Chandra's Aspect Camera Assembly (ACA) measures positions of selected
stars to acquire and hold target pointings, and for post facto aspect
determination. Guide stars are selected from the AGASC. Based
originally on version 1.1 of the Guide Star Catalog for the Hubble
Space Telescope, the AGASC has been extended and refined to include
colors and to improve astrometric information. AGASC merges multiple
entries and combines the best data from Tycho-2 and GSC-ACT to include
one entry per star down to V~14.5. The AGASC also includes ACA mag
predictions based on colors, as well as additional information on
crowding of each star, pertinent to guide star quality. 2MASS data
identify nearby galaxies down to J=12.5.
AGASC1.5: ADASS 2002 paper and
AGASC 1.7: Detailed Information