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When initially filling the cryostat, or topping it up later, there is no substitute for previous experience. The filling is done by using the 75 litre dewar normally and one needs to be confident that the cryostat is full - as indicated by the flow of nitrogen. It is very necessary to have a good routine for checking the cryostat nitrogen and re-filling; this should be done by a resident technician.
The temperature monitor is not intended to warn of nitrogen loss, since it can only indicate when it is really too late!
The cryostat should be checked at least every eight hours and topped-up on a regular schedule at least every 12 hours. If the telescope is tipped and there is significant nitrogen loss, this should be recorded and the cryostat checked rather earlier. The observer is recommended to fill the cryostat just before dinner and just after the end of observing each night.