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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 CCD temperature monitor is not intended to warn of nitrogen loss, since
it can only indicate when it is really too late! If the CCD
temperature
monitor gives an error, stop everything and refill immediately.
Minutes count at this point.
The cryostat should be checked at least every eight hours
and topped-up
on a regular schedule at least every 12 hours. If the turntable is rotated
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.