As an alternative, Adaptive Optics [AO] (Babcock 1953; Rousset et al. 1990) is a real-time
technique which compensates for the random variation of the turbulent wave
front. This compensation enables the recording of high spatial resolution long
exposure images. However, even if
the object's spatial frequencies are preserved up to the diffraction limit
of the telescope, they are often severely attenuated since the AO
correction is only partial. It is therefore necessary to use image processing techniques
to improve the quality of the recovered object
(Thiebaut & Conan 1995; Christou et al. 1997).
We consider here stellar fields. The main problem with images of such objects is that the AO partial correction induces a halo around each star. When two stars are too close, their halos overlap, and photometric and astrometric measurements are difficult. Different post-processing algorithms have been proposed to deal with this problem (CLEAN (Hogbom 1974), DAOPHOT (Stetson 1987), P-Lucy algorithm (Lucy 1994), (Hook & Lucy 1994)) but they all require that the PSF be pre-determined accurately. In certain cases, especially with a short exposure time, very partial AO correction or uncalibrated aberrations, the determination of the PSF is too rough to yield good astrometric and photometric results.
In this paper, we propose a deconvolution method for such star field observations. This deconvolution technique permits the restoration of both the object and the PSF. We call it a "myopic'' deconvolution, because it incorporates the knowledge of the Point Spread Function [PSF] structure and variability (as opposed to "blind'' deconvolution where the PSF is assumed to be completely unknown). The strong knowledge of the object (point-like object) is also used, which allows a sub-pixel and a good photometric restoration.
In section two, an overview of the formation of AO corrected images is presented. In section three, the myopic deconvolution is introduced. In section four, the method used to estimate the average and the variability of the PSF is detailed. This method is based on the processing of the wavefront sensor data recorded simultaneously with the AO image. In section five simulation results are presented. They show that our method is able to accurately recover the object parameters (astrometry and photometry) and the PSF even in unfavorable cases (low flux, very partial AO correction).
In the last section experimental results acquired with the Canada-France-Hawaii Telescope adaptive optics system are presented. The myopic deconvolution allows a good restoration despite the static aberrations present in the true PSF but not measured by the wavefront sensor.
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