arXiv subjects

Guillaume Bourdarot

Publications and source records attributed to Guillaume Bourdarot.

31 records · Page 2Linked to original sources

arXiv2024

Simplified model(s) of the GRAVITY+ adaptive optics system(s) for performance prediction

In the context of the GRAVITY+ upgrade, the adaptive optics (AO) systems of the GRAVITY interferometer are undergoing a major lifting. The current CILAS deformable mirrors (DM, 90 actuators) will be replaced by ALPAO kilo-DMs (43x43, 1432 actuators). On top of the already existing 9x9 Shack-Hartmann wavefront sensors (SH-WFS) for infrared (IR) natural guide star (NGS), new 40x40 SH-WFSs for visible (VIS) NGS will be deployed. Lasers will also be installed on the four units of the Very Large Telescope to provide a laser guide star (LGS) option with 30x30 SH-WFSs and with the choice to either use the 9x9 IR-WFSs or 2x2 VIS-WFSs for low order sensing. Thus, four modes will be available for the GRAVITY+ AO system (GPAO): IR-NGS, IR-LGS, VIS-NGS and VIS-LGS. To prepare the instrument commissioning and help the observers to plan their observations, a tool is needed to predict the performances of the different modes and for different observing conditions (NGS magnitude, science object magnitude, turbulence conditions...) We developed models based on a Mar{é}chal approximation to predict the Strehl ratio of the four GPAO modes in order to feed the already existing tool that simulates the GRAVITY performances. Waiting for commissioning data, our model was validated and calibrated using the TIPTOP toolbox, a Point Spread Function simulator based on the computation of Power Spectrum Densities. In this work, we present our models of the NGS modes of GPAO and their calibration with TIPTOP.

astro-ph.IM↗

arXiv2024

Blind and robust reconstruction of adaptive optics point spread functions for asteroid deconvolution and moon detection

Initially designed to detect and characterize exoplanets, extreme adaptive optics systems (AO) open a new window on the solar system by resolving its small bodies. Nonetheless, despite the always increasing performances of AO systems, the correction is not perfect, degrading their image and producing a bright halo that can hide faint and close moons. Using a reference point spread function (PSF) is not always sufficient due to the random nature of the turbulence. In this work, we present our method to overcome this limitation. It blindly reconstructs the AO-PSF directly in the data of interest, without any prior on the instrument nor the asteroid's shape. This is done by first estimating the PSF core parameters under the assumption of a sharp-edge and flat object, allowing the image of the main body to be deconvolved. Then, the PSF faint extensions are reconstructed with a robust penalization optimization, discarding outliers on-the-fly such as cosmic rays, defective pixels and moons. This allows to properly model and remove the asteroid's halo. Finally, moons can be detected in the residuals, using the reconstructed PSF and the knowledge of the outliers learned with the robust method. We show that our method can be easily applied to different instruments (VLT/SPHERE, Keck/NIRC2), efficiently retrieving the features of AO-PSFs. Compared with state-of-the-art moon enhancement algorithms, moon signal is greatly improved and our robust detection method manages to discriminate faint moons from outliers.

eess.SP↗

arXiv2024

Open loop calibration and closed loop non-perturbative estimation of the lateral errors of an adaptive optics system: examples with GRAVITY+ and CHARA experimental data

Performances of an adaptive optics (AO) system are directly linked with the quality of its alignment. During the instrument calibration, having open loop fast tools with a large capture range are necessary to quickly assess the system misalignment and to drive it towards a state allowing to close the AO loop. During operation, complex systems are prone to misalignments (mechanical flexions, rotation of optical elements, etc.) that potentially degrade the AO performances, creating a need for a monitoring tool to tackle their driftage. In this work, we first present an improved perturbative method to quickly assess large lateral errors in open loop. It uses the spatial correlation of the measured interaction matrix of a limited number of 2D spatial modes with a synthetic model. Then, we introduce a novel solution to finely measure and correct these lateral errors via the closed loop telemetry. Non-perturbative, this method consequently does not impact the science output of the instrument. It is based on the temporal correlation of 2D spatial frequencies in the deformable mirror commands. It is model-free (no need of an interaction matrix model) and sparse in the Fourier space, making it fast and easily scalable to complex systems such as future extremely large telescopes. Finally, we present some results obtained on the development bench of the GRAVITY+ extreme AO system (Cartesian grid, 1432 actuators). In addition, we show with on-sky results gathered with CHARA and GRAVITY/CIAO that the method is adaptable to non-conventional AO geometries (hexagonal grids, 60 actuators).

