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For the last 20 months, the Chandra X-Ray Observatory (Weisskopf et. al. 2000) has been producing X-ray images of the universe in stunning detail. This is due in large part to the excellent post-facto pointing aspect determination for Chandra (Aldcroft et. al. 2000). This aspect determination performance is achieved using elliptical gaussian centroiding techniques. Application of point spread function (PSF) fitting using a true PSF model for the Aspect Camera Assembly (ACA) on Chandra could improve this performance. We have investigated the use of an ACA PSF model in the post-facto centroiding of stars and fiducial lights imaged by the ACA. We will present the methodologies explored for use in determining a model for the ACA PSF and discuss the results of a comparison of PSF fit centroiding and the current method of elliptical gaussian centroiding as they apply to post-facto aspect reconstruction. The first method of recovering the ACA PSF uses a raytrace model of the ACA to generate simulated stellar PSFs. In this method, the MACOS raytracing software package is used to describe each element of the Chandra aspect optical system. The second method investigated is the so called shift and add method whereby we build a high resolution image of the PSF by combining several thousand low resolution images of a single star collected by the ACA while tracking during normal science observations. The programmed dither of the spacecraft slowly sweeps the stellar image across the ACA focal plane, and the many slightly offset images are used to effectively increase the resolution of the resultant image of the star to a fraction of an ACA pixel. In each method, a library of PSF images is built at regularly gridded intervals across the ACA focal plane. This library is then used to interpolate a PSF at any desired position on the focal plane. We have used each method to reprocess the aspect solution of a set of archived Chandra observation and compare the results to one another and to the delivered post-facto aspect solution, currently derived using elliptical gaussian centroiding of ACA star images. Finally, we will present a summary of Chandra's aspect performance achieved to date, and discuss the effect of incorporating a PSF model into the post-facto aspect determination software.
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