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Date submitted: 27-Nov-1995
L. Acton 11/27/95 EFFECTS OF RECENT CCD BAKEOUT A 24 hour bakeout of the CCD took place on 22-23 Nov 1995. Figure A is a post-bakeout diffuser image. Figure B is a difference image between diffuser images taken on 20-Nov-95 18:18 minus 22-NOV-95 15:40, just before bakeout. Figure C is a difference image between 23-NOV-95 14:21 minus 22-NOV-95 15:40. The present location of the sun is indicated by the circle. Figure B is quite flat except for a couple of transient radiation-damage artifacts while figure C shows extensive changes. The bakeout had an obvious effect over much of the CCD but especially in an increase in transmission in the corners. Note particularly the effect on the strange arcs around the solar image. It seems that the mystery of the origin of these arcs has been solved. I surmise that they are caused by a build up of contamination on the CCD and the pattern must have some relation to line of sight to the major source of contamination. The bakeout partially removed this contamination. The effect of the bakeout is clearly revealed in the histograms. The histogram with broken lines is from the difference shown in figure B and the width largely reflects statistical variations plus the radiation damage artifacts. The solid-line histogram of figure C reveals that optical transmission increased for many pixels but that some pixels experienced a difference in transmission. This is characteristic of what we have seen before. This contamination seems to produce a "lensing" of a size of a small number of pixels so that in some cases optical light is concentrated into one or a few pixels. This effect is especially evident in the arc structure. I know of no way to prove that what we see here is contamination related rather than some other change in the CCD induced by heating. However, based upon previous experience where x-ray absorbing blotches were removed by bakeout I think that it is safe to conclude that we are dealing with contamination here -- even 4 years after launch! We should continue to bake out the CCD at least every 6 months. Although this was only a 24 hour bakeout I suggest that bakeouts normally run for 2 days.