Papers by Sandrine Thomas
Creating an inclusive and diverse environment at Vera C. Rubin Observatory
Observatory Operations: Strategies, Processes, and Systems IX
Journal of Astronomical Telescopes, Instruments, and Systems
Direct imaging of exoplanets around multiple star systems
Vera C. Rubin Observatory system integration, test, and commissioning: strategy and status
The Vera C. Rubin Observatory is a joint NSF and DOE construction project with facilities distrib... more The Vera C. Rubin Observatory is a joint NSF and DOE construction project with facilities distributed across multiple sites. These sites include the Summit Facility on Cerro Pachón, Chile; the Base Facility in La Serena, Chile; the Project and Operations Center in Tucson, AZ; the Camera integration and testing laboratories at SLAC National Accelerator Laboratory in Menlo Park, CA; and the data support center based at the National Center for SuperComputing Applications at Urbana-Champaign, IL. The Rubin Observatory construction Project has entered its system integration and testing phase where major subsystem components are coming together and being tested and verified at a system level for the first time. The system integration phase of the Project requires a closely coordinated and organized plan to merge, manage, and be able to adapt the complex set of subsystems and activities across the entire observatory as real effects are discovered. In this paper we present our strategy to s...
Why Alpha Centauri is a Particularly Good Target for Direct Imaging of Exoplanets
We present H-band observations of β Pic with the Gemini Planet Imager’s (GPI’s) polarimetry mode ... more We present H-band observations of β Pic with the Gemini Planet Imager’s (GPI’s) polarimetry mode that reveal the debris disk between ∼0 3 (6 AU) and ∼1 7 (33 AU), while simultaneously detecting β Pic b. The polarized disk image was fit with a dust density model combined with a Henyey–Greenstein scattering phase function. The best-fit model indicates a disk inclined to the line of sight ( 85. 27 0.19 0.26 f = + ) with a position angle (PA) 30. 35 PA 0.28 0.29 q = + (slightly offset from the main outer disk, 29 PA q » ), that extends from an inner disk radius of 23.6 AU 0.6 0.9 + to well outside GPI’s field of view. In addition, we present an updated orbit for β Pic b based on new astrometric measurements taken in GPI’s spectroscopic mode spanning 14 months. The planet has a semimajor axis of a 9.2 AU 0.4 1.5 = + , with an eccentricity e 0.26. The PA of the ascending node is 31. 75 0. 15, W = offset from both the outer main disk and the inner disk seen in the GPI image. T...
The Astronomical Journal, 2019
The ∼ 500 Myr A2IV star HR 1645 has one of the most significant low-amplitude accelerations of ne... more The ∼ 500 Myr A2IV star HR 1645 has one of the most significant low-amplitude accelerations of nearby early-type stars measured from a comparison of the Hipparcos and Gaia astrometric catalogues. This signal is consistent with either a stellar companion with a moderate mass ratio (q ∼ 0.5) on a short period (P < 1 yr), or a substellar companion at a separation wide enough to be resolved with ground-based high contrast imaging instruments; long-period equal mass ratio stellar companions that are also consistent with the measured acceleration are excluded with previous imaging observations. The small but significant amplitude of the acceleration made HR 1645 a promising candidate for targeted searches for brown dwarf and planetary-mass companions around nearby, young stars. In this paper we explore the origin of the astrometric acceleration by modelling the signal induced by a wide-orbit M8 companion discovered with the Gemini Planet Imager, as well as the effects of an inner short-period spectroscopic companion discovered a century ago but not since followed-up. We present the first constraints on the orbit of the inner companion, and demonstrate that it is a plausible cause of the astrometric acceleration. This result demonstrates the importance of vetting of targets with measured astrometric acceleration for short-period stellar companions prior to conducting targeted direct imaging surveys for wide-orbit substellar companions.
