K. Dohlen

Publications (46)5 Total impact

• Article: Apodization in high-contrast long-slit spectroscopy

  A. Vigan, M. N'Diaye, K. Dohlen
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  ABSTRACT: The spectroscopy of faint planetary-mass companions to nearby stars is one of the main challenges that new-generation high-contrast spectro-imagers are going to face. In a previous work we presented a long slit coronagraph (LSC), for which the presence of a slit in the coronagraphic focal plane induces a complex distribution of energy in the Lyot pupil-plane that cannot be easily masked with a binary Lyot stop. To alleviate this concern, we propose to use a pupil apodization to suppress diffraction, creating an apodized long slit coronagraph (ALSC). After describing how the apodization is optimized, we demonstrate its advantages with respect to the CLC in the context of SPHERE/IRDIS long slit spectroscopy (LSS) mode at low-resolution with a 0.12" slit and 0.18" coronagraphic mask. We perform different sets of simulations with and without aberrations, and with and without a slit to demonstrate that the apodization is a more appropriate concept for LSS, at the expense of a significantly reduced throughput (37%) compared to the LSC. Then we perform detailed end-to-end simulations of the LSC and the ALSC that include realistic levels of aberrations to obtain datasets representing 1h of integration time on stars of spectral types A0 to M0 located at 10 pc. We insert spectra of planetary companions at different effective temperatures (Teff) and surface gravities (log g) into the data at angular separations of 0.3" to 1.5" and with contrast ratios from 6 to 18 mag. Using the SD method to subtract the speckles, we show that the ALSC brings a gain in sensitivity of up to 3 mag at 0.3" with respect to the LSC, which leads to a much better spectral extraction below 0.5". In terms of Teff, we demonstrate that at small angular separations the limit with the ALSC is always lower by at least 100K, inducing an increase of sensitivity of a factor up to 1.8 in objects' masses at young ages.
  05/2013;
• Article: Calibration of quasi-static aberrations in exoplanet direct-imaging instruments with a Zernike phase-mask sensor

  M. N'Diaye, K. Dohlen, T. Fusco, B. Paul
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  ABSTRACT: Context. Several exoplanet direct imaging instruments will soon be in operation. They use an extreme adaptive optics (XAO) system to correct the atmospheric turbulence and provide a highly-corrected beam to a near-infrared (NIR) coronagraph for starlight suppression. The performance of the coronagraph is however limited by the non-common path aberrations (NCPA) due to the differential wavefront errors existing between the visible XAO sensing path and the NIR science path, leading to residual speckles in the coronagraphic image. Aims. Several approaches have been developed in the past few years to accurately calibrate the NCPA, correct the quasi-static speckles and allow the observation of exoplanets at least 1e6 fainter than their host star. We propose an approach based on the Zernike phase-contrast method for the measurements of the NCPA between the optical path seen by the visible XAO wavefront sensor and that seen by the near-IR coronagraph. Methods. This approach uses a focal plane phase mask of size {\lambda}/D, where {\lambda} and D denote the wavelength and the telescope aperture diameter, respectively, to measure the quasi-static aberrations in the upstream pupil plane by encoding them into intensity variations in the downstream pupil image. We develop a rigorous formalism, leading to highly accurate measurement of the NCPA, in a quasi-linear way during the observation. Results. For a static phase map of standard deviation 44 nm rms at {\lambda} = 1.625 {\mu}m (0.026 {\lambda}), we estimate a possible reduction of the chromatic NCPA by a factor ranging from 3 to 10 in the presence of AO residuals compared with the expected performance of a typical current-generation system. This would allow a reduction of the level of quasi-static speckles in the detected images by a factor 10 to 100 hence, correspondingly improving the capacity to observe exoplanets.
  05/2013;
• Article: Speckle temporal stability in XAO coronagraphic images II. Refine model for quasi-static speckle temporal evolution for VLT/SPHERE

