Aspect / Aspect Solution
The pointing position of the Chandra telescope vs. time. The Chandra spacecraft has a built in "dither" on its pointing position to average across calibration uncertainties. The position of the telescope's optical axis is determined with gyroscopes and a small star camera (the PCAD) and a system of fiducial lights on the science instrument module (SIM). The fiducial lights are imaged onto the PCAD via a periscope and so tie together the pointing position of the X-ray telescope (HRMA) and the PCAD on the sky. Star positions from Hipparcos and other astrometric surveys are used to put the aspect solution onto a standard reference frame.
See the Proposers' Observatory Guide (POG) for technical details and the CXC Aspect webpages for the accuracy of the current Chandra aspect solution.