NEVER hold things over the top of the telescope with the mirror cover open. It may save 60 seconds not to close the cover when changing diaphragms, exit slots or whatever but the risk is not worth it.

The mirror cover is a plywood slab about 4' above the mirror level. It is operated by 2 buttons (one to open, the other to close) in the center of a grey junction box just to the right of the hour angle dials (Fig. 21). The mirror cover moves only when the buttons are depressed, and the entire motion takes perhaps 5 seconds. If you listen closely you will hear the mirror cover motor. It is advisable to visually verify that the cover has opened/closed completely. If the lights are on you can watch the movement of the mirror cover counterweight arm which is outside the tube on the NE corner at the level of the mirror cover.

Figure 21