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CCD radiation damage & Yohkoh repointing

Image name: Dfsr_diff_26apr94_5jul94.png (click image to enlarge)
Image size: 214.578 KB (512x512)
Date submitted: 14-Jul-1994

To:	SXT Ops Team
From:	L. Acton
Date:	14-Jul-94
Subj:	CCD radiation damage & Yohkoh repointing.

This SHOW_PIX image is the difference between restored SXT diffuser images
taken on 26-Apr-94 and 5-Jul-94.  Here are the data:

   0 26-APR-94  19:26:57  QT/H  Diffu/Open  Half Norm L   4     2.9  512x512
   1 26-APR-94  19:29:03  QT/H  Diffu/Open  Half Norm C   4     2.9  512x512

   0  5-JUL-94  08:39:30  QT/H  Diffu/Open  Half Norm L   4     2.9  512x512
   1  5-JUL-94  08:41:38  QT/H  Diffu/Open  Half Norm C   4     2.9  512x512

The flattened 26-Apr-94 image may be seen in show_pix/calibration image
Diffuser_image_940426.  This difference image was formed as follows

IDL> diff=jul5-(apr26*(total(jul5)/total(apr26)))
IDL> tvscl,diff<40

You will especially note the dark area of increased radiation damage in the
northeast.  The deepest part of this damage amounts to about 50 DN.  This
area is now the most deeply damaged limb area on the CCD.

The white circle is the limb location for the SXT pointing on 5-JUL-94 
08:41:38.  The black circle represents the limb location for a Yohkoh
pointing 1.3 arcmin to the east of the present pointing.  It moves the east
limb away from the most damaged region and places the west limb just to the
left of the least damaged part of the existing west limb damage.  

As the radiation damage in the northeast is only about 15%-20% deeper than
the worst of the west limb damage I do not recommend changing the pointing
at this time -- because of the resulting change in straylight correction
and the probable impact on coverage of the HXA limb sensors (I've not
checked this).  However, the SXT Chief Observer should monitor changes in
the diffuser image from week to week and suggest a pointing change if the
accumulated damage in the northeast becomes more than 10 DN or so worse.

There are a couple of other interesting things to note about this picture.
There are isolated bright pixels in the corners and a distorted ring of
bright pixels where the diffuser signal is enhanced.  I have no clear idea
what causes this effect.  

Also, I note little, if any, change in the "dark specks" (see
show_pix/calibration image Diffuser_specks) between the 2 images.  I was
able to locate a couple of one pixel specks that are present in the 26-Apr
image and not in the 5-Jul image but most of them appear to be quite

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