It may be necessary to change filters or swap filter wheels. This requires removing one or both of the filter wheels from the telescope. The filter wheels are accessible when the telescope is parked at zenith. Because the filter wheels are heavy, it may be necessary to secure the telescope from becoming unbalanced when filter wheels are removed from the telescope (see the Maintenance Manual). Removing, and especially replacing, filter wheels is a difficult process and is best done by two people, especially the first time it is done. The weight (about 9kg) and awkwardness in handling the filter wheels means and extra pair of hands and eyes is quite helpful.
The filter wheels are attached to a sliding drawer for relatively easy access. The drawer is moved out from the guider box by handles located on one of the side panels. Brief instructions for using this access port are printed on the outside of the panel. Maintenance Manuals should be consulted for other details. First of all, a number of cables must be unplugged from the drawer panel. Next, a series of black hand knobs must be loosened. When this is done, the drawer can be pulled out using the handles on the front of the panel. The filter wheels slide outwards from the guider box and, once fully extended, can be rotated into an approximately vertical position if held in place. When the filter wheels are tilted into the vertical postion, take extra precautions to see that the rotating parts clear various electrical connections on the inside of the drawer since the clearance is small. Some thought should be given to finding a way to secure the filter wheels when they are in the vertical position.
The filter wheels are bolted onto a central spindle with hex bolts. Filter wheel 2 must be removed before filter wheel 1 can be accessed. The wheels are heavy, so precautions are warranted. The wheels are best handled by inserting fingers and thumbs into the series of small holes that ring the center of the wheels.
If filters are to be removed from the wheels, it is recommended that a place be prepared on a table (e.g., in the control room) prior to removing the wheel from the telescope. There should be large sheets of lens paper available in the control room which can be put underneath the filter wheel as it lies on the table.