David Le Mignant

French National Centre for Scientific Research, Lutetia Parisorum, Île-de-France, France

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Publications (146)228.79 Total impact

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    ABSTRACT: The Prime Focus Spectrograph (PFS) is an optical/near-infrared multi-fiber spectrograph with 2394 science fibers, which are distributed in 1.3 degree diameter field of view at Subaru 8.2-meter telescope. The simultaneous wide wavelength coverage from 0.38 um to 1.26 um, with the resolving power of 3000, strengthens its ability to target three main survey programs: cosmology, Galactic archaeology, and galaxy/AGN evolution. A medium resolution mode with resolving power of 5000 for 0.71 um to 0.89 um also will be available by simply exchanging dispersers. PFS takes the role for the spectroscopic part of the Subaru Measurement of Images and Redshifts project, while Hyper Suprime-Cam works on the imaging part. To transform the telescope plus WFC focal ratio, a 3-mm thick broad-band coated glass-molded microlens is glued to each fiber tip. A higher transmission fiber is selected for the longest part of cable system, while one with a better FRD performance is selected for the fiber-positioner and fiber-slit components, given the more frequent fiber movements and tightly curved structure. Each Fiber positioner consists of two stages of piezo-electric rotary motors. Its engineering model has been produced and tested. Fiber positioning will be performed iteratively by taking an image of artificially back-illuminated fibers with the Metrology camera located in the Cassegrain container. The camera is carefully designed so that fiber position measurements are unaffected by small amounts of high special-frequency inaccuracies in WFC lens surface shapes. Target light carried through the fiber system reaches one of four identical fast-Schmidt spectrograph modules, each with three arms. Prototype VPH gratings have been optically tested. CCD production is complete, with standard fully-depleted CCDs for red arms and more-challenging thinner fully-depleted CCDs with blue-optimized coating for blue arms.
    08/2014;
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    ABSTRACT: FOCCoS, Fiber Optical Cable and Connector System, has the main function of capturing the direct light from the focal plane of Subaru Telescope using optical fibers, each one with a microlens in its tip, and conducting this light through a route containing connectors to a set of four spectrographs. The optical fiber cable is divided in 3 different segments called Cable A, Cable B and Cable C. Multi-fibers connectors assure precise connection among all optical fibers of the segments, providing flexibility for instrument changes. To assure strong and accurate connection, these sets are arranged inside two types of assemblies: the Tower Connector, for connection between Cable C and Cable B; and the Gang Connector, for connection between Cable B and Cable A. Throughput tests were made to evaluate the efficiency of the connections. A lifetime test connection is in progress. Cable C is installed inside the PFI, Prime Focus Instrument, where each fiber tip with a microlens is bonded to the end of the shaft of a 2-stage piezo-electric rotatory motor positioner; this assembly allows each fiber to be placed anywhere within its patrol region, which is 9.5mm diameter.. Each positioner uses a fiber arm to support the ferrule, the microlens, and the optical fiber. 2400 of these assemblies are arranged on a motor bench plate in a hexagonal-closed-packed disposition.
    08/2014;
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    ABSTRACT: The Prime Focus Spectrograph (PFS) is a new facility instrument for Subaru Telescope which will be installed in around 2017. It is a multi-object spectrograph fed by about 2400 fibers placed at the prime focus covering a hexagonal field-of-view with 1.35 deg diagonals and capable of simultaneously obtaining data of spectra with wavelengths ranging from 0.38 um to 1.26 um. The spectrograph system is composed of four identical modules each receiving the light from 600 fibers. Each module incorporates three channels covering the wavelength ranges 0.38-0.65 mu ("Blue"), 0.63-0.97 mu ("Red"), and 0.94-1.26 mu ("NIR") respectively; with resolving power which progresses fairly smoothly from about 2000 in the blue to about 4000 in the infrared. An additional spectral mode allows reaching a spectral resolution of 5000 at 0.8mu (red). The proposed optical design is based on a Schmidt collimator facing three Schmidt cameras (one per spectral channel). This architecture is very robust, well known and documented. It allows for high image quality with only few simple elements (high throughput) at the expense of the central obscuration, which leads to larger optics. Each module has to be modular in its design to allow for integration and tests and for its safe transport up to the telescope: this is the main driver for the mechanical design. In particular, each module will be firstly fully integrated and validated at LAM (France) before it is shipped to Hawaii. All sub-assemblies will be indexed on the bench to allow for their accurate repositioning. This paper will give an overview of the spectrograph system which has successfully passed the Critical Design Review (CDR) in 2014 March and which is now in the construction phase.
