Subject: HRI/MRI IP TV4 Flash report Date: Mon, 03 Mar 2003 03:39:41 -0500 From: Dennis Wellnitz Hi all, Today (Sunday, March 2) we met at 1 pm MST to plan the day's activities. Because our instruments are so sensitive, we must do most of the radiometric calibrations at night with the lights off to avoid contamination by ambient light. Recognizing that we would be working late into the night, we had decided not to start too early. We worked first, in the afternoon, with the HRI IR spectrometer through the ZnSe window to look further at the effects of the baffle and blocking filter modifications. Since yesterday the IR-FPA had cooled further and had reached a temperature of about 100 K, so the dark current was lower than yesterday and the images looked considerably better than yesterday. Having assured ourselves that we had zeroed the Instrument Platform translation stage within the chamber against the stop and therefore could reproduce the alignments measured before we had closed up the thermal vac chamber, we placed the illumination from the SIRTF black body first into the west side of the HRI aperture (between the secondary mount obscuration and the west edge of the primary mirror) and then into the east side of the HRI aperture (between the secondary mount obscuration and the east side of the HRI aperture). Using the west side illumination we got quite reasonable results, but using the east side illumination gave us some difficulties, because the west side of the HRI telescope had been displaced far enough that it no longer saw the cold interior shroud of the chamber (about 77 K), but instead saw the large fused silica window of the chamber, which was warm enough to saturate the detector at the longer wavelengths (around 4 microns). The fused silica window is nearly opaque longwards of about 2.6 microns, and therefore emits black-body radiation corresponding to its temperature, which is only slightly below room temperature. We found that we needed to place the back of the relatively cold 5-inch flat mirror mount (also in the chamber and therefore quite cold, and on an independent translation stage) ahead of the west side of the HRI telescope to block its view of the large fused silica window. Then we were able to obtain acceptable data on the SIRTF black body. Once again our quick look at the data indicated no sign of the ghosting that the baffle was designed to eliminate. Also, the filter reduces the illumination strongly at long wavelengths and minimally at shorter wavelengths, exactly as designed. We do see some edge effects from the filter, which vary somewhat with wavelength and illumination. Because it was getting to be within an hour of sunset, we then began setting up for the radiometric calibrations of the MRI visible detector. We began with bias images in all modes and then continued with ambient light images to check the levels of ambient light. We checked ambient light in all modes for filter 1 (one of the clear filters), and then all filters for mode 1. Once we were convinced that ambient light was at an acceptable level, we began taking the radiometric calibration data for mode 1. This was followed by data for linearity and flat fields and saturation at many exposure times for filter 1. After taking this data set, we discovered that there was some vignetting (20% at the lower edge of the image), indicating that the MRI was not adequately aligned with the integrating sphere. After correcting this problem we continued taking data, repeating enough measurements to be able to calibrate and use those already taken, instead of having to repeat all of them. We were careful to take some data at very low light levels to investigate non-linearity at low light levels. This exhaustive characterization was done in mode 1, and then we took flats in all modes at moderate light levels. We then moved on to a similar characterization of filter 6 (the other clear filter) in mode 1, and then flats in all modes. We took some additional data in mode 9 for short exposure times because we had noted some unusual results in the analysis of mode 9 data taken on Friday. We ended data taking for the day shortly before midnight, and halted work on analysis tasks a little after midnight. Monday morning we plan to look further at the focus of the MRI and HRI, probably by taking through-focus pinholes for both, and also to set up the large integrating sphere for radiometric calibration of the HRI. Monday afternoon we plan to continue work with the radiometric calibration of the narrow-band MRI filters because to do these we need to set the integrating spheres so bright that we should be able to work even in the daytime. In the evening we hope to finish up MRI and then to start work on the radiometric calibration of HRI. Please let me know if you have any questions or comments. Also please let me know if you would prefer not to be included on this Flash report distribution list, or if there is someone else who should be included on this distribution list. Dennis