eess.SP↗

arXiv2024

Improving constraints on the extended mass distribution in the Galactic Center with stellar orbits

Studying the orbital motion of stars around Sagittarius A* in the Galactic Center provides a unique opportunity to probe the gravitational potential near the supermassive black hole at the heart of our Galaxy. Interferometric data obtained with the GRAVITY instrument at the Very Large Telescope Interferometer (VLTI) since 2016 has allowed us to achieve unprecedented precision in tracking the orbits of these stars. GRAVITY data have been key to detecting the in-plane, prograde Schwarzschild precession of the orbit of the star S2, as predicted by General Relativity. By combining astrometric and spectroscopic data from multiple stars, including S2, S29, S38, and S55 - for which we have data around their time of pericenter passage with GRAVITY - we can now strengthen the significance of this detection to an approximately $10 σ$ confidence level. The prograde precession of S2's orbit provides valuable insights into the potential presence of an extended mass distribution surrounding Sagittarius A*, which could consist of a dynamically relaxed stellar cusp comprised of old stars and stellar remnants, along with a possible dark matter spike. Our analysis, based on two plausible density profiles - a power-law and a Plummer profile - constrains the enclosed mass within the orbit of S2 to be consistent with zero, establishing an upper limit of approximately $1200 \, M_\odot$ with a $1 σ$ confidence level. This significantly improves our constraints on the mass distribution in the Galactic Center. Our upper limit is very close to the expected value from numerical simulations for a stellar cusp in the Galactic Center, leaving little room for a significant enhancement of dark matter density near Sagittarius A*.

astro-ph.GA↗

arXiv2024

General Relativistic effects and the NIR variability of Sgr A* II: A systematic approach to temporal asymmetry

A systematic study, based on the third-moment structure function, of Sgr A*'s variability finds an exponential rise time $τ_{1,\rm{obs}}=14.8^{+0.4}_{-1.5}~\mathrm{minutes}$ and decay time $τ_{2,\rm{obs}}=13.1^{+1.3}_{-1.4}~\mathrm{minutes}$. This symmetry of the flux-density variability is consistent with earlier work, and we interpret it as caused by the dominance of Doppler boosting, as opposed to gravitational lensing, in Sgr~A*'s light curve. A relativistic, semi-physical model of Sgr~A* confirms an inclination angle $i<45$ degrees. The model also shows that the emission of the intrinsic radiative process can have some asymmetry even though the observed emission does not. The third-moment structure function, which is a measure of the skewness of the light-curve increments, may be a useful summary statistic in other contexts of astronomy because it senses only temporal asymmetry, i.e., it averages to zero for any temporally symmetric signal.

astro-ph.HE↗

arXiv2023

2023 Astrophotonics Roadmap: pathways to realizing multi-functional integrated astrophotonic instruments

Photonics offer numerous functionalities that can be used to realize astrophotonic instruments. The most spectacular example to date is the ESO Gravity instrument at the Very Large Telescope in Chile. Integrated astrophotonic devices stand to offer critical advantages for instrument development, including extreme miniaturization, as well as integration, superior thermal and mechanical stabilization owing to the small footprint, and high replicability offering cost savings. Numerous astrophotonic technologies have been developed to address shortcomings of conventional instruments to date, including for example the development of photonic lanterns, complex aperiodic fiber Bragg gratings, complex beam combiners to enable long baseline interferometry, and laser frequency combs for high precision spectral calibration of spectrometers. Despite these successes, the facility implementation of photonic solutions in astronomical instrumentation is currently limited because of (1) low throughputs from coupling to fibers, coupling fibers to chips, propagation and bend losses, device losses, etc, (2) difficulties with scaling to large channel count devices needed for large bandwidths and high resolutions, and (3) efficient integration of photonics with detectors, to name a few. In this roadmap, we identify 24 areas that need further development. We outline the challenges and advances needed across those areas covering design tools, simulation capabilities, fabrication processes, the need for entirely new components, integration and hybridization and the characterization of devices. To realize these advances the astrophotonics community will have to work cooperatively with industrial partners who have more advanced manufacturing capabilities. With the advances described herein, multi-functional instruments will be realized leading to novel observing capabilities for both ground and space platforms.