    08/2014;
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    Optical Engineering 03/2014; · 0.88 Impact Factor
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    ABSTRACT: SPHERE (Spectro-Polarimetric High-contrast Exoplanet Research) is a second-generation instrument for the VLT optimized for very high-contrast imaging around bright stars. Its primary goal is the detection and characterization of new giant planets around nearby stars, together with the observation of early planetary systems and disks. The Infrared Dual Imager and Spectrograph (IRDIS), one of the SPHERE subsystems, will provide dual-band imaging in the near-infrared, among with other observing modes such as long slit spectroscopy, classical imaging and infrared polarimetry. IRDIS is able to achieve very high contrast with the help of extreme-AO turbulence compensation, coronography, exceptional image quality (including non-common-path aberrations compensation), very accurate calibration strategies (including star centring with waffle mode) and very advanced data processing. We will describe the results of performances validations performed with SPHERE. In particular we present the achievable level of contrast based on the latest experimental validations at IPAG.
    12/2013;
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    ABSTRACT: Next year the second generation instrument SPHERE will begin science operations at the Very Large Telecope (ESO). This instrument will be dedicated to the search for exoplanets through the direct imaging techniques, with the new generation extreme adaptive optics. In this poster, we present the performances of one of the focal instruments, the Infra-Red Dual-beam Imaging and Spectroscopy (IRDIS). All the results have been obtained with tests in laboratory, simulating the observing conditions in Paranal. We tested several configurations using the sub-system Integral Field Spectrograph (IFS) in parallel and simulating long coronographic exposures on a star, calibrating instrumental ghosts, checking the performance of the adaptive optics system and reducing data with the consortium pipeline. The contrast one can reach with IRDIS is of the order of 2\times 10^{-6}$ at 0.5 arcsec separation from the central star.
    Proceedings of the International Astronomical Union 07/2013;
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    ABSTRACT: IRDIS filters were designed, manufactured with DIBS technology and tested after integration in the instrument. Spectral and WFE measurements indicated that filters are well within specifications, allowing differential aberrations below 10nm rms.
    Optical Interference Coatings; 06/2013
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    ABSTRACT: In the frame of the future European Extremely Large Telescope, the Laboratoire d’Astrophysique de Marseille is developing manufacturing methods and complex instrumentation for astronomy, based on the active bending of mirrors.
    Latin America Optics and Photonics Conference; 11/2012
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    ABSTRACT: We describe the conceptual design of the spectrograph opto-mechanical concept for the SuMIRe Prime Focus Spectrograph (PFS) being developed for the SUBARU telescope. The SuMIRe PFS will consist of four identical spectrographs, each receiving 600 fibers from a 2400 fiber robotic positioner at the prime focus. Each spectrograph will have three channels covering in total, a wavelength range from 380 nm to 1300 nm. The requirements for the instrument are summarized in Section 1. We present the optical design and the optical performance and analysis in Section 2. Section 3 introduces the mechanical design, its requirements and the proposed concepts. Finally, the AIT phases for the Spectrograph System are described in Section 5.