  P. Martinez, M. Kasper, A. Costille, J. F. Sauvage, K. Dohlen, P. Puget, J. L. Beuzit
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  ABSTRACT: Observing sequences have shown that the major noise source limitation in high-contrast imaging is due to the presence of quasi-static speckles. The timescale on which quasi-static speckles evolve, is determined by various factors, among others mechanical or thermal deformations. Understanding of these time-variable instrumental speckles, and especially their interaction with other aberrations, referred to as the pinning effect, is paramount for the search of faint stellar companions. The temporal evolution of quasi-static speckles is for instance required for a quantification of the gain expected when using angular differential imaging (ADI), and to determine the interval on which speckle nulling techniques must be carried out. Following an early analysis of a time series of adaptively corrected, coronagraphic images obtained in a laboratory condition with the High-Order Test bench (HOT) at ESO Headquarters, we confirm our results with new measurements carried out with the SPHERE instrument during its final test phase in Europe. The analysis of the residual speckle pattern in both direct and differential coronagraphic images enables the characterization of the temporal stability of quasi-static speckles. Data were obtained in a thermally actively controlled environment reproducing realistic conditions encountered at the telescope. The temporal evolution of the quasi-static wavefront error exhibits linear power law, which can be used to model quasi-static speckle evolution in the context of forthcoming high-contrast imaging instruments, with implications for instrumentation (design, observing strategies, data reduction). Such a model can be used for instance to derive the timescale on which non-common path aberrations must be sensed and corrected. We found in our data that quasi-static wavefront error increases with ~0.7 angstrom per minute.
  02/2013;
• Conference Proceeding: The IFS of SPHERE: integration and laboratory performances

  R Claudi, U Anselmi, P Bruno, E Cascone, A Costille, V De Caprio, S Desidera, E Giro, R Gratton, L Lessio, [......], D Fantinel, G Finger, J ~L Lizon, E Sant'Ambrogio, B Salasnich, J Beuzit, K Dohlen, P Puget, M Kasper, N Hubin
  Society of Photo-Optical Instrumentation Engineers (SPIE) Conference Series; 09/2012
• Article: Gearing up the SPHERE

  M Kasper, J -L Beuzit, M Feldt, K Dohlen, D Mouillet, P Puget, F Wildi, L Abe, A Baruffolo, P Baudoz, [......], N Schuhler, A Sevin, R Siebenmorgen, C Soenke, E Stadler, M Suarez, M Turatto, S Udry, A Vigan, G Zins
  The Messenger. 09/2012; 149:17-21.
• Source

  Available from: ArXiv

  Article: Improved achromatization of phase mask coronagraphs using colored apodization

  M. N'Diaye, K. Dohlen, S. Cuevas, R. Soummer, C. Sánchez-Pérez, F. Zamkotsian
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  ABSTRACT: For direct imaging of exoplanets, a stellar coronagraph helps to remove the image of an observed bright star by attenuating the diffraction effects caused by the telescope aperture of diameter D. The Dual Zone Phase Mask (DZPM) coronagraph constitutes a promising concept since it theoretically offers a small inner working angle (IWA \sim \lambda_0/D), good achromaticity and high starlight rejection, typically reaching a 1e6 contrast at 5 \lambda_0/D from the star over a spectral bandwidth \Delta\lambda/\lambda_0 of 25% (similar to H-band). This last value proves to be encouraging for broadband imaging of young and warm Jupiter-like planets. Contrast levels higher than 1e6 are however required for the observation of older and/or less massive companions over a finite spectral bandwidth. An achromatization improvement of the DZPM coronagraph is therefore mandatory to reach such performance. In its design, the DZPM coronagraph uses a grey (or achromatic) apodization. We propose to replace it by a colored apodization to increase the performance of this coronagraphic system over a large spectral range. This innovative concept, called Colored Apodizer Phase Mask (CAPM) coronagraph, is defined with some design parameters optimized to reach the best contrast in the exoplanet search area. Once this done, we study the performance of the CAPM coronagraph in the presence of different errors to evaluate the sensitivity of our concept. A 2.5 mag contrast gain is estimated from the performance provided by the CAPM coronagraph with respect to that of the DZPM coronagraph. A 2.2e-8 intensity level at 5 \lambda_0/D separation is then theoretically achieved with the CAPM coronagraph in the presence of a clear circular aperture and a 25% bandwidth. In addition, our studies show that our concept is less sensitive to low than high-order aberrations for a given value of rms wavefront errors.
  11/2011;
• Conference Proceeding: Optical design and test of the BIGRE-based IFS of SPHERE

