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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.