    Proc SPIE 10/2012;
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    ABSTRACT: The Prime Focus Spectrograph (PFS) of the Subaru Measurement of Images and Redshifts (SuMIRe) project has been endorsed by Japanese community as one of the main future instruments of the Subaru 8.2-meter telescope at Mauna Kea, Hawaii. This optical/near-infrared multi-fiber spectrograph targets cosmology with galaxy surveys, Galactic archaeology, and studies of galaxy/AGN evolution. Taking advantage of Subaru's wide field of view, which is further extended with the recently completed Wide Field Corrector, PFS will enable us to carry out multi-fiber spectroscopy of 2400 targets within 1.3 degree diameter. A microlens is attached at each fiber entrance for F-ratio transformation into a larger one so that difficulties of spectrograph design are eased. Fibers are accurately placed onto target positions by positioners, each of which consists of two stages of piezo-electric rotary motors, through iterations by using back-illuminated fiber position measurements with a wide-field metrology camera. Fibers then carry light to a set of four identical fast-Schmidt spectrographs with three color arms each: the wavelength ranges from 0.38 {\mu}m to 1.3 {\mu}m will be simultaneously observed with an average resolving power of 3000. Before and during the era of extremely large telescopes, PFS will provide the unique capability of obtaining spectra of 2400 cosmological/astrophysical targets simultaneously with an 8-10 meter class telescope. The PFS collaboration, led by IPMU, consists of USP/LNA in Brazil, Caltech/JPL, Princeton, & JHU in USA, LAM in France, ASIAA in Taiwan, and NAOJ/Subaru.
    10/2012;
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    ABSTRACT: IRDIS is one of the science sub-systems of VLT/SPHERE dedicated to the detection and characterization of giant exoplanets at large orbital radii with high-contrast direct imaging. It offers a unique set of observational modes including dual-band imaging (DBI) with very low differential aberrations, and long slit spectroscopy (LSS) coupled with a classical Lyot coronograph that will be used to obtain spectra at low (R = ~50) and medium (R = ~500) resolution. During the past year, IRDIS has been integrated and tested in laboratory in a standalone configuration, and it has recently been integrated on the full SPHERE bench including the calibration unit, the common path optics and the extreme AO system. We present the first analysis of data obtained during laboratory tests of IRDIS in the DBI mode, both in standalone and with the full SPHERE bench, but without simulated seeing and AO correction. We show the first performance estimates of spectral differential imaging with IRDIS in H-band, which is used to attenuate the speckle noise induced by the instrumental aberrations. Similarly, for the LSS mode we present the first application of the spectral deconvolution data analysis method to attenuate the speckle noise on IRDIS data. Finally we compare these results to simulations that were performed during the development phase of the instrument.
    Proc SPIE 09/2012;
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    ABSTRACT: Freeform optics offer additional degrees of freedom that can lead to a simplification of instrument optical designs with compact solutions. In this context, we propose a new mathematical description of freeform surfaces. This new mathematical formalism, based on the "eigen-modes" of Bernstein polynomials was developed for off-axis highly aspherical surfaces modelling. It allows to take into account different kinds of deformations of the optical surface with local influence capabilities. We present the mathematical formalism developed and then we focus on the optical analysis of an innovative instrument design. The advantages provided by this new modelling are examined.
    Proc SPIE 09/2012;
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    The Messenger. 09/2012; 149:17-21.
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    ABSTRACT: The next generation of focal-plane astronomical instruments requires technological breakthroughs to reduce their system complexity while increasing their scientific performances. Applied to the optical systems, recent studies show that the use of freeform reflective optics allows competitive compact systems with less optical components. In this context, our challenge is to supply an active freeform mirror system, using a combination of different active optics techniques. The optical shape will be provided during the fabrication using the mechanical property of metals to plasticize and will be coupled with a specific actuator system to compensate for the residual form errors, during the instrument operation phase. We present in this article the development of an innovative manufacturing process based on cold hydro-forming method, with the aim to adapt it for VIS/NIR requirements in terms of optical surface quality. It can operate on thin and flat polished initial substrates. The realization of a first prototype for a 100 mm optical diameter mirror is in progress, to compare the mechanical behaviours obtained by tests and by Finite Element Analysis (FEA), for different materials. Then, the formed samples will be characterized optically. The opto-mechanical results will allow a fine tuning of FEA parameters to optimize the residual form errors obtained through this process. It concerns the microstructure considerations, the springback effects and the work hardening evolutions of the samples, depending on the initial substrate properties and the boundary conditions applied. Modeling and tests have started with axi-symmetric spherical and aspherical shapes and will continue with highly aspherics and freeforms.
