Subject: ITS Room-Temperature Calibration Flash report Date: Fri, 17 Jan 2003 09:30:26 -0500 From: Dennis Wellnitz Hi all, We started the post-vibration ITS Room-Temperature Performance and Calibration tests on Tuesday, January 14, one day ahead of schedule. On that day we took vertical knife-edge measurements through focus to compare to the pre-vibration vertical knife-edge measurements to discover whether there had been any shift of focus due to vibration of the ITS. Analysis of that data indicated that the focus at the CCD had shifted by no more than 2 thousands of an inch, plus or minus about 2 thousands of an inch, that is, there was no discernable shift of focus. This was consistent with the determination of minimal boresight shift from before to after vibration. However, the location of the focus as determined from these measurements was approximately 15 thousands of an inch from the optimal position. The question then was, is this a sufficiently significant deviation from optimal focus to require a shift of the focus of the ITS before starting thermal vac, considering the risk to the instrument, schedule, and cost of making a focus change? Also, were we sufficiently sure of the direction of the deviation from optimal focus to move the CCD in the right direction if we were to shift the CCD? To investigate these questions further, we took additional data on Wednesday, January 15, including moving a pinhole through focus and adding a horizontal knife edge through focus. We also took images of the Air Force resolution target at best focus, at the central position and displaced to four other locations, to check for image distortion and resolution over larger areas of the array. The results of the analysis of the pinhole through focus and the horizontal knife edge through focus were unsettling. The horizontal knife edge showed a best focus point about 60-70 thousands of an inch away from the best vertical knife edge focus position, but the data were fairly noisy and did not cover a sufficient range beyond the best horizontal focus position to give a unique and defensible focus solution. The pinhole images through focus also showed astigmatism of the same order of magnitude as indicated by the vertical and horizontal knife edge tests considered together. The nominal focus solution, the mean of the vertical and horizontal line foci, was therefore displaced by about 30-35 thousands of an inch from the focus we had been using, which placed it about 15-20 thousands of an inch on the other side of the optimal focus position. The depth of field for maintaining an acceptable deviation from optimal focus is about 30 thousands of an inch, so this analysis moved the apparent focus from one edge of acceptable position of focus to slightly beyond the opposite edge of acceptable position of focus. Today, January 16, we continued the exploration of the ITS focus astigmatism, trying to determine whether it was coming from the ITS or from the test setup. But first, while we were waiting for the results of the analysis of the previous day's data, we took additional data on geometric distortion, moving a pinhole to 13 different locations and measuring each position with a theodolite. This also permitted a calibration of the displacements of the Air Force resolution target imaged the previous day. This accomplished, we took some additional horizontal knife edge data to permit a better solution to the horizontal line focus position. Then we took some additional pinhole data through focus, first with the ITS and then with the MRI. The MRI pinhole data showed very little if any astigmatism, which seemed to indicate that the astigmatism was internal to the ITS. We then took additional pinhole data on the ITS, first in the usual position and then with the ITS rotated about 45 degrees. The astigmatism did not rotate with the ITS, but rather stayed fixed with the test setup. This result seemed to be inconsistent with the results from the MRI. After discussing these results for some time with several people, we decided to rotate the ITS through the range of rotation permitted by the mount, in approximately 45-degree increments. Our working theory was that there is something moving in the ITS that changes due to gravity as the ITS is rotated. If this is confirmed it is an indication that something loosened or weakened during vibration, but only by a small amount. We have just finished taking this data, and on quick look it appears that nothing is changing: the astigmatism just rotates around, staying fixed with the test setup rather than moving or changing with respect to orientation of the ITS. We are calling it a day, and taking some time to think about what to do next to understand this astigmatism. Please let me know if you have any comments or questions. If you would prefer not to receive these Flash reports on the ITS Room-Temperature Calibration, please let me know. Dennis