The breadboard on which WHIRCAM sits has a series of permanent mounts that should ensure WHIRCAM is properly aligned with the optical axis and at a reasonable focus, and your support astronomer will check to make sure that this is the case in the evening twilight of the first WHIRCAM night (see §9 in case alignment is lost). Acquire a 5-8 mag star, and ask your TO to focus it on WHIRCAM (not the autoguider!). Once focussed, the temperature monitors, coupled to the WHT focus drive, should ensure that focus is maintained throughout the night. However, if the seeing does seem to deteriorate, it is always good procedure to re-check the focus.
The internal lenses are made from combinations of LiF and BaF elements, which apart from optimising IR transmission, also ensure very good image quality and only a small shift in focus position over the range 1 - 5 m. To compensate for this wavelength-dependent focus shift, the detector is attached to a movable mount, whose position is automatically controlled by the Vaxstation software, using a lookup table of internal focus positions for each filter. The offset positions, in mm, are J = -1.4, H = -1.1, K = -0.9, and L = -1.0.