The Astrophysical Journal, 2019
We have obtained Hubble Space Telescope (HST) STIS and NICMOS, and Gemini/GPI scattered light ima... more We have obtained Hubble Space Telescope (HST) STIS and NICMOS, and Gemini/GPI scattered light images of the HD 191089 debris disk. We identify two spatial components: a ring resembling Kuiper Belt in radial extent (FWHM: ∼25 au, centered at ∼46 au), and a halo extending to ∼640 au. We find that the halo is significantly bluer than the ring, consistent with the scenario that the ring serves as the "birth ring" for the smaller dust in the halo. We measure the scattering phase functions in the 30 •-150 • scattering angle range and find the halo dust is both more forward-and backward-scattering than the ring dust. We measure a surface density power law index of −0.68 ± 0.04 for the halo, which indicates the slowdown of the radial outward motion of the dust. Using radiative transfer modeling, we attempt to simultaneously reproduce the (visible) total and (near-infrared) polarized intensity images of the birth ring. Our modeling leads to mutually inconsistent results, indicating that more complex models, such as the inclusion of more realistic aggregate particles, are needed.
The Astrophysical Journal, 2019
Major advances in our understanding of the Universe frequently arise from dramatic improvements i... more Major advances in our understanding of the Universe frequently arise from dramatic improvements in our ability to accurately measure astronomical quantities. Aided by rapid progress in information technology, current sky surveys are changing the way we view and study the Universe. Next-generation surveys will maintain this revolutionary progress. We describe here the most ambitious survey currently planned in the optical, the Large Synoptic Survey Telescope (LSST). A vast array of science will be enabled by a single wide-deep-fast sky survey, and LSST will have unique survey capability in the faint time domain. The LSST design is driven by four main science themes: probing dark energy and dark matter, taking an inventory of the Solar System, exploring the transient optical sky, and mapping the Milky Way. LSST will be a large, wide-field ground-based system designed to obtain repeated images covering the sky visible from Cerro Pachón in northern Chile. The telescope will have an 8.4 m (6.5 m effective) primary mirror, a 9.6 deg 2 field of view, and a 3.2 Gigapixel camera. The standard observing sequence will consist of pairs of 15-second exposures in a given field, with two such visits in each pointing in a given night to identify and constrain the orbits of asteroids. With these repeats, the LSST system is capable of imaging about 10,000 square degrees of sky in a single filter in three clear nights. The typical 5σ point-source depth in a single visit in r will be ∼ 24.5 (AB). The system is designed to yield high image quality as well as superb astrometric and photometric accuracy. The project is in the construction phase and will begin regular survey operations by 2022. The survey area
SPIE Proceedings, 2016
We † provide an overview of the Model Based Systems Engineering (MBSE) language, tool, and method... more We † provide an overview of the Model Based Systems Engineering (MBSE) language, tool, and methodology being used in our development of the Operational Plan for Large Synoptic Survey Telescope (LSST) operations. LSST's Systems Engineering (SE) team is using a model-based approach to operational plan development to: 1) capture the topdown stakeholders' needs and functional allocations defining the scope, required tasks, and personnel needed for operations, and 2) capture the bottom-up operations and maintenance activities required to conduct the LSST survey across its distributed operations sites for the full ten year survey duration. To accomplish these complimentary goals and ensure that they result in self-consistent results, we have developed a holistic approach using the Sparx Enterprise Architect modeling tool and Systems Modeling Language (SysML). This approach utilizes SysML Use Cases, Actors, associated relationships, and Activity Diagrams to document and refine all of the major operations and maintenance activities that will be required to successfully operate the observatory and meet stakeholder expectations. We have developed several customized extensions of the SysML language including the creation of a custom stereotyped Use Case element with unique tagged values, as well as unique association connectors and Actor stereotypes. We demonstrate this customized MBSE methodology enables us to define: 1) the rolls each human Actor must take on to successfully carry out the activities associated with the Use Cases; 2) the skills each Actor must possess; 3) the functional allocation of all required stakeholder activities and Use Cases to organizational entities tasked with carrying them out; and 4) the organization structure required to successfully execute the operational survey. Our approach allows for continual refinement utilizing the systems engineering spiral method to expose finer levels of detail as necessary. For example, the bottom-up, Use Case-driven approach will be deployed in the future to develop the detailed work procedures required to successfully execute each operational activity.
