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et al.1985], [ROSAT AO-21991, NRA 91-OSSA-31991],
[Voges1992, Snowden and
During the six-month survey phase, nearly the entire sky was scanned in 2 (5 for the WFC) strips centered on great circles running through the ecliptic poles, roughly perpendicular to the Earth-Sun line. The rotation period was synchronized to the orbital period by the scan control law in such a manner that no Earth occultations occurred and the zenith angle was minimized. The nominal scan rate was 360 of ecliptic latitude per satellite orbit, and the nominal progression rate of 0.986 per day, which was increased to 1.46 per day at the end of the survey.
The PSPC (with a 2 FOV) was in the focal plane of the XRT during the all-sky survey. The S1 and S2 filters were used alternately by the WFC (5 FOV) in order to obtain 2 XUV colours in the 60-200 Å band.
The last week of the scheduled survey phase was lost due to the solar slew on 1991 January 25 described in App. A. However, the unobserved region was surveyed between 1991 February 23-25 and 1991 August 3-13 during breaks in the AO-1 mission phase. Figure 1.3 is an Aitoff projection of the celestial sphere showing the (logarithmic) exposure time of the ROSAT all-sky survey.