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Focusing with WFCAM

Notes for focusing with WFCAM.

  • More often than not, the focus offset changes rapidly in the first two hours and then alters direction, ending the night back roughly at the point where it started.
  • More often than not, the total change is of order 0.1 in size and is goes from positive to negative, then slowly back to positive.
  • The above pattern is not consistent; some nights it is even inverted.
  • From night to night, the pattern shifts in the time axis and you can't assume you're on the same part of the curve at any given time in the night.
  • Magnitudes: 0.1 focus offset corresponds to 0.3-0.4 arcseconds in seeing degradation. Can't ignore any focus change of 0.05 or larger. 0.02 is typical scatter between focus measurements.
  • If a large focus change is indicated by a focus run (+/- 0.05 or greater), the resulting focus is probably not optimal and the focus should be rerun at the new recommended value.
  • On a typical night, focus as follows: every 30 minutes during the first two hours. every hour for the rest of the first half. every two hours for the rest of the night. Additionally focus at the point where the curve appears to have bottomed out after the rapid change at the start. In general, should not be necessary to focus more frequently than this, and bear in mind the typical scatter mentioned in (v) above.
  • Don't accept any focus which relies on less than 15 stars in any given chip.

Contact: Watson P. Varricatt. Updated: Mon Sep 22 10:38:45 HST 2008

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