The dome shutter is opened and closed by the pushbuttons just below the light switches on the circuit breaker panel. These buttons do not need to be held down. If you desire to stop the shutter before it reaches the end of its travel, use the red button. Normally the shutter limit switches stop the motion when fully open or closed. The shutter sometimes binds up, especially when it's very cold and there is ice on the shutter tracks, so it is suggested you stay near the red stop button until the shutter has completed its travel. When all the way open or closed it will (should!) stop by itself. If it has snowed recently, do not attempt to open the shutter unless a telescope technician (ext. 8-0652) tells you it is okay. Before the shutter can be opened safely, snow and ice must be removed from the shutter tracks to prevent it from piling up under the wheels and damaging the mechanism. Do not open if there is loose snow on top of the dome which might blow in on the mirror while you work. If in any doubt, check with a telescope technician or support astronomer.

If the shutter buttons should fail to work, either to open or close, the cause will probably be a rocker limit switch/interlock which has failed to trip. These are located at the bottom of the slit (Fig. 5). The usual failure leaves the center switch level when the dome is open so the shutter does not close. Simply lower the windscreen so you can reach over it and tip the center rocker switch to your left (as you face out the slit), then you should be able to close the dome in the normal manner. If you note these failures in a trouble report to the 3-m telescope operator (ext. 8-0652), the telescope and dome maintenance people will know an adjustment is required.

One you have opened the dome, keep a sharp eye on the weather, which on Mt. Hamilton may go from completely clear to foggy in very little time indeed! Once you have opened the dome, responsibility for protecting the telescope lies wholly with you. For photoelectric work there is usually less reason to worry about stabilizing the mirror temperature than there is for long exposure photographic work, simply because convenient opportunities for refocusing arise more often. Therefore the risk of leaving the mirror exposed and perhaps unwatched for the late afternoon hours is not usually worth it. However, if for some reason you feel you must have the mirror exposed for a few hours before observing, stay alert and in a place where you can observe any changes in the weather; i.e., do not go back to your room and fall asleep! The safest policy is simply not to leave the dome with the shutter open.

The proper stow position for the dome is with the slit pointing roughly southeast, the east dome drive motor within the area marked off with white lines on the floor at the top of the mezzanine stairs, and with the black tape on the dome near the floor (between the dome drive motor and the circuit breaker panel) lined up with the black tape on the east mezzanine floor (Fig. 6).

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6