Search

We describe regularized methods for image reconstruction and focus on the question of hyperparameter and instrument parameter estimation, i.e. unsupervised and myopic problems. We developed a Bayesian framework that is based on the \post density for all unknown quantities, given the observations. This density is explored by a Markov Chain Monte-Carlo sampling technique based on a Gibbs loop and including a Metropolis-Hastings step. The numerical evaluation relies on the SPIRE instrument of the Herschel observatory. Using simulated and real observations, we show that the hyperparameters and instrument parameters are correctly estimated, which opens up many perspectives for imaging in astrophysics.

We investigate super-resolution methods for image reconstruction from data provided by a family of scanning instruments like the Herschel observatory. To do this, we constructed a model of the instrument that faithfully reflects the physical reality, accurately taking the acquisition process into account to explain the data in a reliable manner. The inversion, ie the image reconstruction process, is based on a linear approach resulting from a quadratic regularized criterion and numerical optimization tools. The application concerns the reconstruction of maps for the SPIRE instrument of the Herschel observatory. The numerical evaluation uses simulated and real data to compare the standard tool (coaddition) and the proposed method. The inversion approach is capable to restore spatial frequencies over a bandwidth four times that possible with coaddition and thus to correctly show details invisible on standard maps. The approach is also applied to real data with significant improvement in spatial resolution., Comment: Astronomy & Astrophysics

The Mid Infra Red Instrument (MIRI) is one of the four instruments onboard the James Webb Space Telescope (JWST), providing imaging, coronagraphy and spectroscopy over the 5-28 microns band. To verify the optical performance of the instrument, extensive tests were performed at CEA on the flight model (FM) of the Mid-InfraRed IMager (MIRIM) at cryogenic temperatures and in the infrared. This paper reports on the point spread function (PSF) measurements at 5.6 microns, the shortest operating wavelength for imaging. At 5.6 microns the PSF is not Nyquist-sampled, so we use am original technique that combines a microscanning measurement strategy with a deconvolution algorithm to obtain an over-resolved MIRIM PSF. The microscanning consists in a sub-pixel scan of a point source on the focal plane. A data inversion method is used to reconstruct PSF images that are over-resolved by a factor of 7 compared to the native resolution of MIRI. We show that the FWHM of the high-resolution PSFs were 5-10% wider than that obtained with Zemax simulations. The main cause was identified as an out-of-specification tilt of the M4 mirror. After correction, two additional test campaigns were carried out, and we show that the shape of the PSF is conform to expectations. The FWHM of the PSFs are 0.18-0.20 arcsec, in agreement with simulations. 56.1-59.2% of the total encircled energy (normalized to a 5 arcsec radius) is contained within the first dark Airy ring, over the whole field of view. At longer wavelengths (7.7-25.5 microns), this percentage is 57-68%. MIRIM is thus compliant with the optical quality requirements. This characterization of the MIRIM PSF, as well as the deconvolution method presented here, are of particular importance, not only for the verification of the optical quality and the MIRI calibration, but also for scientific applications., Comment: 13 pages, submitted to SPIE Proceedings vol. 7731, Space Telescopes and Instrumentation 2010: Optical, Infrared, and Millimeter Wave

Context: Sh2-104 is a Galactic H ii region with a bubble morphology, detected at optical and radio wavelengths. It is considered the first observational confirmation of the collect-and-collapse model of triggered star-formation. Aims: We aim to analyze the dust and gas properties of the Sh2-104 region to better constrain its effect on local future generations of stars. In addition, we investigate the relationship between the dust emissivity index {\beta} and the dust temperature, T_dust. Methods: Using Herschel PACS and SPIRE images at 100, 160, 250, 350 and 500 {\mu}m we determine T_dust and {\beta} throughout Sh2-104, fitting the spectral energy distributions (SEDs) obtained from aperture photometry. With the SPIRE Fourier transform spectrometer (FTS) we obtained spectra at different positions in the Sh2-104 region. We detect J-ladders of CO and 13CO, with which we derive the gas temperature and column density. We also detect proxies of ionizing flux as the [NII] 3P1-3P0 and [CI] 3P2-3P1 transitions. Results: We find an average value of {\beta} ~ 1.5 throughout Sh2-104, as well as a T dust difference between the photodissociation region (PDR, ~ 25 K) and the interior (~ 40 K) of the bubble. We recover the anti-correlation between {\beta} and dust temperature reported numerous times in the literature. The relative isotopologue abundances of CO appear to be enhanced above the standard ISM values, but the obtained value is very preliminary and is still affected by large uncertainties., Comment: Accepted by A&A, to be published on the Herschel A&A Special Issue. 5 pages, 5 figures.

We present far-infrared spectra and maps of the DR21 molecular cloud core between 196 and 671 microns, using the Herschel-SPIRE spectrometer. Nineteen molecular lines originating from CO, 13CO, HCO+ and H2O, plus lines of [N II] and [CI] were recorded, including several transitions not previously detected. The CO lines are excited in warm gas with Tkin ~ 125 K and nH2 ~ 7 x 10^4 cm-3, CO column density N(CO) ~ 3.5 x 10^18 cm^-2 and a filling factor of ~ 12%, and appear to trace gas associated with an outflow. The rotational temperature analysis incorporating observations from ground-based telescopes reveals an additional lower excitation CO compoment which has a temperature ~ 78 K and N(CO) ~ 4.5 x 10^21 cm^-2. Astronomy & Astrophysics HERSCHEL special Issue, in press., Comment: 5 pages, 6 figures

We present an original method for reconstructing a three-dimensional object having two spatial dimensions and one spectral dimension from data provided by the infrared slit spectrograph on board the Spitzer Space Telescope. During acquisition, the light flux is deformed by a complex process comprising four main elements (the telescope aperture, the slit, the diffraction grating and optical distortion) before it reaches the two-dimensional sensor. The originality of this work lies in the physical modelling, in integral form, of this process of data formation in continuous variables. The inversion is lso approached with continuous variables in a semi-parametric format decomposing the object into a family of Gaussian functions. The estimate is built in a deterministic regularization framework as the minimizer of a quadratic criterion. These specificities give our method the power to over-resolve. Its performance is illustrated using real and simulated data. We also present a study of the resolution showing a 1.5-fold improvement relative to conventional methods.

We report the detection of correlated anisotropies in the Cosmic Far-Infrared Background at 160 microns. We measure the power spectrum in the Spitzer/SWIRE Lockman Hole field. It reveals unambiguously a strong excess above cirrus and Poisson contributions, at spatial scales between 5 and 30 arcminutes, interpreted as the signature of infrared galaxy clustering. Using our model of infrared galaxy evolution we derive a linear bias b=1.74 \pm 0.16. It is a factor 2 higher than the bias measured for the local IRAS galaxies. Our model indicates that galaxies dominating the 160 microns correlated anisotropies are at z~1. This implies that infrared galaxies at high redshifts are biased tracers of mass, unlike in the local Universe., Comment: ApJ Letters, in press