The cooling power of the PTR is insufficient to keep the instrument cold, so the cold table tank has to be refilled at regular intervals.
In storage orientation, with the PTR at the lower side of the vessel, the time from filling to the tank was empty was measured to be 57 hours. This means that the tank should be re-filled every two days when NOTCam is not mounted on the telescope. If the PTR is removed (not just turned off), the holding time was found to be 44 hours.
When mounted on the telescope, the PTR will be in an orientation where it will give better cooling performance. But as some LN2 will spill out from the tank when the telescope is moved, filling the tank every day is recommended.
When cold, the pressure should be monitored in order to see whether the
vacuum is ok. A typical pressure of 3*
mBar is reached during
the first days when cold, and during the approximately 1 week long cold
periods in the laboratory, the pressure has been seen to continue to decrease
slowly.
If the pressure starts to increase significantly while everything is
cold, it is an indication of a saturated molecular sieve or perhaps a leak.
A pressure above 1*
mBar should be considered unacceptable, and
the dewar must be opened for a baking of the sieve. If the problem continues,
the dewar should be examined for leaks.
In order to properly monitor the pressure and temperature, make certain that a logging program is continuously running. This feature may be built into BIAS or a separate program.
At a regular basis, the entrance window end the optical surfaces visible when looking through the window should be examined for contamination. An effective method is to inspect using a flashlight in an otherwise dark room. Flat field exposures should also be examined for changes.