Chandra's Aspect Camera Assembly (ACA) measures positions of selected stars to acquire and hold target pointings, and for post facto aspect determination. The selection and matching of the guide stars is governed by data in the AXAF (Chandra) Guide and Acquisition Star Catalog (AGASC). Based originally on version 1.1 of the Guide Star Catalog for the Hubble Space Telescope, the AGASC has been extended and refined in several stages, with data from additional catalogs and with recalibrations using on-orbit ACA observations.In 2002 the Chandra X-ray Center (CXC) completed a major upgrade of AGASC. We merged data from three catalogs---Tycho-2, GSC-ACT, and 2MASS. The Tycho-2 data substantially improve the photometric and astrometric measurements of stars as faint as V=12, while the GSC-ACT merge decreases by about half the systematic astrometric errors down to the catalog limit of about V=14.5. The 2MASS data identify galaxies down to J=12.5. These new catalog data enhance the value of AGASC for scientific as well as operational purposes. Specifically for Chandra's operational use of AGASC, we recalibrated the estimated ACA magnitudes based on Chandra on-orbit measurements, and implemented a more sophisticated calculation of the effect of nearby stars on the best-fit centroid of a guide star. This paper presents the rationale for and the process of updating the AGASC. We then discuss the improvements in performance that we expect to result, and introduce the online AGASC 1.5 query tool http://cxc.harvard.edu/agasc.
Astronomical Data Analysis Software and Systems XII
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