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The time variability of x-ray emission can be studied with the HRI since
each detected photon has its time of arrival recorded by the detector.
The accuracy of this time is limited by the electronic resolution of the
HRI processor which is 61 
s relative to the ROSAT spacecraft
clock. The relative arrival times of photons during a single observation
is accurate to this value. The absolute accuracy of the ROSAT spacecraft
clock, and its conversion to UT, is expected to be a few milliseconds.
The HRI has a processing dead time during which events may not be
counted which varies between 0.36 and 1.35 ms per event. The variation
is discussed in § 4.2.2 
and depends on the fine position of
the event. Thus there is a dead time correction that needs to be made for
calculating the true event rate from a source. A mean dead time of
= 0.81 ms can be used for this purpose, and the true rate is
then given by: 
, where n is the observed rate.
The HRI window does not have a strongback so there is no occultation of
a source due to drifting of the pointing direction during an observation
as there is for the PSPC (see § 10.6 ). However for
sources detected near the edge of the field of view, there may be short
time scale (about 1 minute) variations in count rate that are due to
spacecraft motions.
As for the PSPC, the HRI observations are typically interrupted once per orbit, and sometimes as much a three times per orbit. Typical continuous viewing times for a source will be about 2000 s, with some cases lasting up to 4000 s. Long term monitoring of sources on time scales of weeks of more will be limited by the solar view constraints of the satellite. This limits source accessibility to about one month every six months for a source in the elliptic plane, with greater access time for sources closer to the ecliptic poles.
Comparison to the Einstein HRI
A complete comparison of the Einstein HRI performance and that
expected for ROSAT is not trivial since the band passes of the two
observatories are different and the detector photocathode is different.
A plot of the effective area of the Einstein HRI and the ROSAT HRI
is given in Figure 11.8 which serves to illustrate in a qualitative
manner the difference in sensitivity. The ROSAT HRI effective area
exceeds that for the Einstein HRI for almost all of the ROSAT
energy band. This is due to the increased area of the ROSAT telescope,
and also to the use of CsI as the HRI photocathode which has higher
quantum efficiency than the MgF 
used on the Einstein HRI.
Typically the expected count rate for a ROSAT observation
will be a factor of 3 to 4 greater than the same observation carried out
with the Einstein HRI. The exact value of this ratio depends on
the source spectrum and the interstellar absorption.
The ROSAT XMA has better scattering performance and a smaller point response function than the Einstein telescope. This allows us to use a 5'' detect cell as compared to the 12'' cell used with Einstein. The smaller cell size is a factor in the increased sensitivity of the ROSAT HRI since less background falls in the smaller region.
