Showing papers by "Éric Thiébaut published in 2006"

STECMAP: STEllar Content from high-resolution galactic spectra via Maximum A Posteriori

Abstract: In this paper we describe STECMAP (STEllar Content via Maximum A Posteriori), a flexible, non-parametric inversion method for the interpretation of the integrated light spectra of galaxies, based on synthetic spectra of single stellar populations (SSPs). We focus on the recovery of a galaxy's star formation history and stellar age–metallicity relation. We use the high-resolution SSPs produced by pegase-hr to quantify the informational content of the wavelength range λλ= 4000–6800. Regularization of the inversion is achieved by requiring that the solutions are relatively smooth functions of age. The smoothness parameter is set automatically via generalized cross validation. A detailed investigation of the properties of the corresponding simplified linear problem is performed using singular value decomposition. It turns out to be a powerful tool for explaining and predicting the behaviour of the inversion, and may help designing SSP models in the future. We provide means of quantifying the fundamental limitations of the problem considering the intrinsic properties of the SSPs in the spectral range of interest, as well as the noise in these models and in the data. We demonstrate that the information relative to the stellar content is relatively evenly distributed within the optical spectrum. We show that one should not attempt to recover more than about eight characteristic episodes in the star formation history from the wavelength domain we consider. STECMAP preserves optimal (in the cross validation sense) freedom in the characterization of these episodes for each spectrum. We performed a systematic simulation campaign and found that, when the time elapsed between two bursts of star formation is larger than 0.8 dex, the properties of each episode can be constrained with a precision of 0.02 dex in age and 0.04 dex in metallicity from high-quality data [R= 10 000, signal-to-noise ratio (SNR) = 100 per pixel], not taking model errors into account. We also found that the spectral resolution has little effect on population separation provided low- and high-resolution experiments are performed with the same SNR per A. However, higher spectral resolution does improve the accuracy of metallicity and age estimates in double-burst separation experiments. When the fluxes of the data are properly calibrated, extinction can be estimated; otherwise the continuum can be discarded or used to estimate flux correction factors. The described methods and error estimates will be useful in the design and in the analysis of extragalactic spectroscopic surveys.

STECKMAP: STEllar Content and Kinematics from high resolution galactic spectra via Maximum A Posteriori

Abstract: We introduce STECKMAP (STEllar Content and Kinematics via Maximum A Posteriori likelihood), a method for recovering the kinematic properties of a galaxy simultaneously with its stellar content from integrated light spectra. It is an extension of STECMAP (presented recently by Ocvirk et al.) to the general case where the velocity distribution of the underlying stars is also unknown. The reconstructions of the stellar age distribution, the age‐metallicity relation and the line-of-sight velocity distribution (LOSVD) are all non-parametric, i.e. no specific shape is assumed. The only a priori conditions that we use are positivity and the requirement that the solution is smooth enough. The smoothness parameter can be set by generalized cross-validation according to the level of noise in the data in order to avoid overinterpretation. We use single stellar populations (SSPs) from P´-HR (R = 10 000, λ = 4 000‐6 800 A, from Le Borgne et al.) to test the method through realistic simulations. Non-Gaussianities in LOSVDs are reliably recovered with signal-to-noise ratio (SNR) as low as 20 per 0.2 A pixel. It turns out that the recovery of the stellar content is not degraded by the simultaneous recovery of the kinematic distribution, so that the resolution in age and error estimates given in Ocvirk et al. remain appropriate when used with STECKMAP. We also explore the case of age-dependent kinematics (i.e. when each stellar component has its own LOSVD). We separate the bulge and disc components of an idealized simplified spiral galaxy in integrated light from high-quality pseudo-data (SNR = 100 per pixel, R = 10 000), and constrain the kinematics (mean projected velocity, projected velocity dispersion) and age of both components. Ke yw ords: methods: data analysis ‐ methods: statistical ‐ techniques: spectroscopic ‐ galaxies: abundances ‐ galaxies: kinematics and dynamics ‐ galaxies: stellar content.

