Filling the cryostat

The cryostat has a hold time of at least 12 hours with the telescope stationary at zenith. If the telescope is tipped to the least favourable inclination then nitrogen loss will occur and the hold time can be reduced to less than 12 hours. Standard practice is that the cryostat gets a refill of liquid nitrogen at the start of the night, at the end of the night and at midday. The refill at midday (approximately 14:00) is the responsibility of ING staff. The refill at the start and end of observing is the responsibility of the visiting astronomer. Your Support Astronomer will give you instructions on how to fill the cryostat.

You can check the CCD temperature on the CCD-STATUS window on lpss12.

In order to minimize 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.

If you forget to refill the cryostat and the dewar warms up, it will take about 10 hours to let the nitrogen evaporate, to pump it to vacuum, and to cool it down again. In other words: you will lose observing time!

If you leave the JKT early in the night to go to bed, for instance when the weather is bad, then you cannot fill the cryostat in the morning. Please request the day-time staff to fill the cryostat for you, by submitting a FAULT report. This is the only way to communicate with day time staff, and to make sure that they are aware of the situation first thing in the morning.

If you find that the hold time of the cryostat is not sufficient to get you through the night without an extra refill, leave a fault report. This usually means that the cryostat is losing its vacuum and that you will have the same problems for the following nights as well. In the fault report you can request the cryostat to be checked or pumped.

You are advised to use the protective gloves and mask when filling the cryostat.