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On-orbit measurements have been made of the HRI background, which is decomposed into several components:
The internal background is believed to
mainly come from residual radioactivity in the MCP glass and the
surrounding detector structure. This was measured on the ground and in
orbit, and is 
counts s 
over the entire device.
This appears to be a stable component of the
background and is uniform over the detector.
The externally induced background from trapped particles, cosmic rays,
and scattered solar radiation was about
0.5 counts s minimum on the Einstein HRI. This was occasionally
two to three times greater for a small number of HRI observations.
For ROSAT the trapped particle fluxes are
higher than for Einstein due to the higher inclination
of the orbit and the higher altitude, although there are parts of the
ROSAT orbit where the particle flux is as low as that for Einstein.
Based on the on-orbit calibration data that has been analyzed thus far, the
external background rate for the ROSAT HRI will vary from a minimum of
2 counts s to about 10 counts s at which point the HRI high
voltage is reduced to protect the detector from overload.
The X-ray background (XRB) contribution consists of a galactic
component and an extragalactic component. Since the overall XRB flux
in the ROSAT energy band is not well known, it is not possible to
accurately calculate the contribution of these sources to the detector
counting rate. An estimate for this contribution can be made based on
an assumed XRB model; a detailed discussion of this is given in
§ 11.1 
. An estimate from the on-orbit
calibration data corrected for the internal and particle induced
rates is 
counts s over the detector.
Thus the total ROSAT HRI background rate is estimated to be at least
counts s over the entire field of view.
Table 7.2 summarizes the
background sources, where the expected values are listed as well and the
ranges of these values. The table also gives the total background in various
units.
