### FOCUS

FOCUS measurements are done to optimize the position of the subreflector (secondary) along the telescope axis by maximizing the intensity of the radiation focussed into the receiver(s). It is best done on a strong point source, e.g., on a planet if or when its angular diameter is less than the beam width at the frequency to be observed. It is strongly recommended to do a POINTING on the same source before a FOCUS.

FOCUS is normally used with BEAM SWITCHING or WOBBLER SWITCHING. (With the bolometer FOCUS is done with WOBBLER SWITCHING).

A calibration is not needed for FOCUS, anyhow it will probably already have been done before the POINTING before the FOCUS!

After a focus the data processing software displays the results and you can enter a correction for the observed focus offset with the command

SET FOCUS focusCorrection

Note that this is the total correction, i.e., the previous correction plus the additional offset found with the FOCUS observation.

The optiomal focus correction for different receiver bands may be slightly different, by a few times 0.1 mm, and the observer can decide to optimize for one particular band or use a compromise value.

After you enter a focus setup manually, we recommend to “save” it, optionally to a named file:

SAVE FOCUS [/FILE focus-1]

It can then at any later time be reloaded with @ focus[-1], see HELP SAVE.