The JHU/APL TASS Observatory has had a long term problem with TEC cooling. There has been a great deal of discussion on the TASS e-mail list as to the reason why we are not able to get the cooling temperature down to the same levels we were able to achieve over a year ago. The consensus from the TASS e-mail list was that the reason was due to a combination of weather conditions, and water temperature. The problem was reported during the summer months, when there was a larger ambient heat load to overcome. The water temperature is normally at room temperature, averaging at about 25 degrees C.

The TASS control software is normally run in an un-attended mode. For that reason, the TEC current setting is set via the TASS.RC file to 1050 and not modified during the run. It should be noted that the recommended operating procedure by Tom Droege was to gradually reduce the TEC setting from a high number to the operational number while monitoring the current setting. The desired target current was about 1 amp.

With the data collection that occurred during the month of October, we now have two sets of data, exactly a year apart.   A MATLAB FITS reader was used to input the test files, and the water temperature, CCD temperature and TEC current was plotted for both sets of data files. The first plot shows the CCD and water temperature from October 20, 1997

The second set of data was collected on October 31, 1998. At this time, we have remote control of the TEC power supply through a RS-232 controlled UPS unit, and a special water pump designed by Bernie Kluga (JHU/APL). As a result, we are able to cycle the water pump for 5 minutes every hour during daylight hours in order to insure there are no air bubbles in the system, and we are able to program the computer to turn on power at least 30 minutes prior to data collection. It should also be noted that both data sets were collected with identical TEC current settings (1050), and that the same control software was used (TM3GET11).  The following plot is the CCD temperature and water temperature for October 31, 1998:

The next plot is the TEC current plot for October 31, 1998:

CONCLUSIONS:
The ambient temperatures of these two data sets were approximately the same. The water temperatures were also very close (4.7 degree difference). The TEC current for Oct 20, 1997 shows a control circuit properly cooling down the CCD unit. The TEC current of Oct 31, 1998 shows a saturated control circuit that is not able to cool the unit below 2 degrees C. The combination of current readout, and the CCD temperature are consistent with a unit not able to drive the temperature to the desired -10 degrees. As a next step, once the linux software is able to correctly read the status temperatures and currents, a programmable TEC current experiment will be setup. The TEC current setting will be gradually adjusted until it reaches the lowest CCD temperature setting.

It is quite possible that there is a hardware failure either in the TEC unit, or on its control circuitry that is causing the TEC cooling failure.

For more information, contact: Dr. Nicholas Beser

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