The Auxiliary focus is a facility provided in the Cassegrain A&G box
to allow a direct image to be taken on-axis at the Cassegrain focus, at
the expense of an extra (45 
) reflection. The original idea
of the system was that it could be used for short direct images of
objects which were part of a mainly spectroscopic programme, but it
has emerged as the major instrument for a number of projects for which
the high throughput in the ultraviolet provided by an all-reflecting
optical system, or the 0.1 arcsec pixel size, are a particular
advantage. The image quality at Cassegrain is illustrated by
Figure 4 which shows spot diagrams for this focus.
Figure 4: Spot
diagrams for the WHT Cassegrain Focus