LSST Telescope and site status
SPIE Proceedings, 2014
ABSTRACT The Large Synoptic Survey Telescope (LSST) has recently completed its Final Design Revie... more ABSTRACT The Large Synoptic Survey Telescope (LSST) has recently completed its Final Design Review and the Project is preparing for a 2014 construction authorization. The telescope system design supports the LSST mission to conduct a wide, fast, deep survey via a 3-mirror wide field of view optical design, a 3.2-Gpixel camera, and an automated data processing system. The observatory will be constructed in Chile on the summit of Cerro Pachón. This paper summarizes the status of the Telescope and Site group. This group is tasked with design, analysis, and construction of the summit and base facilities and infrastructure necessary to control the survey, capture the light, and calibrate the data. Several early procurements of major telescope subsystems have been completed and awarded to vendors, including the mirror systems, telescope mount assembly, hexapod and rotator systems, and the summit facility. These early contracts provide for the final design of interfaces based upon vendor specific approaches and will enable swift transition into construction. The status of these subsystems and future LSST plans during construction are presented.
The Gemini Planet Imager (GPI) entered on-sky commissioning phase, and had its First Light at the... more The Gemini Planet Imager (GPI) entered on-sky commissioning phase, and had its First Light at the Gemini South telescope in November 2013. Meanwhile, the fast loops for atmospheric correction of the Extreme Adaptive Optics (XAO) system have been closed on many dozen stars at different magnitudes (I=4-8), elevation angles and a variety of seeing conditions, and a stable loop performance was achieved from the beginning. Ultimate contrast performance requires a very low residual wavefront error (design goal 60 nm RMS), and optimization of the planet finding instrument on different ends has just begun to deepen and widen its dark hole region. Laboratory raw contrast benchmarks are in the order of 10^-6 or smaller. In the telescope environment and in standard operations new challenges are faced (changing gravity, temperature, vibrations) that are tackled by a variety of techniques such as Kalman filtering, open-loop models to keep alignment to within 5 mas, speckle nulling, and a calibra...
We present H-band observations of β Pic with the Gemini Planet Imager's (GPI's) polarimet... more We present H-band observations of β Pic with the Gemini Planet Imager's (GPI's) polarimetry mode that reveal the debris disk between 0.3" ( 6 AU) and 1.7" ( 33 AU), while simultaneously detecting β Pic b. The polarized disk image was fit with a dust density model combined with a Henyey-Greenstein scattering phase function. The best fit model indicates a disk inclined to the line of sight (ϕ=85.27^+0.26_-0.19) with a position angle θ_PA=30.35^+0.29_-0.28 (slightly offset from the main outer disk, θ_PA≈29), that extends from an inner disk radius of 23.6^+0.9_-0.6 AU to well outside GPI's field of view. In addition, we present an updated orbit for β Pic b based on new astrometric measurements taken in GPI's spectroscopic mode spanning 14 months. The planet has a semi-major axis of a=9.2^+1.5_-0.4AU, with an eccentricity e≤ 0.26. The position angle of the ascending node is Ω=31.75±0.15, offset from both the outer main disk and the inner disk seen in the GPI ima...
A principal scientific goal of the Gemini Planet Imager (GPI) is obtaining milliarcsecond astrome... more A principal scientific goal of the Gemini Planet Imager (GPI) is obtaining milliarcsecond astrometry to constrain exoplanet orbits. However, astrometry of directly imaged exoplanets is subject to biases, systematic errors, and speckle noise. Here we describe an analytical procedure to forward model the signal of an exoplanet that accounts for both the observing strategy (angular and spectral differential imaging) and the data reduction method (Karhunen-Loève Image Projection algorithm). We use this forward model to measure the position of an exoplanet in a Bayesian framework employing Gaussian processes and Markov chain Monte Carlo (MCMC) to account for correlated noise. In the case of GPI data on β Pic b, this technique, which we call Bayesian KLIP-FM Astrometry (BKA), outperforms previous techniques and yields 1σ-errors at or below the one milliarcsecond level. We validate BKA by fitting a Keplerian orbit to twelve GPI observations along with previous astrometry from other instrum...
An Atmospheric Dispersion Corrector (ADC) uses a double-prism arrangement to nullify the vertical... more An Atmospheric Dispersion Corrector (ADC) uses a double-prism arrangement to nullify the vertical chromatic dispersion introduced by the atmosphere at non-zero zenith distances. The ADC installed in the Gemini Planet Imager (GPI) was first tested in August 2012 while the instrument was in the laboratory. GPI was installed at the Gemini South telescope in August 2013 and first light occurred later that year on November 11th. In this paper, we give an overview of the characterizations and performance of this ADC unit obtained in the laboratory and on sky, as well as the structure of its control software.
