The WFC has no internal focus mechanism: you focus up using the telescope's focus drive. The relative focus of the science array and the autoguider is fixed. There are adjustments for tilt and rotation on the cryostat but, since the WFC can only be used at INT prime-focus, these should never need to be adjusted.
The telescope focus is changed using the focus command at the TO> or SYS> prompt (i.e. via the system computer) or at the USER> prompt (i.e. direct to the TCS). The focus cannot be driven from the WFC GUI.
The TCS changes the focus to compensate for temperature changes and also applies focus offsets (when ordered to by the ICS) to compensate for filters of different thickness. The value displayed in the observer's Infodisp window is the nominal value before correction for temperature and filter thickness.
When the TCS is started, the focus is reset to the nominal value of 46.0mm, as determined at the commissioning of April 1998. This nominal value was determined on the assumption that the Harris R filter has zero offset and the other filter offsets are measured relative to R.
The observing-system command focusrun takes a multiple exposure with a single CCD readout. Between each exposure there is a specified step in focus and in position ofthe telescope. These frames can be analyzed manually using the IRAF command imexam or semi-automatically using starfocus. The image scale is 0.333 arcseconds per pixel.