    Proc SPIE 09/2012;
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    ABSTRACT: SPHERE (Spectro-Polarimetric High-contrast Exoplanet Research) is a second generation instrument for the VLT optimized for very high-contrast imaging around bright stars. Its primary science goal is the detection and characterization of giant planets, together with observation of circumstellar environment. The infrared differential imager and spectrograph (IRDIS), one of the three science instruments for SPHERE, provides simultaneous differential imaging in the near infrared, among with long slit spectroscopy, classical imaging and infrared polarimetry. IRDIS is designed to achieve very high contrast with the help of extreme-AO (Strehl < 90%), coronography, exceptional image quality (including non-common-path aberrations compensation), very accurate calibration strategies and very advanced data processing for speckle suppression. In this paper, we report on the latest experimental characterizations of IRDIS performed with SPHERE/SAXO before the preliminary acceptance in Europe.
    Proc SPIE 07/2012;
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    ABSTRACT: The Seeing-limited, large multiplex, optical/near-IR spectrograph, Optimos-Dioramas, studied by a Consortium of Institutes from France, Italy, and Switzerland, is one of the possible candidates for first light on the E-ELT Telescope. The spectograph is designed to maximize the field of view and cover in two-shot the spectral range (0.37micron - 1.6micron). This paper describes the studies performed to establish a base-line conceptual design of the Slit Masks System for the Optimos-Dioramas spectrograph. This unit has been designed in order to better satisfy the limits of the allowed volume on the Nasmyth E-ELT platform, and it is also able to guarantee all the optical specifications needed to cover the overall field of view (measures 1468x1468mm - divided in 4 quadrants). In order to take and position the masks in the focal plane, the system is fully robotic and able to load/unload the masks in the proper quadrant.
    Memorie della Societa Astronomica Italiana Supplementi. 01/2012;
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    ABSTRACT: We developed a new mathematical formalism to model highly aspherical optical surfaces opening the possibility to explore innovative optical designs. This formalism is based on Bernstein polynomials allowing to describe from low to high order deformations of the optical surface. It has been implemented into Zemax making use of the User-Defined Surface (UDS-DLL) Zemax capability. In this case, the mathematical definition of the surface is imported into Zemax then allowing to apply classical optimization and analysis functionalities. This paper presents the UDS-DLL tool based on Bernstein polynomials together with an initial optical analysis performed to evaluate the gain obtained in using such a new formalism.
    Proc SPIE 09/2011; 8167:81670B.
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    ABSTRACT: The evolution of astrophysical needs in the era of extremely large telescopes calls more and more complex instrumental systems and sub-systems. A promising solution would be to propose compact reflective optical systems, with less optical surfaces than classical optical designs. This is made possible if the designs are not limited by the use of known conics or symmetrical optical surfaces. Recent studies have shown that the use of highly aspherics could strongly reduce the number of optical surfaces and also the size of instruments, while improving the global system performances. The aim of this article is to study the feasibility of the design and manufacturing of highly aspheric optical mirrors, toward freeform mirrors, thanks to the combination of different active optics techniques: stress deformations up to plasticization phenomenon to provide the optical shape during the manufacturing and actuator corrections to compensate for residual errors during the operation phase of the instrument. A first step consists in structural mechanics analysis to understand as possible the non-linear behaviors of materials and its particular effects which depend on the material chosen, the global dimensions and the boundary conditions parameterized for the manufacturing process.