  R Claudi, E Giro, U Anselmi, D Mesa, J Antichi, R Gratton, S Desidera, V Caprio, L Lessio, S Scuderi, P Bruno, D Fantinel, B Salasnich, E Cascone, J ~L Beuzit, K Dohlen, F Wildi, P Puget, M Kasper, N Hubin
  Society of Photo-Optical Instrumentation Engineers (SPIE) Conference Series; 09/2011
• Conference Proceeding: Thermal baffling of SPHERE IFS

  R ~G Gratton, R ~U Claudi, E Giro, U Anselmi, D Mesa, L Lessio, V Caprio, E Mattaini, S Incorvaia, E Santambrogio, S Scuderi, J ~L Beuzit, K Dohlen, P Puget, J ~L Lizon, R ~J Dorn, M Kasper
  Society of Photo-Optical Instrumentation Engineers (SPIE) Conference Series; 09/2011
• Article: Simulation of planet detection with the SPHERE integral field spectrograph

  D Mesa, R Gratton, A Berton, J Antichi, C Verinaud, A Boccaletti, M Kasper, R ~U Claudi, S Desidera, E Giro, J -L Beuzit, K Dohlen, M Feldt, D Mouillet, G Chauvin, A Vigan
  åp. 05/2011; 529:A131.
• Source

  Available from: Jacopo Antichi

  Article: Simulation of planet detection with the SPHERE IFS

  D. Mesa, R. Gratton, A. Berton, J. Antichi, C. Verinaud, A. Boccaletti, M Kasper, R. U. Claudi, S. Desidera, E. Giro, J. L. Beuzit, K. Dohlen, M. Feldt, D. Mouillet, G. Chauvin, A. Vigan
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  ABSTRACT: Aims. We present simulations of the perfomances of the future SPHERE IFS instrument designed for imaging extrasolar planets in the near infrared (Y, J, and H bands). Methods. We used the IDL package code for adaptive optics simulation (CAOS) to prepare a series of input point spread functions (PSF). These feed an IDL tool (CSP) that we designed to simulate the datacube resulting from the SPHERE IFS. We performed simulations under different conditions to evaluate the contrast that IFS will be able to reach and to verify the impact of physical propagation within the limits of the near field of the aperture approximation (i.e. Fresnel propagation). We then performed a series of simulations containing planet images to test the capability of our instrument to correctly classify the found objects. To this purpose we developed a separated IDL tool. Results. We found that using the SPHERE IFS instrument and appropriate analysis techniques, such as multiple spectral differential imaging (MDI), spectral deconvolution (SD), and angular differential imaging (ADI), we should be able to image companion objects down to a luminosity contrast of ? 10-7 with respect to the central star in favorable cases. Spectral deconvolution resulted in the most effective method for reducing the speckle noise. We were then able to find most of the simulated planets (more than 90% with the Y-J-mode and more than the 95% with the Y-H-mode) for contrasts down to 3 \times 10-7 and separations between 0.3 and 1.0 arcsec. The spectral classification is accurate but seems to be more precise for late T-type spectra than for earlier spectral types. A possible degeneracy between early L-type companion objects and field objects (flat spectra) is highlighted. The spectral classification seems to work better using the Y-H-mode than with the Y-J-mode.
  03/2011;
• Article: SPHERE: a planet imager for the VLT

