The dewar has an inner can to cool the detector (center hole) and an outter
can to cool the shield. The inner can has a hold time of more than 24 hours,
so you should to re-fill (with LN2) it every day, say in the afternoon,
at about the same time. The outter can has a hold time of less than 12
hours, so you should re-fill it at the same time as the inner can, plus
at the end of the night.
At the start of your run, follow standard procedures posted elsewhere to
pump the dewar. I strongly recommend "baking" the dewar :
Get a first ~ 5mTorr vacuum in the dewar cavity.
After isolating from the pump test port, put some dry N2 in the dewar
cavity, to improve heat
Use the heating electrical tape with the variac set at 30-40 volts.
Watch the temperature of the
dewar. If you can touch the dewar with your hands and keep them there
for several seconds, you
are probably not meltin the NICMOS detector. About three hours.
Then resune pumping, with electrical tape still on.
Whenever we follow this procedure, we achieve a good vacuum in the dewar,
about 5x10-7 Torr.
The detector takes about 2 hours to reach very close to 77 K after you
start cooling it.
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