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Thermal properties

The cryostat should be filled to at least 1.2 litre capacity initially, and if kept at Zenith position say, gives about 16 hours hold time. If the telescope is tipped to the least favourable inclination then nitrogen loss will occur and the hold time can be reduced to 10 or 12 hours.

In order to minimise nitrogen boil-off and give some temperature control the detector assembly (and temperature sensor) have an intrinsic time constant. This explains the 2-3 hour cool-down period. A further effect is that if the liquid nitrogen runs out completely then it may take an hour or so before any temperature rise is detected; the temperature rise may then be rapid.