The Cassegrain Auxiliary port is on the side of the A&G box, and is fed by the small feed flat. A small filter box is bolted to the Auxiliary port; this contains a standard filter wheel which has 6 positions for 50mm square filters. Because the cryostat and filter box must be removed in order to access the wheel, the maximum number of filters which can be used in any one night at this focus is 6. The filter wheel and shutter are driven remotely by the A&G box 4ms computer and the CCD controller respectively.
Because the optics before the filter wheel consist of three reflecting surfaces the Auxiliary port has the highest (efficiency collecting area) in the ultraviolet of all ING imaging systems, and is particularly recommended for deep U band imaging. As inserting the small feed flat takes only a few seconds it is also very useful when imaging observations form a small part of a mainly spectroscopic programme. It also gives the best spatial sampling of the available imaging options, and is recommended when resolution is of the highest priority.