  J.-L. Beuzit, M. Feldt, D. Mouillet, K. Dohlen, P. Puget, F. Wildi, SPHERE Consortium
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  ABSTRACT: Direct detection and spectral characterization of extra-solar planets is one of the most exciting but also one of the most challenging areas in modern astronomy due to the very large contrast between the host star and the planet at very small angular separations. SPHERE (Spectro-Polarimetric High-contrast Exo- planet Research in Europe) is a second-generation instrument for the ESO VLT dedicated to this scientific objective. It combines an extreme adaptive optics system, various coronagraphic devices and a suite of focal instruments providing imaging, integral field spectroscopy and polarimetry capabilities in the visible and near-infrared spectral ranges. We will present the science objectives, instrument design and expected performance of the SPHERE instrument and report on the status of the project, now in the integration and test phase for a first light foreseen for mid of 2012.
  09/2010; -1:44.
• Conference Proceeding: Manufacturing and integration of the IFS integral spectrograph

  V Caprio, E Giro, R Claudi, U Anselmi, P Bruno, E Cascone, S Desidera, D Fantinel, R Gratton, S Incorvaia, [......], E Sant'ambrogio, S Scuderi, E Stadler, M Turatto, K Dohlen, J ~L Beuzit, J Antichi, N Hubin, F Wildi, P Puget
  Society of Photo-Optical Instrumentation Engineers (SPIE) Conference Series; 07/2010
• Conference Proceeding: SPHERE IFS: the spectro differential imager of the VLT for exoplanets search

  R ~U Claudi, M Turatto, E Giro, D Mesa, U Anselmi, P Bruno, E Cascone, V Caprio, S Desidera, R Dorn, [......], L Lessio, J ~L Lizon, B Salasnic, S Scuderi, K Dohlen, J ~L Beuzit, P Puget, J Antichi, N Hubin, M Kasper
  Society of Photo-Optical Instrumentation Engineers (SPIE) Conference Series; 07/2010
• Source

  Available from: ArXiv

  Article: Design, analysis, and testing of a microdot apodizer for the Apodized Pupil Lyot Coronagraph. II. The dot size impact. (Research Note)

  P. Martinez, C. Dorrer, M Kasper, A. Boccaletti, K. Dohlen
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  ABSTRACT: The Apodized Pupil Lyot Coronagraph (APLC) is a promising coronagraphic device for direct exoplanets detection on the European-Extremely Large Telescope. We present new near-IR laboratory results using binary apodizers -- the so-called microdots apodizer -- which represent a very attractive and advantageous solution for the APLC. Microdots apodizers introduce high-frequency noise whose characteristics depend on the pixel size. The aim of this work is to characterize the impact of the pixel size on the coronagraphic image. Estimation of both the noise intensity and its localization in the field of view is the objective of this study. Stray-light diffraction introduced by the finite pixel size was measured during experiment. Intensity decreases, and radial distance increases, when the pixel size gets smaller. The physical properties of these microdots apodizers have been demonstrated in laboratory. The microdots apodizer is a suitable solution for any coronagraph using pupil amplitude apodization if properly designed.
  05/2009;
• Article: Design, analysis, and testing of a microdot apodizer for the apodized pupil Lyot coronagraph. II. Impact of the dot size

  P. Martinez, C. Dorrer, M. Kasper, A. Boccaletti, K. Dohlen
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  ABSTRACT: Context: The Apodized Pupil Lyot Coronagraph (APLC) is a promising coronagraphic device for direct exoplanet detection with the European Extremely Large Telescope. This concept features amplitude apodization in the entrance aperture, and a small opaque Lyot mask in the focal plane. We present new near-IR laboratory results using binary apodizers - the so-called microdot apodizer - which represent a very attractive and advantageous solution for the APLC. Aims: Microdot apodizers introduce high-frequency noise whose characteristics depend on the pixel size. The aim of this work is to characterize the impact of the pixel size on the coronagraphic image. We aim to estimate both the noise intensity and its localization in the field of view. Methods: The microdot apodizer, consisting of an array of pixels with spatially variable density that are either opaque or transparent, was manufactured by lithography of a light-blocking metal layer deposited on a transparent substrate. A set of 5 masks has been designed with different pixel sizes, tested in the near-IR, and their behavior compared to theoretical models. Results: Stray light diffraction introduced by the finite pixel size was measured during experiments. The intensity decreases, and radial distance increases, when the pixel size gets smaller. Conclusions: The physical properties of these microdot apodizers have been demonstrated in the laboratory. The microdot apodizer is a suitable solution for any coronagraph using pupil amplitude apodization, if properly designed.
  Astronomy & Astrophysics - ASTRON ASTROPHYS. 01/2009; 500(3):1281-1285.
• Chapter: SPHERE: A ‘Planet Finder’ Instrument for the VLT

