Date: Mon, 10 Apr 1995 17:35:49 -0400 (EDT) From: MURCHIE@s1pvx7.jhuapl.edu To: near-s-msinis@aplmail.jhuapl.edu Message-Id: <950410173549.37800226@S1PVX7.JHUAPL.EDU> Subject: more updates To: MSI-NIS team From: Scott Murchie re: more updates (1) I have updated the files of measurements of the calibration lamps using calibrated photodiodes, imcorporating measurements through all the neutral density filters. (2) There have been more questions about hte nature and cleaning of the lubricant contaminant on the CCD. The mass spectrum most closely matches that expected for the solvent of an oil-based paint. There is no component of the detector array or any associated hardware that should produce such a substance. The likely candidates (so far) are a C-clamp on the inside of the FPU characterization chamber and the vacuum pump. Results appear negative so far for both. When this contamination was discovered, all surfaces exposed in the chamber were cleaned following manufacturer's instructions with pure ethanol, and then the whole unit was baked out at 80C for 30 hrs. This procedure should insure that there are no remnants that volatilize again later, since the CCD was the lowest-temperature surface inside the chamber at the time of measurements, and it would have acted as a cold finger. (3) We think we know the source of the "unfocused" images through the 450 nm filter. You will recall from my last update that the other color filters have spot sizes of about 1 pixel, whereas for the 450-nm filter it is closer to 2-3 pixels. Recall that each filter acts to correct a chromatic aberration of the camera, using the thickness of the filter to bring light into focus on the CCD for the center wavelength of the filter. The aberration is more severe over broader wavelength ranges, and at shorter wavelengths. (This is the reason why the clear, broad filter was not going to be as well focused.) Our theory is that the larger blur circle for the 450 nm filter is the result of the broadness of this filter (100 nm) and its short wavelength. Terry Harris's ray trace program in fact predicted (which I didn't realize) that this filter would not have as good a focus as the very narrow ones. For a narrower 450-nm filter (50 nm), the blur circle is predicted to be about 1 pixel in size. Joe has been made aware of these facts, and asks for the following course of action. First, Terry is now analyzing the results of the focus test to demonstrate that his predictions about blur circle sizes are corroborated by the test results for the other filters. This corroboration would be hard evidence that we have identified the source of the poorer focus in the violet filter. Second, he will predict what the improvement would be in the narrower 450 nm filter (50 nm wide). Based on these data to be supplied to Joe, the team can make an informed decision about what to do about this filter. Third, pending that decision, the engineers are going to switch to the narrower 450 nm filter and do some quick focus tests to acquire empirical evidence on the effect on focus of the filter width. (4) I've discussed the issue of the scratch on the CCD at some length with Joe. In a nutshell, it is about 100 pixels long in the lower left of the field. It probably formed during the cleaning of the CCD prior to detector-level calibration measurements. It shows up relatively well in these data because the parallel rays cause shadowing. However it is quite small, below the level of what can be seen with the naked eye. Ed Hawkins did some calculations and believes that in converging rays its effects will be spread over about 30 pixels, such that its effect on brightness levels will be attenuated by 30 times. So if in the parallel rays it appears, say, 30% darker than surroundings, mounted in the camera its effect will be 1%. Right now we don't have really enough data to determine to what extent and how it will affect images. Joe is requesting of Rob Gold a little time back in the OCF to image a variety of targets to acquire more information. I have tracked down 1 image, of graph paper, where there is some illuminated target with a regular pattern in the area of the scratch. So far, nothing I can do in the way of edge enhancements, etc., brings out anything at all.