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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.