The ROSAT HRI
The ROSAT High Resolution Imager (HRI) is very similar to the Einstein Observatory HRI and is comprised of two cascaded microchannel plates (MCPs) with a crossed grid position readout system. The HRI provides a 38 arcminutes (on a side) square field-of-view, with ~ 2 arcsec spatial resolution (FWHM). The HRI has negligible energy resolution but provides relative time resolution down to 61 microseconds.
Two significant changes from the Einstein instrument that have affected the performance of the detector are: the substitution of CsI for MgF as the detector photocathode, and the introduction of a thin aluminum-coated plastic membrane (the ``electrostatic shield'') near the front MCP (in addition to the UV/Ion shield which is situated farther in front of the detector).
Observations of the Andromeda galaxy (M31) allowed detection of numerous point sources. With 12 identified sources distributed over the HRI field from this observation (data courtesy of F. Primini), the mean plate scale determined with a standard astrometric plate reduction method was found to be 0.499 +/- 0.001 arc seconds per pixel.
The relationship between Einstein and ROSAT pixels was also measured (using 23 M31 sources that were matched to Einstein HRI detections) and found to be:
1 ROSAT pixel = 1.0050 0.0007 Einstein pixel
The process of determining boresight offsets (systematic zero-point shifts found by comparing measured and known positions of catalogued objects) for the HRI has led to the conclusion that there are time-varying errors in the attitude solutions used to correct event positions. Although most known objects are detected at positions within 10 arcseconds of their catalog positions, residual discrepancies can be present even after application of the boresight corrections. This can be particularly evident in a comparison of observations of the same object taken six months apart (and hence at spacecraft roll angles differing by 180 degrees).
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