  D. Mouillet, J.-L. Beuzit, M. Feldt, K. Dohlen, P. Puget, F. Wildi, A. Boccaletti, T. Henning, C. Moutou, H. M. Schmid, [......], J. Charton, R. Claudi, T. Fusco, R. Gratton, N. Hubin, M. Kasper, M. Langlois, J. Pragt, R. Roelfsema, M. Saisse
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  ABSTRACT: SPHERE (Spectro-Polarimetric High-contrast Exoplanet Research) is a second generation instrument for the VLT optimized for the very high-contrast imaging around bright stars[J.-L. Beuzit, M. Feldt, K. Dohlen et al. in Messenger 125, 29 (2006)]. The primary goal is the detection and characterization of new giant planets around a variety of nearby stars. Together with the observation of early planetary systems and disks, and in association with the results of other planet search techniques, SPHERE will be a primary contributor to get a complete picture of the variety of planetary systems and to better understand their mechanisms of formation and evolution. Such results will be obtained before even more ambitious projects for the direct imaging of planets either from the ground with ELTs or from space.
  12/2008: pages 337-341;
• Source

  Available from: Bruno Merín

  Article: Diversity among other worlds: characterization of exoplanets by direct detection

  J Schneider, A. Boccaletti, A. Aylward, P. Baudoz, J. L. Beuzit, R Brown, J. Cho, K. Dohlen, M. Ferrari, R. Galicher, [......], P. Shore, Ch. Sotin, A. Sozzetti, D. Stam, J. Surdej, F. Tamburini, G. Tinetti, S. Udry, C. Verinaud, D Walker
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  ABSTRACT: The physical characterization of exoplanets will require to take spectra at several orbital positions. For that purpose, a direct imaging capability is necessary. Direct imaging requires an efficient stellar suppression mechanism, associated with an ultrasmooth telescope. We show that before future large space missions (interferometer, 4-8 m class coronograph, external occulter or Fresnel imager), direct imaging of giant planets and close-by super-Earth are at the cross-road of a high scientific interest and a reasonable feasibility. The scientific interest lies in the fact that super-Earths share common geophysical attributes with Earths. They already begin to be detected by radial velocity (RV) and, together with giant planets, they have a larger area than Earths, making them detectable with a 1.5-2 m class telescope in reflected light. We propose such a (space) telescope be a first step before large direct imaging missions.
  12/2008;
• Source

  Available from: ArXiv

  Article: Design, analysis and test of a microdots apodizer for the Apodized Pupil Lyot Coronagraph

  P. Martinez, C. Dorrer, E. Aller-Carpentier, M Kasper, A. Boccaletti, K. Dohlen, N. Yaitskova
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  ABSTRACT: Coronagraphic techniques are required to detect exoplanets with future Extremely Large Telescopes. One concept, the Apodized Pupil Lyot Coronagraph (APLC), is combining an apodizer in the entrance aperture and a Lyot opaque mask in the focal plane. This paper presents the manufacturing and tests of a microdots apodizer optimized for the near IR. The intent of this work is to demonstrate the feasibility and performance of binary apodizers for the APLC. This study is also relevant for any coronagraph using amplitude pupil apodization. A binary apodizer has been designed using a halftone dot process, where the binary array of pixels with either 0% or 100% transmission is calculated to fit the required continuous transmission, i.e. local transmission control is obtained by varying the relative density of the opaque and transparent pixels. An error diffusion algorithm was used to optimize the distribution of pixels that best approximates the required field transmission. The prototype was tested with a coronagraphic setup in the near IR. The transmission profile of the prototype agrees with the theoretical shape within 3% and is achromatic. The observed apodized and coronagraphic images are consistent with theory. However, binary apodizers introduce high frequency noise that is a function of the pixel size. Numerical simulations were used to specify pixel size in order to minimize this effect, and validated by experiment. This paper demonstrates that binary apodizers are well suited for being used in high contrast imaging coronagraphs. The correct choice of pixel size is important and must be adressed considering the scientific field of view. Comment: A&A accepted, 8 pages
  10/2008;
• Article: SPHERE: exo-planets science with the new frontier of high contrast imaging

