Attitude Measurement and Control System

The ROSAT XRT has two CCD star trackers (one of which failed early in the mission). The WFC has an independent star tracker. The data from this star tracker are being incorporated into XRT attitude solutions in the Rev 2 data re-processing.

During PSPC observations the ROSAT spacecraft is wobbled with an amplitude of typically $\pm$ 3 arcminutes at a period of around 400 seconds. This avoids having the sharply focussed X-ray image of a point source hitting a detector wire throughout the observation. HRI images are also wobbled, but with a lower amplitude of typically $\pm$ 1.5 arcminutes. Although the wobble is strictly periodic, the path of an image across the detector varies from cycle to cycle.

Attitude reconstruction has not met specification. This can manifest itself in distortion of the on-axis PSF seen with the HRI, and as offsets of up to 30 arcseconds between the positions of objects observed after an interval of months.