B. Truax 6/15/00 (with guidance from John Geary)
Low quantum efficiency on the CCD probably means that the back surface has lost its O- ion charge. There are different methods for recovering the dewar depending on who you talk to and what caused the problem. Assuming that the Ion pump was not on when the dewar warmed above 0C, you can probably use one of the first two methods below. If the Ion pump was on as the dewar warmed, it probably ionized a bunch of water molecules and the H2+ ions are probably stuck to the Pt coating on the rear of the chip. I this case, John Geary recommends technique number 3.
1. The simplest way to solve the problem is to pump the dewar and let it warm to ambient temperature. Once the dewar has reached ambient, pump for 24 hours and then fill with LN2.
2. If method 1 does not work or if you want to skip method 1 then allow the dewar to warm well above freezing (within 5 degrees of ambient if possible and definitely above the current dew point). Slowly vent the dewar to atmosphere, if possible, an oxygen rich gas mixture (try to avoid pure oxygen). Allow the dewar to soak at atmosphere for at least 12 hours and then pump and fill according to normal procedures.
3. If all else fails, pump and bake the dewar at 50C for 12 hours, then vent to atmosphere while hot. Allow to sit hot at atmospheric pressure for 2 hours. Pump while hot overnight. Disconnect the heater and fill with LN2.
Never turn on the Ion Pump while the dewar is warm, it can destroy the CCD QE (see above). The Ion Pump should only be turned on while the backing pump is running and the dewar has been filled with LN2 for at least one hour.
It means that the ion pump gauge is off scale. This does not mean that the Ion Pump is not working. If the pump will not stay on scale you should probably pump more with the backing pump. Check the pressure by measuring the gauge voltage on the Ion Pump electrical connector (see the Kernco manual for pin numbers). If the voltage is above 5.5 volts and you have pumped well with the backing pump you probably need to perform a thorough baking and cleaning of the dewar (see What do I do of the dewar will not hold a good vacuum?)
No. If the outside of the dewar feels colder than the other metal on the telescope, or if there is condensation on the dewar wall or you notice condensation on the dewar window, the dewar pressure is too high and it needs to be pumped more.
The gauge should read 2-3 lights below the top of the scale (~1 x 10e-6)
We will give you the benefit of the doubt and assume that you closed the valve tightly. Assuming this is true, the dewar either has a leak or is has a lot of adsorbed gas on the dewar walls. A leak is unlikely if the dewar has not been opened recently, so the problem is probably gas and water contamination in the dewar. To clean the dewar perform the following steps:
1. Bake the outside of the dewar and the focal plane at 50C while pumping the dewar with the backing pump (do not use the Ion Pump). Do this for at least 12 hours.
2. While the dewar is hot, backfill the dewar with dry N2 and pump again to about 3 x 10e-6
3. Perform step 2 a total of 3 times.
4. After the final N2 backfill, pump overnight (or for 12 hours). Turn off the heaters and fill with LN2.
5. One hour after filling turn on the power to the ion pump and pump for one more hour.
6. Close the dewar valve and turn off the backing pump.
The Ion Pump should be turned off before the dewar is allowed to warm above the freezing point of water. To be safe, the Ion Pump should be shut off well before the CCD temperature sensor reaches 0C (-50 C is a safe number). This will avoid outgassing of water vapor from the surfaces and the charcoal and the resulting generation of H2+ ions by the Ion Pump.
It should be pumped for at least 24 hours. In addition, try to observe the rule that the backing pump gauge pressure is less than 2 x 10e-6 before filling with LN2. If you cannot achieve this pressure in 24 hours then the dewar probably needs to be baked as described above.