High dynamic range imaging by pupil single‐mode filtering and remapping

Abstract: Because of atmospheric turbulence, obtaining high angular resolution images with a high dynamic range is difficult even in the near-infrared domain of wavelengths. We propose a novel technique to overcome this issue. The fundamental idea is to apply techniques developed for long baseline interferometry to the case of a single-aperture telescope. The pupil of the telescope is broken down into coherent subapertures each feeding a single-mode fibre. A remapping of the exit pupil allows interfering all subapertures non-redundantly. A diffraction-limited image with very high dynamic range is reconstructed from the fringe pattern analysis with aperture synthesis techniques, free of speckle noise. The performances of the technique are demonstrated with simulations in the visible range with an 8-m telescope. Raw dynamic ranges of 1:10 6 can be obtained in only a few tens of seconds of integration time for bright objects.

MATISSE: perspective of imaging in the mid-infrared at the VLTI

Abstract: MATISSE is foreseen as a mid-infrared spectro-interferometer combining the beams of up to four UTs/ATs of the Very Large Telescope Interferometer (VLTI) of the European Southern Observatory. The related science case study demonstrates the enormous capability of a new generation mid-infrared beam combiner. MATISSE will constitute an evolution of the two-beam interferometric instrument MIDI. MIDI is a very successful instrument which offers a perfect combination of spectral and angular resolution. New characteristics present in MATISSE will give access to the mapping and the distribution of the material (typically dust) in the circumstellar environments by using a wide mid-infrared band coverage extended to L, M and N spectral bands. The four beam combination of MATISSE provides an efficient UV-coverage : 6 visibility points are measured in one set and 4 closure phase relations which can provide aperture synthesis images in the mid-infrared spectral regime.

2006 interferometry imaging beauty contest

Abstract: We present a formal comparison of the performance of algorithms used for synthesis imaging with optical/infrared long-baseline interferometers. Five different algorithms are evaluated based on their performance with simulated test data. Each set of test data is formatted in the OI-FITS format. The data are calibrated power spectra and bispectra measured with an array intended to be typical of existing imaging interferometers. The strengths and limitations of each algorithm are discussed.

High dynamic range imaging with a single-mode pupil remapping system : a self-calibration algorithm for redundant interferometric arrays

Abstract: The correction of the influence of phase corrugation in the pupil plane is a fundamental issue in achieving high dynamic range imaging. In this paper, we investigate an instrumental setup which consists in applying interferometric techniques on a single telescope, by filtering and dividing the pupil with an array of single-mode fibers. We developed a new algorithm, which makes use of the fact that we have a redundant interferometric array, to completely disentangle the astronomical object from the atmospheric perturbations (phase and scintillation). This self-calibrating algorithm can also be applied to any - diluted or not - redundant interferometric setup. On an 8 meter telescope observing at a wavelength of 630 nm, our simulations show that a single mode pupil remapping system could achieve, at a few resolution elements from the central star, a raw dynamic range up to 10^6; depending on the brightness of the source. The self calibration algorithm proved to be very efficient, allowing image reconstruction of faint sources (mag = 15) even though the signal-to-noise ratio of individual spatial frequencies are of the order of 0.1. We finally note that the instrument could be more sensitive by combining this setup with an adaptive optics system. The dynamic range would however be limited by the noise of the small, high frequency, displacements of the deformable mirror.

FRIM: minimum-variance reconstructor with a fractal iterative method

Abstract: Adaptive optics (AO) systems under study for the future generation of telescopes have to cope with a huge number of degrees of freedom. This number N is typically 2 orders of magnitude larger than for the currently existing AO systems. An iterative method using a fractal preconditioning, has recently been suggested for a minimum-variance reconstruction in O ( N ) operations. We analyze the efficiency of this algorithm for both the open-loop and the closed-loop configurations. We present the formalism and illustrate the assets of this method with simulations. While the number of iterations for convergence is around 10 in open-loop, the closed-loop configuration induces a reduction of the required number of iterations by a factor of 3 typically. This analysis also enhances the importance of introducing priors to ensure an optimal command. Closed-loop simulations demonstrate the loss of performance when no temporal priors are used. Besides, we discuss the importance of an accurate model for both the system and its uncertainties, so as to ensure a stable behavior in closed-loop.

