In an XTERM or CMDTOOL window, the following command is entered to do a focus loop: ``focus 1 1 7 5''. The first two numbers are the x and y binning, respectively (usually always ``1'' for both), the third number is the number of exposures to take, and the last number is the exposure time for each exposure (in seconds).
You will manually vary the telescope focus between exposures using the paddle (and then hit any key to take the next exposure). It is suggested to use 0.2 mm steps to get a rough focus, 0.1 mm steps in good seeing, and 0.05 mm steps only in the very best seeing. It is also recommended that you always increase the focus value between steps in order to compensate for backlash. Part of the CCD is read out between exposures.
The spacing between the first and second exposures is twice as big as between the remainder; this is opposite to the convention used by the VAX CCD focus routines for the standard Palomar system.
Approximate focus for COSMIC in the reimaged mode is 29.0 mm for the r filter. Approximate focus for COSMIC in the direct mode is 28.1 mm for the r filter. Other filters (and filterless) foci relevant to the r filter are given here for the standard filters. The CCD13 focus has been given as ~20.4 mm, and is not parfocal with the guider camera (i.e. the guider will be out of focus if you use CCD13 in the direct mode).