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Plate Scale

The Andromeda galaxy (M31) was observed for 50 ksec, with numerous point sources detected. 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

(Note that there are 8192 such ``pixels'' in the ``sky-coordinate'' event lists distributed to ROSAT HRI observers.)  

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