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On AXIS point source
If you are background subtracting a point source close to the optical
axis then you can't go far wrong. Select a background area,
preferably in an annulus around the source, but if the field is too
crowded, in any blank region of the image. The program corrects for
the vignetting effects between the source and background boxes. The
only possible problem is if the background box has to be taken at a
large distance from the source and if there is a strong gradient in
the background. Such a gradient, may be due to either galactic
background effects (for sources at low galactic latitudes) or solar
scattered X-rays. It is trivial to determine whether
such gradients exist, by performing an ICIRCSTATS at various points
on the image. If you HAVE got a gradient, then you may produce two
background files at THE SAME OFF-AXIS angle from the centre and with
the SAME BOX SIZE , but on opposite sides of your source and average
them using MEANDAT. Before using the result in XRTSUB.
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