Data processing in nulling interferometry: case of the Darwin mission

Abstract: This paper first gives an introduction to the broad features of the Darwin mission of ESA and then describes in some detail the data processing that is necessary to detect planets and spectrally characterize them. The proposed data processing method is validated by means of simulated data.

Recent progress and future prospects of the GI2T interferometer

Abstract: We give an overview of recent results obtained with the GI2T interferometer. On the technical side, great improvements have been obtained on photon counting detectors, especially in terms of quantum efficiency and of photon centroiding algorithms. Piston measurements with the GI2T dispersed fringes have been made during coordinate observations with the Generalized Seeing Monitor GSM. These observations have lead to wavefront outer scale determinations. The last topic we will present concerns the polarimetric measurements done with the SPIN device on the GI2T spectrograph. We conclude this paper by a summary of the results obtained with the GI2T during its scientific life.

Polychromatic phase retrieval with Kolmogorov self-adapting prior constraints

Abstract: Phase retrieval is a very promising approach for wavefront sensing in the focal plane of ground-based large telescopes. Itis a non-linear problem that must be solved by means of global optimization. Currently only multi-focal phase diversityalgorithms are used in adaptive optics. They enable the correction of static aberrations. For speckle imaging the problem isincreasingly multi-modal with the ratio D/r 0 . Yet thanks to an iterative Newton algorithm with self-adapting Kolmogorovprior information, we show from consistent modeling and simulations, that we could efciently sense short exposurewavefronts at high D/r 0 from a single focal plane. We show that using data at different wavelengths with a properpolychromatic model would even enforce the convergence, thus making it an envisageable method to sense the returnedux of a polychromatic laser guide star (PLGS). For instance, we show that if we suppose the PLGS is not resolved, phaseretrieval would enable an improvement in the centroid estimation in agreement with the Cramer-Rao lower bound. As apost-processingtechnique,ouralgorithmalreadyhasnumerouspotentialapplicationsforastronomyandforotherdomains.Thanks to the improvement of computing workstations and the optimization of the algorithm, applications involving real-time wavefront corrections should be soon possible.Keywords: phase retrieval, wavefront sensing, speckle images, global optimization, prior constraints, tip-tilt, centroiding,polychromatic laser guide star

Performances of CPNG: a new generation photon-counting camera with real-time dedicated optimal processing

Abstract: Since 1974 we develop photon-counting imaging devices for high angular resolution in the visible by means of speckle and optical interferometry. Our last generation photon-counting camera, CPNG, has been built to benefit from the recent advances in photonic commercial components. CPNG is an ICCD which uses electron multiplication in microchannel plates to overcome the readout noise of fast CCD. We achieve optimal performances (sensitivity and resolution) by proper optical design, by cooling of the first stage photocathode and by careful data processing. Thanks to the power of current workstations, the processing of the CCD signal can be done by elaborated real-time software at frame rates as high as 262 Hz (72 Mpixel/s). The real-time software is in charge of detecting occurences of photon-events and estimating their positions. We explain how our dedicated processing improves the detection sensitivity to reach an effective quantum efficiency of 35%. We also show that our unbiased recentering of detected photons can avoid spurious high-energy events and nevertheless achieve sub-pixel resolution. In practice, our resolution is limited by the size of a microchannel to about 2000×2000 effective pixels for our 516×532 CCD. The very good performances of CPNG open us new classes of objects and have proven to be useful for other applications. For instance, several versions of our camera have been developped (with different spectral ranges) to cover the common needs in Astronomy and biological imaging for an extremely low-light level and fast imaging detector.