Dewar Cooldown

Temperature Sensors

The temperature sensor box on the side of the dewar displays the temperature at 7 points inside the dewar. Connect the box to the Monitor connector on the top of the dewar to activate the sensors.

There are two types of sensors inside the dewar. Connected to the optical bench surfaces that run at liquid (or solid) nitrogen temperature are four diodes at points called: Saddlebags (optical bench near bottom on LN2 can), Work Surface (near top of optical bench), LN Shield (nitrogen shield above helium can), and GSM (grating scan mechanism carousel). When the switch on the temperature box is set to one of these positions, the indicator reads the temperature in degrees Kelvin directly. Because these diodes fail at liquid helium temperatures, however, three resistors are attached to points called Detector (outside of detector housing), Filter Wheel (bottom of filter wheel cover), and 128^2 (detector substrate) which run at less than 10 K. When the switch is set to any of these sensors, refer to the conversion chart in Figure 2.1 to convert the displayed voltage to a temperature.

Figure 2.1:   Liquid helium temperature sensor conversion chart.

Cooldown timeline

Cooling down SpectroCam-10 requires a minimum of 48 hours before the dewar is ready for observing. The following timeline is approximate:

1. 0 hours -- Vacuum Pumping.

   The pumpout procedure for SpectroCam is similar to that of any cryogenic instrument, except that the process takes longer because of the dewar's large size.

   1. Attach the pump-out valve assembly, including the valve itself and the cold trap, to the fitting on the dewar top plate, then connect a hose from the valve to a pump (preferably the pump that normally lives in the Cornell Lab). If you don't use the pump station, put the pinch valve over the hose so you can regulate the pumping. Note that no cold trap is required on the pumping station; the one mounted on the dewar valve is much more effective because of its proximity to the dewar. We rarely use a diffusion pump on this dewar -- a rough pump together with the valve's cold trap is sufficient.

   2. Attach the vacuum gauge to its connector on the top plate to monitor pressure. Check the dewar's pressure before opening any valves. If the dewar is at atmospheric pressure there may be a serious leak unless the dewar has just been closed up.

   3. The next step depends on the pressure inside the dewar. The dewar's pump-out valve is of positive-pressure relief design: if the inside of the dewar is at greater pressure than outside, the valve will open.

      If the dewar is at atmospheric pressure, open the dewar valve all the way (as it will open anyways as soon as the pump starts pulling a vacuum). Now turn on the pump and slowly open the pump's valve over a few minutes to gradually bring down the dewar pressure.

      If the dewar is already under vacuum, turn on the pump and bring the hoses down to a pressure below the inside of the dewar, then open the dewar valve.

      To open the dewar valve, push the shaft all the way in, turn the handle counterclockwise until the screw stops backing out, then pull up on the shaft. Reverse the procedure to close the valve. On the new Palomar pump station, Valve 2 controls the rough pumping and is the only pump station valve you need to operate.

   4. When the dewar pressure drops below about 2000 mTorr (typically 10 to 20 minutes after starting the rough pump), attach a liquid nitrogen dewar to the valve's cold trap and fill it. The pressure should soon start dropping rapidly. We usually let the dewar pump for at least a few hours like this to evacuate the multiple layers of superinsulation and clean out the getters as well as possible. In a hurry, the liquid nitrogen can be filled when the dewar reaches about 200 mTorr. Pressures as low as 20 mTorr can be achieved after 3 to 5 hours, depending on the pump.

      Do not let the cold trap run out of nitrogen --- its presence keeps pump oil fumes from backflowing into the dewar.

      In Case of emergency: If the pump fails, a belt breaks, or a hose comes loose, the FIRST thing to do is close the dewar pump-out valve. The first priority is not necessarily to ``save'' as much vacuum as possible, but to keep pump oil and other contaminants out of the dewar!

2. 6 hours -- Liquid Nitrogen Fill
   1. Get a full 50 liter LN2 storage dewar ready. A complete cooldown of SpectroCam-10 will use at least the entire 50 l.

   2. Close the dewar pump-out valve, making sure the shaft is pulled all the way up once the valve has been screwed shut. Remove the small LN2 dewar from the cold trap and allow the trap to warm up while you proceed to the next step. You may warm the trap with a heat gun if necessary. When the trap is warm, turn off the pump, then remove the valve assembly from the dewar and replace the screw-on cap.

   3. Fill the outer can with liquid nitrogen. Use the long fill tube that extends well into the outer can neck to minimize cooling of the O-rings. The outer can capacity is 15 liters, and 15 -- 30 minutes are required to fill it the first time, depending on the pressure in the storage dewar.

   4. When the outer can is full, put extension elbows on both necks to vent the cold gas over the side and keep the dewar top plate and O-rings from getting too cold.

