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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 stable.