astro-ph.IM↗

arXiv2023

On-sky demonstration at Palomar Observatory of the near-IR, high-resolution VIPA spectrometer

A near-IR high-resolution, R=80000 spectrometer has been developed at IPAG to directly characterize the atmosphere of exoplanets using adaptive optics (AO) assisted telescopes, and a single-mode fiber-injection unit. A first technical test with the 200' Hale telescope at Palomar Observatory occurred in March 2022 using the PALM3000 AO system offered by this telescope. Observations have also been made at the same time with the PARVI spectrometer so that a direct comparison can be made between the two instruments. This spectrometer uses a virtually imaged phased array (VIPA) instead of an echelle grating, resulting in a very compact optical layout that fits in a 0.25m3 cryostat. Using a quarter of an H2RG detector, the spectrometer analyses the middle part of the H-band, from 1.57 to 1.7 microns for 2 sources whose light is transferred from the telescope to the spectrometer using single-mode fibers. By design, the transmission of the spectrometer is expected to be 40-50%, which is 2-3 times higher than the transmission of current high-resolution spectrometers such as CRIRES+ and NIRSPEC. A damaged cross-disperser limited it to 21%, however. A replacement grating with a correct, twice as high efficiency has been procured after the on-sky demonstration. In addition to recalling the main specifications of the VIPA spectrometer, this paper presents the control software, the calibration process, and the reduction pipeline that have been developed for the instrument. It also presents the results of the on-sky technical test with the Hale telescope, as well as measurements of the effective resolution and transmission, along with a comparison of a spectrum of the sun obtained with the spectrometer with the BASS2000 reference spectrum. Planned modifications are also discussed. That includes the integration of a new dedicated H2RG detector, and of K-band optics.

astro-ph.IM↗

arXiv2023

The GRAVITY+ Project: Towards All-sky, Faint-Science, High-Contrast Near-Infrared Interferometry at the VLTI

The GRAVITY instrument has been revolutionary for near-infrared interferometry by pushing sensitivity and precision to previously unknown limits. With the upgrade of GRAVITY and the Very Large Telescope Interferometer (VLTI) in GRAVITY+, these limits will be pushed even further, with vastly improved sky coverage, as well as faint-science and high-contrast capabilities. This upgrade includes the implementation of wide-field off-axis fringe-tracking, new adaptive optics systems on all Unit Telescopes, and laser guide stars in an upgraded facility. GRAVITY+ will open up the sky to the measurement of black hole masses across cosmic time in hundreds of active galactic nuclei, use the faint stars in the Galactic centre to probe General Relativity, and enable the characterisation of dozens of young exoplanets to study their formation, bearing the promise of another scientific revolution to come at the VLTI.

astro-ph.IM↗

arXiv2022

AMICal Sat: A sparse RGB imager on board a 2U cubesat to study the aurora

AMICal sat, a dedicated 2U cubesat, has been developed, in order to monitor the auroral emissions, with a dedicated imager. It aims to help to reconstruct the low energy electrons fluxes up to 30 keV in Earth auroral regions. It includes an imager entirely designed in Grenoble University Space Center. The imager uses a 1.3 Mpixels sparse RGB CMOS detector and a wide field objective (f=22.5 mm). The satellite platform has been built by the polish company Satrevolution. Launched September, 3rd, 2020 from Kuru (French Guyana) on board the Vega flight 16, it produces its first images in October 2020. The aim of this paper is to describe the design of the payload especially the optics and the proximity electronics, to describe the use of the payload for space weather purpose. A preliminary analysis of a first image showing the relevance of such an instrument for auroral monitoring is performed. This analysis allowed to reconstruct from one of the first images the local electron input flux at the top of the atmosphere during the exposure time.

astro-ph.IM↗

arXiv2020

Toward a large bandwidth photonic correlator for infrared heterodyne interferometry

