The filter cut-out is designed to take 2.0 inch square filters (in a 51 mm square hole); a 47 mm clear aperture is provided.
In addition, filters up to 56 mm diameter can be accommodated, although in this case light can ``leak" around four corners unless a mask is fitted.
Any smaller filters should be mounted within a 2.0 inch square frame which is then readily fitted to the wheel.
The maximum thickness of filter allowed is 12 mm; filters of thickness 3 mm will require packing material to stop them moving about within the 3 mm deep cut-out hole.
Each filter is held in position by three adjustable clamps, each of which consists of three parts: a flat-headed screw, which holds a threaded brass post to the filter wheel under pressure from a coil spring, and the main clamp body which screws on to the threaded brass post.
When mounting a filter in the filter wheel, there should be no need to touch the central screw on each clamp. To mount the filter, turn the clamps anticlockwise on the threaded brass posts (which winds the clamps away from the filter wheel) until the clamps will just clear the filter mounted in the recess. The threaded brass posts should not themselves turn. When the filter is in position, wind the clamps clockwise until they just clamp the filter under light pressure from the springs.
Problems may be experienced with extra thick (>10mm thick) filters. In this case, do not slacken the central screw, but consult one of the La Palma-based support astronomers.
Also, the filter wheel for the Prime Focus should be balanced as well as possible. I.e., if a heavier filter is used in one position, it should be matched by a heavy one in the opposite position if possible. This will help to avoid reluctance to move from one filter position to another, or even filter movement during exposures.