   5. Attach the bayonet fitting to the inner can refill neck, and blow dry nitrogen gas (preferably from a liquid nitrogen storage dewar) into the refill neck for about 1 minute to clear out any water in the tube.

   6. Fill the inner can with liquid nitrogen through its main fill neck. You should detect a plume coming out of the refill line; if there is none then the tube may be plugged. This is serious problem and must be dealt with by warming up the inner can and clearing the plug.

   7. Connect the main and refill necks to a y-fitting through two rubber hoses, and connect a third hose to the y-fitting to drain cold gas over the side of the dewar. These connections are needed to avoid forming condensation or a plug in the necks.

   8. The dewar should now cold-soak for about 24 hours. Use the temperature sensor box to monitor the cooldown rate. The first fill will last for 2 to 2.5 hours before the cans need to be topped off. The hold time of the outer can once everything is cold is a few days. About 50 liters of LN2 are required to get from fully warm to fully cold with both cans full.

3. 30 hours -- Liquid Helium fill.
   1. Remove the hoses and elbows from the outer can necks and the main inner can neck. Take the hose of the inner can refill elbow but leave the elbow in place.

   2. Turn the dewar upside-down in the cart to dump the cryogens in both the inner and outer cans in preparation for the LHe fill. It is very important to completely empty the inner can. Slowly swing the dewar back and forth about 30 degrees and watch for spurts of LN2 coming from the inner (Helium) can. Keep doing this until there are no more spurts (about 5 minutes).

   3. Using the innertube and clip assembly on a liquid helium storage dewar, blow dry helium gas into the refill neck at a few psi for a few minutes to clear out any liquid trapped in the thin tube.

   4. At least partially fill the outer can with liquid nitrogen to prevent the dewar from warming up.

   5. Fill the inner can with liquid helium through the main fill neck, using a standard transfer line. This takes about 10 minutes, depending on the pressure in the storage dewar. We usually keep the pressure fairly low, one or two psi. Normally the neck frosts up pretty well almost down to its base. If the neck is iced up all the way to its base, then a heat gun should be used to keep the O-ring from freezing. Excessive icing may indicate a serious problem with the dewar or transfer line. The bayonet fitting on the refill neck should show a noticeable plume after a few minutes -- indicating that the refill tube is not plugged.

   6. Attach an elbow fitting to the main neck, and connect it to the refill line bayonet fitting with tubing and a Y-fitting. The bayonet fitting should not be plugged because oscillations in the liquid helium will result. Connect the third side of the Y-fitting to a long hose to prevent backflow, and to the blowoff meter, if desired. The blowoff meter only works when it is vertical.

   7. Finish refilling the outer can with liquid nitrogen if necessary.

   8. Put the elbows back on the outer can necks.

   9. The first helium fill may last only a few hours depending on how fast the transfer was done. Following fills last for about 12 hours if the liquid nitrogen is NOT pumped, about 24 hours if pumped. The equivalent gaseous capacity of the helium can (as measured on the blowoff meter) is about 2400 liters.

4. 32 Hours -- Initial tests: About 2 hours after filling the helium can, the detector is cold enough to turn on and operate (but be sure to check the temperatures first -- the detector housing should be < 10 K [< 280 on the meter]). At this time it may be wise to connect the detector and GSM electronics and try some initial tests (See sections Electronics and Cabling through Testing Everything) to ensure the instrument is operating properly.

5. 34 Hours -- LN2 Can Pumping: If the instrument checks out OK, then the process of pumping on the outer (liquid nitrogen) can should be started as soon as possible. Pumping reduces the temperature of the nitrogen from 77 to about 60 K, reducing the thermal radiation from the interior surfaces and increasing the helium hold time from 12 to > 24 hours. The pumping process should be started at least 12 hours before mounting on the telescope to allow internal temperatures to stabilize.
   1. Inspect the ``slurpee'' assembly and the pump out elbow - pinch valve assembly, including the O-rings, pinch valve hose, and hose clamps. Once the pumping has starting, it can be very difficult and dangerous to break the vacuum to fix problems with these pieces, so make sure they are OK before proceeding.

   2. Attach the ``slurpee'' assembly to the neck on the left side of the dewar (the side farthest from the window). Close the valve.

   3. Attach the elbow fitting to the open neck on the right side of the dewar, and connect the pinch valve hose to the pump with a second hose.

   4. Close the pinch valve all the way if it's not closed already.

   5. Turn on the pump. Slowly open the pinch valve to begin pumping on the nitrogen. Don't open the valve too quickly or you might suck all the liquid out of the dewar before it freezes.

   6. Monitor and log the temperature sensor readings. After several hours, the Saddlebag temperature should drop to about 60 K.

6. 48 hours: By now, 42 hours after the nitrogen transfer and 16 hours after the helium transfer, the dewar should be ready to be mounted on the telescope. Some slight additional cooling may be noticed over the next 24 hours, but the effect of this on dewar performance is small.