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Using the 75 litre dewar.

Visiting observers who have not used this dewar before must obtain instruction from a member of the Technical group or a Support Astronomer.

1. Ensure that all valves apart from No 4 (vent valve) are shut.
2. Insert the fill tube into the cryostat
3. Close Valve No 4 - this allows the dewar to pressurise.
4. Open Valve No 3 (the dispensing valve) and Valve No 1 (the by-pass valve). Check that the pressure does not rise above 10 psi on the gauge. If it does, reduce it by opening Valve No 4 a small amount. If it takes a long time to build up pressure then open Valve No 5 for a short period, again checking the pressure on the gauge. Close No 5 as soon as pressure has built up.
5. When the cryostat is full, close Valve No 1 and 3 and then open No 4. As the pressure is released there will be a rush of gas out of this valve.
6. Check that all valves EXCEPT No 4 are closed.
7. Remove the fill tube when it becomes flexible again and place the dewar away from the telescope.