Why were the HRI observations wobbled, anyway?
All HRI GO observations have been taken with the wobble on. A few calibration observations were taken with the wobble off. There was never any reason to wobble HRI observations. MPE decided to wobble both PSPC and HRI data, even though only the PSPC needed to be wobbled because of the wire grids.
During PSPC observations, the telescope's pointing direction is slowly moved back and forth by +/- 3 arcmin on a timescale of ~400 second, in a diagonal direction to the coarse wire mesh of the PSPC window support structure, in order to prevent unwanted shadowing of X-ray sources behind this mesh. The amplitude of the wobble is not always the same, and was smaller (about half the normal amplitude) during the 'Reduced Pointing' mode of 1991 May 18- Nov 04. A wobble of lower amplitude is performed during pointed HRI observations. +/- 1 arcmin on a timescale of ~400 seconds, to smooth out variations across the microchannel plate gaps.
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