Background

DEIMOS uses air pressure to drive certain stages (CCD shutter, grating clamps, slitmask insertion/removal system, front hatch). This document describes the consequences of a loss of air pressure during observing.

Purge System

To remove water from the DEIMOS air lines, DEIMOS includes a water trap at the point where the observatory air supply enters at the back of DEIMOS. This water trap is cleared once per day by a purge system that is activated by a hardware-store lamp timer. This system is set to fire off at around 8:30am daily and will cause a momentary loss of air pressure. If necessary to clear additional water, the purge system may fire off one or more additional times, following at approximately 30-minute intervals from the intial purge.

Although we hope that the purge cycle will never occur during nighttime observing, it is possible that it could happen if the time clock malfunctions or is accidentally reset. It is also possible that we could lose air pressure during observing due to the failure of some other components (e.g., air compresser fails).

Loss of Air Pressure

What happens if we lose air pressure then? The Tricorder panel will show that the barrel and cradle air pressure state is failed. To launch the tricorder panel, on DEIMOS instrument control GUI, click on the Tricorder label (bottom right on the main GUI) to activate the details button, and then click on the Details... button. (Need to implement yellow alarm box on the DEIMOS control GUI to alert users that the air pressure has failed.) Unforunately, if you had an exposure in progress, it may aleady be spoiled. Ideally, one would pause the exposure (which includes closing the shutter) just as soon as such a loss of air pressure was detected and wait for the air pressure to be restored. But since the shutter is operated by compressed air, you probably won't be able to command it closed when you detect that the air pressure has failed.

So at this point you have a shutter that is most likely in an indeterminate state (i.e., partly open or closed), a grating slider in an indeterminate state (i.e., grating clamps partially open or closed, so slider is no longer tightly clamped against the grating box), a slitmask in an indeterminate state (i.e., partially inserted but probably not properly registered in the form), and a hatch that is partly open or closed. In short, if you lose air pressure in the middle of exposure, your exposure is probably toast.

Remedial Action

What to do next? Since PAUSING the exposure really doesn't help any, it is probably best to ask the exposure to STOP AND READOUT, in the hope that maybe you can salvage something in the image. If, after reading out, you determine the exposure is garbage, you can always delete it.

After dealing with the exposure, check to see whether air pressure has been restored by clicking the Tricorder button on the main Dashboard GUI. This will pop up the Tricorder panel and the Air Pressure indicators for the cradle and barrel at the middle right should show you whether the air pressure remains off. If it has returned, suspect that the purge system fired. If it is not the normal time for the purge system to fire (between 8:30 and 9:30am), ask summit personnel to inspect the timer for accuracy. If air pressure remains off, it could be that the observatory air is off. Again, ask summit personnel to investigate.