  R. Claudi, J.-L. Beuzit, M. Feldt, D. Mouillet, K. Dohlen, P. Puget, F. Wildi, A. Baruffolo, J. Charton, J. Antichi, [......], P. Raboub, R. Roelfsema, M. Saisse, H.-M. Schmid, M. Turatto, C. Moutou, T. Henning, S. Udry, F. Vakili, R. Waters
  [show abstract] [hide abstract]
  ABSTRACT: ABSTRACT High contrast imaging will be the new frontier of exoplanets search providing the opportunity to have at once a deep glance in the neighborhood of the target star. In addition, coupling integral field spectrographs to extreme adaptive optics module at the focus of 8m telescope class and in future to ELTs, gives also the possibility to have a first order characterization of the exoplanets itself. SPHERE, second generation instrument for VLT, is an exo-solar planet imager, which goal is to detect giant exo-solar planets in the vicinity of bright stars and to characterize them through spectroscopic and polarimetric observations. It is a complete system with a core made of an extreme-Adaptive Optics (AO) turbulence correction, pupil tracker and interferential coronagraphs. At its back end, a differential dual imaging camera (IRDIS) and an integral field spectrograph (IFS) work in the Near Infrared (NIR) Y, J, H and Ks bands (0.95-2.32 μm) and a high resolution polarization camera (ZIMPOL) covers the visible (0.6 - 0.9 μm). The three instruments could work simultaneously. As matter of fact, as the instrument has been thought and designed, It should be considered more like an experiment than a typical ancillary instrumentation. The prime objective of SPHERE is the discovery and study of new planets orbiting stars by direct imaging of the circumstellar environment. The challenge consists in the very large contrast of luminosity between the star and the planet (larger than " 12.5 magnitudes or " 105 flux ratio), at very small angular separations, typically inside the seeing halo. The whole design of SPHERE is therefore optimized towards high contrast performance in a limited field of view and at short distances from the central star. Both evolved and young planetary systems will be detected, respectively through their reflected light (mostly by ZIMPOL) and through the intrinsic planet emission (IRDIS+IFS modes). Both components of the near-infrared arm of SPHERE will provide complementary detection capacities and characterization potential, in terms of field of view, contrast, and spectral domain. The number of planets expected to be detected is a very strong function of the (assumed) distribution of planet separation. Extending the semi-major axis distribution up to P=250 yr (about 40 AU) yield a number of planet detections about 3.5 larger than for the same distribution truncated at P=70 yr (about 17 AU). Several tens of planet detection (details depend on target number and selection criteria) are then expected between 20 and 40 AU if planets are there. SPHERE has clearly the potential for an accurate determination of the frequency of planets in wide orbits. Note that while giant planets are not expected to be found in large number at very wide separation (a >50-100 AU), brown dwarfs might instead be present. In this paper a brief description of the whole instrument is given. Furthermore, an analysis of the performances of the instrument with its foreseen ability in discovering and characterize warm planets is also given. Last, but not least, SPHERE and its USA counter part: GPI, open the path towards new high contrast istrumentation for ELT like EPICS.
  08/2008; -1:875.
• Conference Proceeding: BIGRE: a new double microlens array for the integral field spectrograph of SPHERE

  E Giro, R ~U Claudi, J Antichi, P Bruno, E Cascone, V De Caprio, S Desidera, R ~G Gratton, D Mesa, S Scuderi, M Turatto, J ~L Beuzit, K Dohlen, P Puget
  Society of Photo-Optical Instrumentation Engineers (SPIE) Conference Series; 08/2008