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
- 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
- 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