5. The CCD System

The CCD system is a SITe back-illuminated 2kx4k chip with 15 micron (1.5 arcsec) pixels, run by a Leach version 2 controller. Its long (4096 pixel) axis is oriented N/S.

The CCD currently has one working amplifier. Readout time is about a minute and a half. Gain is fixed and is very close to 2 electrons per ADU. Full well capacity is at least 100,000 electrons. The ADC is 16 bits, ie from 0 to 65K. Bias level is about 7000 counts. Readout noise is around 12 electrons.

5.1 Filling the CCD dewar

The CCD needs to be cooled to a temperature of --70 degrees C. [The temperature can be checked from within CCDtool (see Section 6). The ``Read CCD Temp'' button is located at the top right corner of the ``Expose'' window ( Figure 6.3).] Its dewar should be filled daily (or more often if required) by the observer with liquid N2. The LN2 is stored in the brown 35-liter storage dewars in the dome (Peter Mack will make sure that one of these is filled 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:

  • Wheel the 35 liter storage dewar which is currently in use under the CCD dewar on the telescope.
  • 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.
  • 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).
  • 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 5 -- 20 minutes.
  • 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 (if storage dewar was pressurized).
  • 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.
  • The CCD temperature can be checked using CCDTool: the button is at the top right of the expose window.

    5.2 Dry Nitrogen Bottles

    We use dry nitrogen in the dome for two reasons: (i) when filling the dewar, it is sometimes necessary to pressurize the liquid nitrogen storage tanks. When the tanks are full there may not be sufficient gas volume to maintain pressure during a fill. (ii) to keep the dewar window free of moisture, dry nitrogen flows across the window. This is only necessary in humid conditions. In this case the flow rate of 0.5 on the flowmeter near the south pier is sufficient.

    The main valve on the dry nitrogen tanks downstrairs (located at the bottom of the spiral staircase) should be turned off whenever dry nitrogen is not being used. It is particularly important to remember to turn the main valve off at the end of your observing run.

    When the nitrogen tank is empty, mark it empty with chalk or masking tape and pen. Turn off the main valve on the top of the bottle. Use the wrench provided to remove the regulator from the empty bottle, and attach it to the new one.

    If you are down to the last bottle, please inform Peter Mack.

    4. Computers at the Burrell Schmidt    6. CCD Acquisition