The ACA (Ball Aerospace & Technologies Corp) is a 4-inch defocused telescope with 5" pixels and a 2 deg field of view, which provides about 0.1" centroids for single stars down to a magnitude of < 10.2 in its own red (unfiltered CCD) bandpass. Chandra observations typically use up to 8 acquisition stars and 5 guide stars for every pointing. After the data reach the ground, the guide star observations are used for aspect reconstruction - to assemble the dithered X-ray events into a final high spatial resolution X-ray image. If the ACA magnitude estimates in AGASC are good, a high fraction of stars are acquired. If the star positions in the AGASC are accurate, the spatial resolution and astrometric accuracy of reconstructed X-ray images are optimized. With the latest updates to the AGASC, more than 95% of stars are acquired, and the aspect reconstruction achieves excellent (< 1") relative and absolute astrometry. For most Chandra observers, the full thread of acquisition and guide star selection and aspect reconstruction is completely transparent, and the full aspect solution is provided by the CXC as a standard data product.
The AGASC was originally based on the Hubble Space Telescope Guide Star Catalog v1.1, containing 19 million stars. 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. The GSC-ACT data merge decreases by about half the systematic astrometric errors down to the catalog limit of about V=14.5. The 2MASS data identify 41855 galaxies down to J=12.5. All these new 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. We also flag any known multiple or variable stars, as well as those within 3r20 of a 2MASS galaxy. (r20 is the K-band 20 mag/arcsec2 isophotal elliptical radius in arcseconds.)
AGASC 1.5 currently represents the only available catalog that combines high quality astrometry on the ICRS system down to 14.5 mag with detailed color and proper motion information down to about 12th. In addition, a unique feature are the "spoiler codes" that indicate the likely perturbation any stellar centroid might suffer from nearby objects. These properties make AGASC1.5 a potentially important catalog for guide star selection for other space- and ground-based telescopes (e.g., for telescope guiding or Adaptive Optics). AGASC1.5 should also prove useful for a variety of scientific purposes. More information about the catalog, its constituents and construction are available on the web.
Paul Green and Dennis Schmidt