The MOSCA web page

Focusing MOSCA:

To focus the telescope with MOSCA is done easiest if you select a Focus Fields or (a Landolt standard star field ) close to the meridian, use scripts focusexpose [focus-start-value] [focus step-size] [tint] and mosca.focus-auto [focus step-size] [file name].
The default telescope focus without filter is 22153, and you should start the sequence below this value and end above.

!!!NOTE Do not auto-guide!!!!
!!!Make sure the instrument field-rotation has the default value (-90)!!!!

When you focus without a filter, start with e.g. "focusexpose 21900 100 12"
however "focusexpose 22000 70 12" should work fine.
The mosca.focus-auto-script computes the best focus value from a focus sequence image taken with the command 'focusexpose'
e.g. "mosca.focus-auto 100" for the last obtained image or "mosca.focus-auto 100 MOsd19/MOsd190109.fits"

Set the telescope focus on the TCS by (e.g.)"foc-pos 22130".

Unknown focus offset:

First, focus the telescope without filter.
Then try e.g. "focusexpose 21800 150 12" and
take a second sequence with 50-75 focus step-size centered with the "best focus" from the first guess. Calculate the focus offset:
focus-offset=focus(with filter) - focus(no filter)

The focusexpose-script:

For the "focusexpose"-script you have to give the initial TCS focus value and the focus step size. If the exposure time is not provided, the script will use the MOSCA default integration time (tint).

The mosca.focus-auto-script:

Computes the best focus value from a focus sequence image taken with the command 'focusexpose'. If no image name is given, the last obtained image will be used. The TCS focus step size must be given on the command line.

Additional info

Note that there are three focus scripts for MOSCA:

focus3expose [focus-start-value] [focus step-size] [tint]
focusexpose [focus-start-value] [focus step-size] [tint]
focus7expose [focus-start-value] [focus step-size] [tint]
which make 3,5 and 7 images, respectively, but the mosca.focus-auto-script works only for focusexpose (a.k.a. focus5expose).

Alternatively you can analyse the image in a similar way as the StanCam focus sequence image (point "4")
(see the test image " /data/mosca/EXfocus.fits")