From: Mike Malin (malin@msss.com)
Subject: The Scratch, the Spots, and Other Attributes of the New MSI Detector
Date: 10 April 1995 As some of you may be aware, there has been additional problems with the new CCD for the MSI. Sometime during the piecepart testing, a lubricant of some type became smeared on the optical window of the CCD. Mark Robinson and Scott Murchie have been taking the lead in exchanging comments on the topic, but with the rapid movement APL is taking towards completing these tests, it was bumped up to Veverka, who called me to find out what's going on. I've been fighting a battle with another project, but took a quick look at the problem, and that's shown in this page.
Figure 1 shows four "flat" images taken through the new CCD. They were taken at room temperature (upper left), -30 deg C (upper right), -35 deg C (lower left), and -20 deg C (lower right) on successive days. The arrows in the room temperature image point out the scratch and other blemishes (dirt, pits in the cover glass?) on the detector prior to testing. Note that these remain visible in the other images.
My first concern was that testing proceeded with a scratched cover glass and with dirt remaining on the CCD cover glass. Cleaning CCDs is not hard and I would have thought that some cleaning effort was expended prior to testing. I think Scott believes this detector was cleaned prior to testing, but these spots seem to indicate that the cleaning wasn't thorough.
I am also surprised that the detector cover glass was scratched. This is hard to do. However, once it was scratched, I don't think we should have proceeded with testing until it was decided whether or not we could live with the scratch. The most recent memo from Scott indicates that the scratch is easily visible owing to the illumination. However, I think that the orientation of the crack is almost radial to the illumination as seen in the images, and so we don't really know what it'll look like in other illumination.
The APL engineers are suggesting that the effect of the scratch on the use of the detector is going to be quite small (see #4 in Scott's message. However, it isn't clear we want to accept this without additional testing and/or proof. Unfortunately, there isn't much they can do with the bare focal plane, and once its mated with the optics they're going to be less likely to want to change the detector again.
My purpose in posting this stuff is to insure we're all looking at the same things and calling them the same thing.
I have recommended to Joe that we ask APL to replace this detector,
and to send us images taken through the next detector before they do
any more testing. It shouldn't take more than a few hours for us to
tell if they're next detector is okay.