    Proc SPIE 09/2011;
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    ABSTRACT: The future generation of Extremely Large Telescopes will require a complex combination of technologies for adaptive optics (AO) systems assisted by laser guide stars (LGS). In this context, the distance from the LGS spot to the telescope pupil ranges from about 80 to 200 km, depending on the Sodium layer altitude and the elevation of the telescope. This variation leads to a defocusing effect on the LGS wave-front sensor which needs to be compensated. We propose an active mirror able to compensate for this variation, based on an original optical design including this active optics component. This LGS Variable Curvature Mirror (LGS-VCM) is a 120 mm spherical active mirror able to achieve 820 mum deflection sag with an optical quality better than 150 nm RMS, allowing the radius of curvature variation from F/12 to F/2. Based on elasticity theory, the deformation of the metallic mirror is provided by an air pressure applied on a thin meniscus with a variable thickness distribution. In this article, we detail the analytical development leading to the specific geometry of the active component, the results of finite element analysis and the expected performances in terms of surface error versus the range of refocalisation. Three prototypes have been manufactured to compare the real behavior of the mirror and the simulations data. Results obtained on the prototypes are detailed, showing that the deformation of the VCM is very close to the simulation, and leads to a realistic active concept.
    Proc SPIE 09/2011;
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    ABSTRACT: IRDIS (Infra Red Dual Imager and Spectrograph) is one of the scientific sub-systems for the SPHERE (Spectro-Polarimetric High-contrast Exoplanet REsearch) instrument, to be mounted on one of the four VLT 8-m telescopes in Paranal (Chile) in 2012. IRDIS and two other scientific sub systems will analyze the resulting high-contrast image with the aim of direct detection of extrasolar planets. IRDIS covers the near infrared bands Y, J, H and Ks (950-2300nm) and works at cryogenic temperature. The main observational mode of IRDIS is Dual Band Imaging, where the same object is observed simultaneously in two adjacent wavebands. For this mode, differential aberrations between the two channels are critical and filter optical quality is crucial. In this paper, we focus on the design, production and tests of the IRDIS filters. The deposition technique involves DIBS (Dual Ion Beam Sputtering) and leads to very compact coatings, with material properties close to those of bulk material, making these filters well suited for cryogenic applications. The use of an in-situ optical monitoring system in visible and near infrared range (up to 2500nm) permits to reach the demanding spectral filter specifications (bandwidth, rise and fall widths, peak transmission, wide band blocking) and to have a good agreement with the theoretical design. Spectral measurements at ambient and cryogenic temperatures are then presented.
    Proc SPIE 09/2011;

Publication Stats

1k Citations
228.79 Total Impact Points

Institutions

  • 2010
    • French National Centre for Scientific Research
      Lutetia Parisorum, Île-de-France, France
    • National Institute of Astrophysics
      • Institute of Space Astrophysics and Cosmic Physics IASF - Rome
      Roma, Latium, Italy
  • 2009
    • Aix-Marseille Université
      • Laboratory of Astrophysics of Marseille (UMR 7326 LAM)
      Marseille, Provence-Alpes-Cote d'Azur, France
  • 2008
    • San Diego State University
      • Department of Astronomy
      San Diego, CA, United States
    • University of California, Santa Cruz
      • Center for Adaptive Optics
      Santa Cruz, CA, United States
  • 2002–2008
    • W. M. Keck Observatory
      Hilo, Hawaii, United States
  • 2004
    • CSU Mentor
      Long Beach, California, United States
  • 2003
    • University of California, Irvine
      • Department of Physics and Astronomy
      Irvine, CA, United States
    • California Institute of Technology
      Pasadena, California, United States
  • 1999
    • Université Paris-Sud 11
      • Institut d'Astrophysique Spatiale
      Orsay, Île-de-France, France
  • 1998
    • European Southern Observatory
      Arching, Bavaria, Germany