Direct imaging of extra-solar planets has now become a reality, especially with the deployment an... more Direct imaging of extra-solar planets has now become a reality, especially with the deployment and commissioning of the first generation of specialized ground-based instruments such as the GPI, SPHERE, P1640 and SCExAO. These systems will allow detection of planets 1e7 times fainter than their host star. For space-based missions, such as EXCEDE, EXO-C, EXO-S, WFIRST-AFTA, different teams have shown in laboratories contrasts reaching 1e-10 within a few diffraction limits from the star using a combination of a coronagraph to suppress light coming from the host star and a wavefront control system. These demonstrations use a deformable mirror (DM) to remove residual starlight (speckles) created by the imperfections of telescope. However, all these current and future systems focus on detecting faint planets around a single host star or unresolved binaries/multiples, while several targets or planet candidates are located around nearby binary stars such as our neighbor star Alpha Centauri....
The application of laser guide stars to large aperture telescopes has spurred many new studies in... more The application of laser guide stars to large aperture telescopes has spurred many new studies in wavefront sensing. For the particular case of the Shack-HartmannWFS, wavefront sensing is prone to new sources of errors not previously considered. The primary source of error is spot elongation resulting from the resolution of the finite thickness of the sodium layer. The elongation spreads the signal over several pixels, resulting in a decrease of the signal to noise ratio and an increase of non-linearity. Also, the SHWFS performance becomes sensitive to temporal and spatial variations of the density of the sodium atoms. Among the different centroid algorithms to be used with this WFS, the most powerful methods (correlation, matched filter, WCoG) require a reference. Although straightforward for a point source, the use of a resolved laser guide star is more cumbersome because the sodium layer variations affect their performance. In this paper we look at the impact of the sodium layer ...
We present new observations of the planet beta Pictoris b from 2018 with GPI, the first GPI obser... more We present new observations of the planet beta Pictoris b from 2018 with GPI, the first GPI observations following conjunction. Based on these new measurements, we perform a joint orbit fit to the available relative astrometry from ground-based imaging, the Hipparcos Intermediate Astrometric Data (IAD), and the Gaia DR2 position, and demonstrate how to incorporate the IAD into direct imaging orbit fits. We find a mass consistent with predictions of hot-start evolutionary models and previous works following similar methods, though with larger uncertainties: 12.8 [+5.3, -3.2] M_Jup. Our eccentricity determination of 0.12 [+0.04, -0.03] disfavors circular orbits. We consider orbit fits to several different imaging datasets, and find generally similar posteriors on the mass for each combination of imaging data. Our analysis underscores the importance of performing joint fits to the absolute and relative astrometry simultaneously, given the strong covariance between orbital elements. Tim...
V&V planning and execution in an integrated model-based engineering environment using MagicDraw, Syndeia, and Jira
This paper describes the evolution of the processes, methodologies and tools developed and utiliz... more This paper describes the evolution of the processes, methodologies and tools developed and utilized on the Large Synoptic Survey Telescope (LSST) project that provide a complete end-to-end environment for verification planning, execution, and reporting. LSST utilizes No Magic’s MagicDraw Cameo Systems Modeler tool as the core tool for systems modeling, a Jira-based test case/test procedure/test plan tool called Test Management for Jira for verification execution, and Intercax’s Syndeia tool for bi-directional synchronization of data between Cameo Systems Modeler and Jira. Several additional supporting tools and services are also described to round out a complete solution. The paper describes the project’s needs, overall software platform architecture, and customizations developed to provide the end to- end solution.
We present a new matched-filter algorithm for direct detection of point sources in the immediate ... more We present a new matched-filter algorithm for direct detection of point sources in the immediate vicinity of bright stars. The stellar point-spread function (PSF) is first subtracted using a Karhunen-Loéve image processing (KLIP) algorithm with angular and spectral differential imaging (ADI and SDI). The KLIP-induced distortion of the astrophysical signal is included in the matched-filter template by computing a forward model of the PSF at every position in the image. To optimize the performance of the algorithm, we conduct extensive planet injection and recovery tests and tune the exoplanet spectra template and KLIP reduction aggressiveness to maximize the signalto-noise ratio (S/N) of the recovered planets. We show that only two spectral templates are necessary to recover any young Jovian exoplanets with minimal S/N loss. We also developed a complete pipeline for the automated detection of point-source candidates, the calculation of receiver operating characteristics (ROC), contra...
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Papers by Sandrine Thomas