Prime Focus Adjustment when using Filters

  The filters in front of the CCD have different optical thicknesses and the software compensates for these by refocusing the telescope when a filter is changed. The thickness values are communicated to the instrumentation computer by the ADAM command WHEEL (see section 3.2.4 of the INT Prime Focus Manual). WHEEL is run at the beginning of each Prime Focus run or whenever a filter wheel is changed.

Therefore, after a focus exposure (previous section) and before changing filter, type:

SETFOCUS value

where value is your best estimate for the focus in mm as determined by FOCUS. This command enables the automatic focal adjustment, and the specified focus position value will now be use as a reference position by the software. The focus will from now on be driven automatically to the correct position, and adjusted as you change filters in future, and as the temperature changes.

The automatic focus adjustment can be disabled by simply typing:

SETFOCUS DISABLE