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Grant M. Hill December 23, 1999 Introduction It is assumed the reader is already familiar with the burst/anti-burst alignment technique. Unless explicitly stated, all angular measurements are in sky coordinates. During alignment of the HET mirrors it is necessary to align half of the mirrors at a time since the tracker can obscure many of them. The tracker is moved far enough to one side that all the mirrors on one side of the array are not blocked. These mirrors are aligned, the tracker is slewed the same distance to the other side of the array and the remaining mirrors are aligned. During the slew, considerable telescope shake is visible via the Pulnix tv camera that views the CCAS faceplate. An obvious concern is that this shake may be degrading the alignment achieved for the first half of the mirror array. We have examined this point in the following way.
On six separate occasions, half of the mirrors were aligned in the normal fashion. Three of these were left side stacks and three were right side stacks. After stacking, 3 images were obtained of the resulting stack and EE(50) and EE(80) were measured. The tracker was slewed across and back and 3 more images were obtained and EE(50) and EE(80) measured. This constituted part one of the test.
The same half of the mirror was then restacked again, 3 images obtained,
EE(50) and EE(80) measured. After waiting the same amount of time
as elapsed during part one (described above), 3 more images were obtained
and EE(50) and EE(80) measured again. In other words instead of
slewing, the mirrors were allowed to just sit for the same amount of
time. Results and Conclusion The average change in EE(50) and EE(80) during part one (after slewing) was - 0.018 ± 0.019 and - 0.033 ± 0.022 arcseconds respectively. The quoted errors are sigma/sqrt(6). The average change in EE(50) and EE(80) during part two (after just sitting) was 0.032 ± 0.037 and 0.040 ± 0.054 arcseconds respectively. Two of the tests were conducted with the tracker at Y = 0 and four were
conducted with the tracker at Y = 1500. In one test, the mean truss temperature
was constant throughout the entire test to within 0.1 degree F. In the other
5 tests, temperature data is incomplete but variations were probably less
than 0.2 or 0.3 degrees. No significant difference was noted between any
of these sub-sets of the data. It seems safe to conclude that at the level
we are currently measuring, the shake we see when slewing is not a problem
with respect to stack quality.
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