Infrared heterodyne interferometry has been proposed as a practical alternative for recombining a large number of telescopes over kilometric baselines in the mid-infrared. However, the current limited correlation capacities impose strong restrictions on the sensitivity of this appealing technique. In this paper, we propose to address the problem of transport and correlation of wide-bandwidth signals over kilometric distances by introducing photonic processing in infrared heterodyne interferometry. We describe the architecture of a photonic double-sideband correlator for two telescopes, along with the experimental demonstration of this concept on a proof-of-principle test bed. We demonstrate the \textit{a posteriori} correlation of two infrared signals previously generated on a two-telescope simulator in a double-sideband photonic correlator. A degradation of the signal-to-noise ratio of $13\%$, equivalent to a noise factor $\text{NF}=1.15$, is obtained through the correlator, and the temporal coherence properties of our input signals are retrieved from these measurements. Our results demonstrate that photonic processing can be used to correlate heterodyne signals with a potentially large increase of detection bandwidth. These developments open the way to photonic processing of wide bandwidth signals for mid-infrared heterodyne interferometry, in particular for a large number of telescopes and for direct imager recombiners.

astro-ph.IM↗

arXiv2018

Experimental test of a 40 cm-long R=100 000 spectrometer for exoplanet characterisation

High-resolution spectroscopy is a key element for present and future astronomical instrumentation. In particular, coupled to high contrast imagers and coronagraphs, high spectral resolution enables higher contrast and has been identified as a very powerful combination to characterise exoplanets, starting from giant planets now, up to Earth-like planet eventually for the future instruments. In this context, we propose the implementation of an innovative echelle spectrometer based on the use of VIPA (Virtually Imaged Phased Array, Shirasaki 1996). The VIPA itself is a particular kind of Fabry-Perot interferometer, used as an angular disperser with much greater dispersive power than common diffraction grating. The VIPA is an efficient, small component (3 cm x 2.4 cm), that takes the very advantage of single mode injection in a versatile design. The overall instrument presented here is a proof-of-concept of a compact, high-resolution (R > 80 000) spectrometer, dedicated to the H and K bands, in the context of the project High-Dispersion Coronagrahy developed at IPAG. The optical bench has a foot-print of 40 cm x 26 cm ; it is fed by two Single-Mode Fibers (SMF), one dedicated to the companion, and one to the star and/or to a calibration channel, and is cooled down to 80 K. This communication first presents the scientific and instrumental context of the project, and the principal merit of single-mode operations in high-resolution spectrometry. After recalling the physical structure of the VIPA and its implementation in an echelle-spectrometer design, it then details the optical design of the spectrometer. In conclusion, further steps (integration, calibration, coupling with adaptive optics) and possible optimization are briefly presented. Keywords: Echelle Spectrometer, High Spectral Resolution, Exoplanets, High-Dispersion Coronography, Infrared, Adaptive Optics, SPHERE

astro-ph.IM↗

arXiv2017

Nanobob: A Cubesat Mission Concept For Quantum Communication Experiments In An Uplink Configuration

We present a ground-to-space quantum key distribution (QKD) mission concept and the accompanying feasibility study for the development of the low earth orbit CubeSat payload. The quantum information is carried by single photons with the binary codes represented by polarization states of the photons. Distribution of entangled photons between the ground and the satellite can be used to certify the quantum nature of the link: a guarantee that no eavesdropping can take place. By placing the entangled photon source on the ground, the space segments contains only the less complex detection system, enabling its implementation in a compact enclosure, compatible with the 12U CubeSat standard (12 dm3). This reduces the overall cost of the project, making it an ideal choice as a pathfinder for future European quantum communication satellite missions. The space segment is also more versatile than one that contains the source since it is compatible with a multiple of QKD protocols (not restricted to entangled photon schemes) and can be used in quantum physics experiments, such as the investigation of entanglement decoherence. Other possible experiments include atmospheric transmission/turbulence characterization, dark area mapping, fine pointing and tracking, and accurate clock synchronization; all crucial for future global scale quantum communication efforts.

quant-ph↗

arXiv2016

Experimental demonstration of a crossed cubes nuller for coronagraphy and interferometry

In this communication we present the first experimental results obtained on the Crossed-cubes nuller (CCN), that is a new type of Achromatic phase shifter (APS) based on a pair of crossed beamsplitter cubes. We review the general principle of the CCN, now restricted to two interferometric outputs for achieving better performance, and describe the experimental apparatus developed in our laboratory. It is cheap, compact, and easy to align. The results demonstrate a high extinction rate in monochromatic light and confirm that the device is insensitive to its polarization state. Finally, the first lessons from the experiment are summarized and discussed in view of future space missions searching for extra-solar planets located in the habitable zone, either based on a coronagraphic telescope or a sparse-aperture nulling interferometer

astro-ph.IM↗