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Primarily [Aschenbach
et al.1985], [ROSAT AO-21991, NRA 91-OSSA-31991]
The ROSAT attitude measurement and control system (AMCS)
achieves 3-axes stabilized pointing using up to 3 gyros (out of 4 gyros,
denoted as X-, Y-, and Z-gyros, plus the cross-coupled S-gyro)
and three reaction wheels (RW-1, RW-2, and RW-3)
in connection with the sun sensor and the magnetometer.
The high torque of the momentum wheels allows fast slews
( 
in 
15 minutes) such that ROSAT can observe
two targets on opposite hemispheres each orbit.
(It is uneconomical to observe more than two targets per orbit
due to slewing overheads.)
The AMCS pointing is accurate to 
, the pointing stability is
with a 
jitter radius.
The XRT boresight calibration is discussed in
Sect. 2.2.
PSPC pointed observations are usually carried out in a special
wobble mode
(see Sect. 1.3.3)
whereby the telescope's pointing direction is slowly moved back and forth
by 
with a period of 400s.
The direction is diagonal to the coarse wire mesh of the PSPC
window support structure and is designed to prevent unwanted shadowing
of X-ray sources behind this mesh.
The on-axis point-response function of the XRT is comparable to
the wire diameter of the coarse mesh.
HRI observations were also wobbled in order to smooth out
quantum-efficiency variations in the microchannel-plate detector.