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The telescope focus depends on the optical path. It is also a function of temperature. This is why the numbers given in Table 4 are only to be used as starting points. The focus will also change as the flexure on the telescope and instrument changes. Such flexure results in minor changes in the optical path. It is thus very useful to measure the response of the system focus to flexure. I have conducted two sets of tests to study this.
The first test was designed to see if the zenith focus is robust after large cyclic changes in telescope pointing. I began by focussing at the zenith. I then slewed the telescope one hour to the west and back to the zenith. I checked the focus, and found it unchanged. I repeated this test with slews in the other cardinal directions. I then repeated the test again in all four directions with slews of first two hours and then three hours. The result of this set of tests is that the system focus is robust to large slews and returns.
The second test was designed to see if there is significant, systematic variation in telescope focus as a function of azimuth and zenith angles. Once again, I began by focussing at the zenith. I then slewed the telescope to a star close to the zenith declination, but approximately an hour east of the meridian. I checked the focus on that star, and found it not significantly changed. I then slewed to stars two and three hours east of the meridian, and repeated the focus check. I then returned to the zenith, and repeated this procedure with slews along the meridian, stepping south in approximatley 15 degree increments down to a declination of about -4 degrees. I repeated this procedure, slewing the telescope to the west and to the north. In no case was there any substantial focus change. There were minor changes of 1 or 2 focus units, but there was no evidence of any systematic focus variation with telescope pointing.
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