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.
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
Loss of Air Pressure
What happens if we lose air pressure then? The Dashboard GUI
will pop up a yellow alarm box alerting you that the air
pressure 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.
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.