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SFC difference images

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Date submitted: 06-Aug-1997

It is sometimes found that an SFD image has both over-corrected and 
under-corrected areas.  This may be due to imperfect straylight corrections, 
which we do subtracting a sunset image (called terminator) taken nearly at 
the same pointing as the X-ray image in question.  The straylight pattern
changes with the satellite pointing, and the dependence is most 
outstanding in the thin Al images.  

In order to assess the quality of the terminator images used for straylight
corrections, I have studied the difference of two images taken at a very
small separation.  The figure shows difference images of 6 such pairs.  
The captions give information on the individual as well as the difference
images.  They all come from the normal (not largely offset) pointings.

These difference images show that the terminator images taken at close
pointings are occasionally quite different.  This applies not only to the
areas surrounding the pinholes, but we to other areas, especially close
to the center, where a northeast-southwest ("retina") pattern is quite 
strong in the three pairs shown. 

Nariaki Nitta    6-Aug-97

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