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Primarily [ROSAT AO-21991, NRA 91-OSSA-31991]
During both PSPC and HRI pointed observations, the telescope's pointing direction was in general slowly moved back and forth (by with a period of 400 seconds) in a diagonal direction to the coarse wire mesh of the PSPC window support structure. This was done for the PSPC in order to prevent unwanted shadowing of X-ray sources behind this mesh. The amplitude of the wobble was not always the same, and was smaller (about half the normal amplitude) during the reduced-pointing mode of 1991 May 18 - November 4.