CAT User's Guide

Opening and Closing the Telescope

Opening and closing the telescope is done from the coude slit room, located in the south part of the 120-inch's basement. Before opening any part of the telescope, be sure to check that weather conditions are within the prescribed telescope limits.

The order in which the various parts of the telescope are opened and closed is important. Please follow the directions carefully.

Siderostat Housing and Mirror Cover Control Panels
(These panels are located in slit room.
Note that dimmer pots must be turned up to see indicator lights)

Opening the Telescope

1. Turn on the control power from the toggle switch on the right-hand side of the panel.

2. Begin retracting the roof of the siderostat by pushing the "open" button on the right-hand side of the panel.

3. While the roof is retracting, hold the upper shutter toggle switch up until the green "open" indicator lights.

While waiting for the roof to retract, open the primary mirror cover by pushing and holding the "open" button on the small red box (not illustrated) labeled "CAT MIRROR COVER", located on the ledge above and to the right of the guide camera. Continue holding the button until the "open" indicator lights. You will also hear a pronounced "clunk" when the primary mirror cover is fully open. Opening the primary cover may require a minute or more.

At this time you may also open the portal cover by pulling down on and turning the red "T" handle, hanging from the ceiling above the slit. You may also swing the pick-off mirror (mirror #5) into place by loosening the thumbscrew on the collar around the post, above and to the left of the slit, from which the mirror is suspended, and gently swinging the mirror into place above the slit until it comes to rest on its stop. Carefully open the hinged cover on the pick-off mirror.

By now, the siderostat shed roof should be fully retracted. This is indicated by two green lights to the right of the "open" button on the control panel. Check that all green lights are illuminated across the top of the right-hand side of the control panel and proceed to the step #4.

4. When the roof is fully retracted, open the siderostat mirror cover by pushing the "open" button on the left-hand side of the panel.

The CAT is now fully open.

Closing the Telescope

To close the telescope, reverse the procedure for opening. Always close the siderostat mirror cover before closing the shed roof over it. Always check that the tracking has been turned off and that the siderostat has been stowed at hour angle 0 and declination +30, before closing the shed roof. See Driving the Telescope for instructions on how to park the siderostat.


At the end of each night, the CAT observer should call the 120-inch control room (5952 or 5951) to report any problems or to report that no problems occured. The CAT control room is not routinely checked for notes left by observers. Problems or special needs wanting the attention of the daytime crew must be communicated to the 120-inch operator.