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 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) that 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 more details of the PCAD. See the CXC Aspect WWW pages for reports on the accuracy of the current Chandra aspect solution.