Focus the telescope using the image of a standard star in the Idle
Display. Make sure that the display is not saturated using the
CASPIR/IZMAX=... command, and use the F19 key in that display
to measure the FWHM of the stellar image. 
due the pixel sampling. Increments of 5
focus numbers usually produce satisfactory results. Smaller
increments are required in very good seeing.
Alternatively, you can use any of the three focussing masks installed
in the dewar: 1) When using the fast camera, use the focussing mask in
position 5 of the Utility Wheel (see Table 18).
This places two small holes at the pupil position. A prism over one
hole displaces its image by 
10 
on the array.
The result is two separated images of the star which move in the
direction perpendicular to their separation (i.e., vertically on the
Idle Display) as the telescope focus is adjusted. Correct focus
corresponds to the minimum separation of these images (i.e., the two
images lie in the same image row). 2) When using the fast camera, it
is also possible to use the focussing mask in position 4 of the
Utility Wheel (see Table 18). This is identical
to the position 5 mask, but does not use a displacing prism. Correct
focus is obtained by measuring the separation of the two images for at
least two different focus values, and calculating the focus value
corresponding to zero separation of the images. This mask is
primarily used to calibrate the position 5 mask. 3) The Utility Wheel
masks cannot be used with the slow camera which is also located in the
Utility Wheel. To focus the slow camera, use the focus mask in
position 16 of the Upper Filter Wheel (see Table
17). This is a two hole mask without
displacing prism, like the second mask just described.
CASPIR has been designed so that usually there should be no need to adjust the focus for different filters. However, filters in the lower filter wheel operate in a converging beam when using the slow camera, so some refocussing may be needed. Any focus offset will be small.
The telescope focus is known to change with internal air temperature. Figure 2 shows the measured variation in manual/automatic focus numbers with inside air temperature. The fitted line is
Small deviations from the fitted line may arise because of small
mounting differencesfor the dewar, but relative focus changes can be
accurately tracked using the fitted slope of 3.24 focus numbers per
C for temperatures above at least 
C.
Figure 2: Temperature variation of telescope focus. The solid symbols
are current measurements made since the Tip-Tilt system was
installed. The open symbols are pre-Tip-Tilt measurements and have
been offset by a constant in focus value to bring them into
approximate agreement with the post-Tip-Tilt values. The solid line
is a fit to the post-Tip-Tilt points only. The change in slope
below C may not be real.
In principle, the temperature dependence of the telescope focus can be removed by operating the focus control in compensated mode. This is achieved by typing the following command into the telescope console terminal:
CONFIGURE/FOCUS_CONTROL=COMPENSATED
or including its equivalent in your telescope startup procedure. In practice, 2.3 m telescope focus mechanism tends to lock-up when operated in this way, so its use is not recommended.