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The CCD is focussed by making a multiple exposure, stepping
both the focus and the telescope between exposures and only
reading out the chip at the end of the operation. The multiple
images are examined to establish the best focus (accurate to +/
0.05 mm or so). The exposure is controlled by an automatic
focus procedure on the instrumentation computer. Make sure
that there are no very bright stars in the field of the chip
as these saturate and so are no good for focusing: the plan
is to make exposures of > 5 seconds in order to smooth out
seeing fluctuations. To start the procedure type:
Adam:> FOCUS
and follow the prompts. The parameters are the number of
exposures, the exposure time in seconds and the direction
of motion of the image on the chip (N,S,E or W). The focus
is stepped by +/0.1 mm between exposures, depending on
the sign of the first parameter and starting at
Current focus +/ 0.1(n
1) mm,
so that the current focus is in the middle of the range.
The telescope is offset (20 arcsec is the default) between
shots: make sure that there is enough room by looking at a
GLANCE run or on the finder display before you select the
direction. Check the exposure time by using PHOTOM on the
result of a glance. At the beginning of the night, try seven
exposures (+/0.1 mm around the nominal value). (The default
increment in focus is 0.1 mm but this can be changed useing the FOCINC command
(see II.3.11).An approximate value for focus at a given temperature can be
obtained from Figure 8. Use the selector switch on top of the desk to
determine the current inside dome temperature. The approximate focus can be read
off from the graph. It is intended to introduce automatic
focus compensation for temperature in the near future. Fine-tuning
during the night can be done with 5 exposures: differences in
focus of 0.1 mm are easily distinguishable under good
conditions. The telescope and focus will be returned to
their starting position at the end of the run.
A double step between the first and second image
establishes which image corresponds to which focus position.
It will normally be obvious which images are closest to focus
and there should be measured using the PROFILE option of
PHOTOM and the seeing noted from the focused image.