Subject: APO's focus

From: Krzysztof Stanek

Submitted: Mon, 26 Feb 1996 16:52:55 -0500 (EST)

Message number: 23 (previous: 22, next: 24 up: Index)

Some of the things I write below are quite trivial, but I don't think
they are otherwise available:

1) Focus is very sensitive to the temperature changes and when the
   temperature drops, the focus goes up (so when it it warm, like
   10 C or so, focus would be -300 or so, and when the temperature
   is close to 0 C, focus would be more like 0 - please treat this
   numbers very suspiciously and if you have time, always take your
   own focus sequence).

2) One thing I was experimenting with, during my recent observing run
   using guided exposures on DSC, was to focus the DSC first,
   than to focus the guider (or rather asking Eddie to do it)
   and then monitor the guider output and the temperature value
   to see if the focus holds. So, when the focus would change,
   the HWHM of the guiding star would increase and the image
   on the guider would no longer be round (resulting in different
   values of HWHM for x and y axis of the guiding star image).
   This allows one to keep the telescope in focus when the temperature
   is changing without taking a lot of focus sequences, but of course
   requires doing a guided exposures (guiding works great, by the way).


I hope this will be useful,


Kris Stanek
Princeton University Observatory
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