Evacuating and Cooling the CCD dewar

When you arrive for your night's observing run you will find the CCD dewar in one of a few possible states. Hopefully you are on an extended observing run and you have been using the LN2 'Auto-fill' system. In this case your dewar is probably ready to go, except for a possible top up of LN2 before you begin, though this shouldn't be necessary as a rule.

If you find yourself arriving to a situation that is unknown it is quite possible that you will have to pump the dewar and possibly even do this in conjunction with warming up the outside casing of the dewar.

YOU CANNOT CONTINUE without knowing the CCD temperature.

WARNING: You should not be attempting any of these procedures without having been personally instructed by someone who knows. This is especially true of evacuation. In addition, it is standard procedure to wear safety goggles and gloves when working with the liquid nitrogen (LN2).







How to determine whether the dewar needs cooling?

The CCD dewar can not be cooled unless it has first been properly evacuated. It may well be the case the CCD has already been pumped down and cooled.

You will want to determine the CCD temperature before you can decide on the need to evacuate.

Knowing the temperature, please click on one of the following scenarios;







How to determine whether the dewar needs to be evacuated?

Pumping is only required if the CCD temperature is at or near ambient temperature. This applies to when ambient is above as well as below zero degrees C.

A dewar which is near ambient is considered WARM .

Please carefully read the evacuation procedures before pumping down.







Dewar is at target temperature (-100 C)

If the dewar temperature reads -100 C or colder you may only find that you want to top-up with LN2 before you begin.

Don't forget that if you have just turned on the controller and that the CCD temperature is below -100 C, it will take possibly 20 minutes to attain equilibrium at the target temperature.







Dewar temp. < -10C, but is not at target (-100C)

If the dewar temperature reads -10 C or colder, but is not down at the target temperature of -100C it is safe to simply do a fill up with LN2.









Filling with Liquid Nitrogen (LN2)

Follow these steps to top-up the dewar with LN2:

  • Park the telescope pointing towards the ZENITH.
  • Wheel out the LN2 storage dewar and the pressure pump from the equipment room.
  • Insert the LN2 fill tube to the inlet tube at the back of the dewar, with overflow tube facing down.
  • Check the pressure inside the storage dewar. It should be between 2-4???. If not operate pressure pump to achieve this. Do not forget to open the inlet valve on top of the 35 litre dewar. Close the valve when you reach the desired pressure. You should maintain this pressure during fill sequence.
  • Open the outlet valve which is on top of the 35 litre dewar. Make sure that the valve is opened completely.
  • You will see the flow of LN2 initially. Do not shake or touch to the fill tube during fill after it freeze.
  • Continue to fill the CCD dewar until you see LN2 dense drops comming out the overflow tube.
  • Close the outlet valve of the storage dewar.
  • Wait about 10 minutes until fill tube softens again.
  • Disconnect the fill tube and wheel the storage tank to the equipment room.







    Dewar is at or near ambient temperature

    It will be necessary to pump down the dewar. You may need to heat up outside of the yellow CCD dewar!!

    Three steps to 'pumping down' (evacuating) the dewar:


    1. Setup of Vacuum Pump

    Check the configuration

    • Wheel the mechanical vacuum pump out of the instrument room and park it underneath the CCD system.

    • First, make sure that the BLACK valve on the CCD dewar is CLOSED.
      Attach the vacuum hose flange to the dewar flange.
      Check the vacuum grese between.
      Connect with the clamp and tighten firmly.

    2. Checking the existing vacuum

    This section describes how to evacuate the vacuum hose and to use this to check on the existing vacuum in the dewar. We do this because there is not a vacuum head attached to the dewar itself. The closest gauge head is located near the mechanical pump.

    Follow these steps to check the existing dewar vacuum:

    • Make sure that both the ??GREEN valve and BLACK dewar valve are closed.
    • Turn on the power to the pump and vacuum gauge meter.
    • Open the GREEN valve in order to evacuate the hose. Let it run until about 0.06 to 0.05 mbar.
    • Now close the GREEN valve.
    • Open the BLACK dewar valve. Observe the meter reading. If it goes above 0.1 mbar, then you should perform a complete pump-down of the dewar for at least 30 minutes.

    3. Doing a complete pump-down

    Assuming that both the GREEN valve on the pump and the BLACK vacuum valve on the dewar are closed to begin with. Here are the steps to get a good vacuum:

    • Slowly open the GREEN vacuum valve. Wait about 3 minutes.
    • Slowly open the BLACK vacuum valve on the dewar. Let the vacuum build for 15 to 30 seconds before fully opening it.
    • Pump to about 0.05 mbar (5 x 10**-2 mbar).
      This should take approximately 30 minutes to an hour.
    • Close the BLACK dewar valve.
    • Turn off the power to the vacuum pump and meter.
    • Remove the vacuum hose from the dewar and put back the blank flange end.


      NOTE TO QUALIFIED TECHNICIAN: The preliminary evacuation of the dewar could best be done while the instrument is still in the equipment room.








    Cold-finger Temperature Readout

    With the CCDTool program, temperature readout is a simple point and click operation after initialization procedure and CCD power-on.
    Simply go the the EXPOSE window and click on the Rd CCD Temp icon. The temperature of the CCD (actually of the cold finger to which the CCD is mounted) is displayed at the bottom of the EXPOSE window. The readout is not exact, so you may want to read the temperature out a few times in succession in order to understand the type of inaccuracy that exists.

    It is also possible to readout the temperature manually as follows:

    1. First you must use the CCDTool DSP menu to readout the binary value (Hexadecimal) of the cold finger temperature.
      The image below indicates proper settings for the DSP window:

      After setting the DSP window for Utility, RDM, Y, 0c, you can click on the Send Command menu button. This will ask the CCD controller to reply with the cold-finger temperature, which will be displayed at the bottom of the DSP window.

    2. In another Xterm or xgterm window, enter this hexadecimal number into the 'dew_temp' conversion routine as follows: 
      
> dew_temp b8d
dewar temperature:    -102 C

    Some typical conversion values are shown below.
    Cold Finger Temperature Conversion Values
    DSP temp. readout (Hex) Temperature ( C)
    a3a 20
    a75 0
    b08 -50.0
    b8d -95.2 (old target)
    ba0 -102 (new target)

    NOTE: It may take a couple of clicks on the Send Command button before you get back the 'correct' temperature. It takes a bit of experience to see how consistently (or rather inconsistently) the temperatures get reported.











    Cooling Time

    When a warm dewar has been evacuated and freshly filled with LN2, it will take approximately 4 hours before the setpoint temperature of -102 C is reached.

    Here is a typical cooling curve. Note the discontinuity at the point where an extra top-up of LN2 was applied.

    Cooling Lifetime

    A full well cooled dewar, with CCD controller 'ON' should stay at setpoint of -102 C for approximately 18 hours during the summer months. In fall and winter the lifetime can reach 24 hours.

    With the temperature controller OFF the dewar may stay cold for as long as 24 hours.

    When the dewar has been warm the first LN2 fill may only last about 8 hours, so be careful to top it up for a second time before observing.

    CCD Linearity

    The linearity of the CCD device was tested as follows. Flat field lamps of increasing intensity were taken, up to the point near which the ADC saturates (65536 counts). At a present gain of 2 electrons / ADCU, this is approximately 130000 electrons. The highest intensity exposure reached approximaely 52000 ADCU, or 104000 electrons, the CCD showing very good linearity through the entire range of exposures.

    Here is a graphical depiction of the results.

    • Umit Kiziloglu
    Last modified: Jan 2000