Filling the CCD dewar

The CCD needs to be cooled to a temperature of -130 degrees C. The temperature can be checked from within voodoo. The dewar has a hold time of about 24 hours, and is cooled with liquid nitrogen. The LN2 is stored in the brown 35-liter storage dewars in the dome (Dawn Clemons can be contacted at dclemons@noao.edu to ask her to fill one of these before the observing run).

Because the geometry of the dewar makes leakage of liquid N2 possible for certain telescope orientations, we recommend that the dewar be filled at the end of the night or at lunchtime rather than just before observing starts to minimize wastage of liquid N2.

Filling instructions:

1. Wheel the 35 liter storage dewar which is currently in use under the CCD dewar on the telescope.
2. The long thin rod (stinger) stored on the side of the storage dewar should be already connected to its liquid output valve. Take the stinger assembly and carefully insert the stinger into the fitting at the bottom of the CCD dewar. There is an obstruction part way up the pipe and the stinger tip needs to be well centered in the pipe to get past it. Screw the stinger fitting closed; hand tight is sufficient.
3. Note that especially talented astronomers have managed to place the dewar so that the flexible filling tube, which freezes during filling, is underneath the liquid N2 valve and so prevents closing of the valve. Make sure that it will be possible to close the valve once the hose freezes solid!
4. Especially when the storage dewar is full, the gas pressure inside it may not be sufficient to push the liquid N2 out. About 10 psi (read on the gauge on the top of the dewar) is optimum. The dewar safety valves will let go at 12 psi. If the storage dewar pressure is less than about 5 psi, you will need to add dry N2 to increase the pressure. There are two dry N2 outlets on the S pier of the telescope. Use the tubing connected to the outlet without a flow-meter attached. Connect this tubing to the vent valve on the storage dewar. Turn on the dry N2 and and open the storage dewar valve. (Dry N2 pressure on the tank downstairs should be about 15 psi).
5. Open the liquid valve on the storage dewar (move handle vertical) so that liquid N2 flows into the CCD dewar. Fill until liquid N2 rather than gas flows from the overflow spout on the stinger connection. This should take just a few minutes or so.
6. Turn off the liquid N2 valve on the storage dewar, the vent valve on the storage dewar, and the dry N2 on the S pier and the N2 cylinder downstairs (if storage dewar was pressurized).
7. Wait until the fill assembly warms, and then unscrew connection to CCD dewar, remove stinger and replace in tube on the side of the storage dewar. If you are in a hurry, you can use the hair dryer gently to warm the fill assembly before disconnecting it.

Swapping the fill heads on the LN2 tanks

There are 3 ways to relive the pressure, none of them ideal. We need to add a bleed valve on the gas line at the head of the tank use a snap fitting like we use for the compressor of the N2 gas connection.

• The quickest way to relieve pressure is to disconnect the N2 gas line and open the gas valve (slowly). However this requires a wrench to undo the fitting, and it will leak if we routinely undo it.
• Another way is to hold open one of the spring loaded safety valves there are 2 approx 3/8" diameter silver fittings with brass end plug. You often hear gas coming out of these when you have the N2 gas line connected. (if there is much pressure you finger will get cold before it bleeds off.
• The default way if you didnt think about it. MAKE SURE THE WIRE CABLE IS CONNECTED TO THE RING with the dog lead like hook. Slowly unscrew the black knob, on the clamp that holds the head to the storage dewar. Gas should bleed out around the Oring as the clamp is loosened. I think there is approx 1" of thread for the knob to unscrew, the seal should have broken before the knob unscrews halfway.