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3.3 Dewar Design and Maintenance

The CCDs are mounted in KPNO designed and manufactured universal dewars. The design of these dewars is such that they can be used in any position due to the arrangement of fill and vent tubes. Approximate LN2 hold times are 12 hours. The dewar should not be allowed to warm up (run out of LN2) during the time it is on the spectrograph. The mountain Technical Assistants will normally fill the CCD dewar with LN2 twice a day, in the morning and in the evening just before dinner.

The dewar should be filled in the following manner:

1. Unscrew the brass cap on the filler port.

2. Connect the metal hose from the LN2 storage tank to the dewar filling port finger tight. Back-off the fitting 1/2 turn and tighten only after the fitting is cold. This prevents the fitting from contracting tightly around the filling port and permits easy removal.

3. Open the valve on the storage tank, pressurizing with dry N2 if necessary. The dewar should be filled rapidly to prevent freezing the seal around the filler port.

4. When the LN2 starts spraying out the vent holes, the dewar is full.

5. Remove the metal filling hose. If it has shrunk tightly around the filling port, either warm it up with a heat gun or wait a few minutes for it to expand. DO NOT FORCE IT OFF WITH A WRENCH!

6. Screw on the filling port cap. This is important to prevent the formation of an ice plug in the fill port.

The large gold box attached to the dewar contains video processing electronics, precision voltage references, precision clocks, and an electronic temperature controller.

The mounting of the CCD dewar to either camera 5 or 6 is accomplished by use of either of two mounting plates which attaches to a bracket on each camera. This mounting system allows rotation and motion perpendicular to the dispersion to facilitate spectrum alignment parallel to chip columns and proper centering. Centering in the direction of dispersion (selecting central lambda) is done by moving the grating under computer control.