exoplanet, located 37mas from its host star, corresponding to 2λ/D at λ = 750nm. To achieve this goal, a raw contrast of less than 10−4 at 2λ/D and a Strehl ratio greater than 70% are required, necessitating an extreme adaptive optics system (XAO) for the spectrograph. To meet the performance requirements for RISTRETTO, high sensitivity to low-order wavefront aberrations and petal modes is essential. Therefore, unmodulated Pyramid wavefront sensors (PWFS) and Zernike wavefront sensors (ZWFS) are under consideration. However, these sensors exhibit non-linearities and have a limited dynamic range, requiring different strategies to optimize their performance. The dynamic range of the sensors increases at longer wavelengths. Thus, in this study, we compare the performance of the 3-sided unmodulated PWFS, the 4-sided unmodulated PWFS, and the Zernike WFS at different wavelengths in the visible and near-infrared regime.
The objective of the coronagraphic IFU of RISTRETTO is to enable High Dispersion Coronagraphy of planets at a distance of 2λ/D from their star, without compromising on transmission. The new idea of a PIAA Nuller (PIAAN) allows contrast down to 10−5 over large bandwidth ≥ 25%, with high transmission ≥ 70% at the distance of 2λ/D. While RISTRETTO will be installed on a VLT, this development is of tremendous importance for fully exploiting future ELTs XAO. We will discuss our PIAAN prototyping activities. This covers 1) the characterisation of our 2nd set of IFU bundles, with 3D-printed MLAs; 2) the characterisation of our first PIAA optics; 3) the integration of a high contrast bench, planned for prototyping of Front-End control strategies; 4) the characterisation of the PIAAN system on the bench.
RISTRETTO is a visible high-resolution spectrograph fed by an extreme adaptive optics (AO) system, to be proposed as a visitor instrument on ESO VLT. The main science goal of RISTRETTO is to pioneer the detection and atmospheric characterisation of exoplanets in reflected light, in particular the temperate rocky planet Proxima b. RISTRETTO will be able to measure albedos and detect atmospheric features in a number of exoplanets orbiting nearby stars for the first time. It will do so by combining a high-contrast AO system working at the diffraction limit of the telescope to a high-resolution spectrograph, via a 7-spaxel integral-field unit (IFU) feeding single-mode fibers. Further science cases for RISTRETTO include the study of accreting protoplanets such as PDS70b/c through spectrally-resolved H-alpha emission, and spatially-resolved studies of Solar System objects such as icy moons and the ice giants Uranus and Neptune. The project is in the manufacturing phase for the spectrograph sub-system, and the preliminary design phase for the AO front-end. Specific developments for RISTRETTO include a novel coronagraphic IFU combining a phase-induced amplitude apodizer (PIAA) to a 3D-printed microlens array feeding a bundle of single-mode fibers. It also features an XAO system with a dual wavefront sensor aiming at high robustness and sensitivity, including to pupil fragmentation. RISTRETTO is a pathfinder instrument in view of similar developments at the ELT, in particular the SCAO-IFU mode of ELT-ANDES and the future ELT-PCS instrument.
The High-contrast End-to-End Performance Simulator (HEEPS) is an open-source python-based software with a modular and extensible architecture, that creates end-to-end simulations of high contrast imaging (HCI) instruments. It uses the wavefront Fresnel propagation package PROPER, the telescope instrument data simulator ScopeSim, and the HCI image processing package VIP. In this paper, we present the design of HEEPS, and motivate its baseline structure with the implementation of the Mid-infrared ELT Imager and Spectrograph (METIS) HCI modes, including coronagraphic components such as vortex phase masks, ring apodizers, and apodizing phase plates. Then, we present the key results of our thorough end-to-end simulations starting from 1-hour AO residual phase screens produced with the end-to-end AO simulator COMPASS. We analyze various undesirable effects such as pupil effects (stability, uniformity, drift) and noncommon path phase and amplitude errors. Finally, the coronagraphic performance including all effects is shown for all the METIS HCI modes as 5-sigma sensitivity contrast curves after ADI post-processing.
The Mid-infrared ELT Imager and Spectrograph (METIS) is among the first three scientific instruments commissioned at the ELT. It will implement vortex coronagraphy to achieve high-contrast imaging (HCI) at small angular separations from bright, nearby stars. An important unresolved problem with vortex coronagraphy is the vortex center glow (VCG) effect, where the thermal emission from the warm environment around the entrance pupil is partially diffracted into the image of the pupil by the vortex phase mask (VPM), which shows up as a diffuse bright spot in the center of the image. This effect has proven to be a significant nuisance in previous mid-infrared observations. Here, we use physical optics propagation to model the VCG for the first time and evaluate its strength with respect to the background flux in standard noncoronagraphic imaging in the context of ELT/METIS. Through our end-to-end simulations we find that the VCG peaks at about 70% of the standard background flux at an angular separation of 1 λ/D from the star and reduces to about 20% at 5 λ/D from the star. We apply the same method to model the VCG for the VLT/VISIR configuration, and show our model to be in agreement with the actual VCG measured in VISIR data, where the peak of the VCG is about twice as bright as the thermal background. In case the VCG turns out to be larger than anticipated in METIS, we propose two methods to mitigate it: (i) adding pupil stops in the pupil plane upstream to the VPM to block all of the thermal emission, and (ii) adding undersized Lyot stops in the image plane to block part of the diffracted light.
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