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Staying Focused at the 2.1-m (1Dec93)
(from KPNO, NOAO Newsletter No. 36, 1 December 1993)

Users of the 2.1-m f/7.5 CCD imager have often reported focus drifts
and/or jumps during the night. Is it the focus readout that drifts
while the secondary stays put? Does the secondary drift while the
readout correctly reflects its position?

During the course of a five-night run I was blessed with subarcsec
seeing (0.8 arcsec) on each and every single night and was therefore
able to investigate the focus behavior in some detail. These findings
are consistent with investigations made by Fred Forbes with his
wave-front sensor camera. I find three effects:

1) There is a focus shift with temperature that is 40 microns (40
"units" on the display) per degree F, at least over the range of 52-72
F. (The "depth-of-field" of the focus is about 25 microns.) This is in
the sense that as the temperature drops during the night you want to
increase the focus value.

2) There is a focus shift with zenith distance that appears to be
independent of azimuth; it amounts to about 130 microns per unit
airmass. This is in the sense that as you go to higher airmass you want
to decrease the focus value.

3) There is also a "wild card" factor. If you run a series of
consecutive focus sequences at zenith you will usually get the same
value. But occasionally (10-20% of the time) you will get
somethingsubstantially different: perhaps 200 to 300 microns off. Thus,
something acts as if it is sticking between the focus encoder and the
secondary. (If you then repeat the focus sequence you will get the
original value.) I believe this is the origin of the "focus jump"
reports, for if one were to run a focus frame, set the focus to the
seemingly correct value, and then take an exposure, there is a finite
probability that the images will be substantially out of focus on that
frame. At this point the poor observer will refocus and either find the
same focus value as before (if the focus motion "stuck" on the program
frame) or a substantially different focus (if the focus "stuck" on the
original focus frame). Reports will then drift downtown that the focus
"jumped."

Users are warned to be on the alert for these three effects.  At the
moment the only way of reading the temperature is checking the analog
device ("thermometer") directly outside the control room door.

The following approximation will serve as a rough guide to the focus
through the BVR filters:

24200 - 41.5 * T(dF) - 130 * (airmass - 1.0)

The focus through the "U" filter differs from the above by +150 microns.

Phil Massey

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