The Camera

Certain maintainence should be a routine. A schedule can be established, but an alternative is to routinely monitor the system for problems:

1. Check the level and constancy of the CCD temperature. Each time an image is taken, the temperature is written in the image header. A rising temperature (to warmer than -95C) indicates a softening dewar vacuum. A rapidly-fluctuating temperature (by many 10's of degrees C) indicates condensates or dirt has contaminated the cryogen lines. Pumping the dewar for better cooling or applying a dryer/filter tank to the cryogen lines has proven to resolve this problem. The dewar may require pumping every 3 months or so. High background in images (from dark current) will also accompany images taken with a warm dewar.
2. Checking diskspace on the computers will head off problems of a full hard drive.
3. When at the observatory, check the pressure on the back of the Cryotiger compressor. It should be in the 15-30 psi range when in operation and fully cooled. We have historically seen the pressure near 15 psi. If the compressor is off and the system has warmed up, the pressure should be close to 240 psi, depending on the temperature. We see something near 253 psi, which is a little bit high.
4. Relatively high condensation or cold temperatures on the dewar might indicate flatness or point to the location of a leak.
5. Check images, especially flat fields, for dark blotches that indicate foreign objects lying on the dewar window. It will probably be necessary to clean the dewar window to remove this material. Ordinary condensation should be ruled out first, however. Condensation forms in the central parts of the dewar window in times of high humidity. Beads of water will appear as a cluster of round obscurations. A finer film of condensation tends to leave a dark circular patch in the center of the images. The most remarkable aspect of condensation on the dewar window is its positioning in the center of the dewar window and flucuations with humidity level.
6. Donuts seen in images probably indicate dust on the dewar window. Similar patterns due to dust on the filters would be very large and difficult to discern.