Detector

The amount of cold pixels has slowly increased (from 0.2% to 0.4% of the array) over the period 2008 - 2009. It was getting difficult to find clean vertical areas for spectroscopy which is typically done dithering the objected along the slit. When NOTCam was opened in January the detector plate was dismounted and dry air blown across it in an attempt to clean it. Also, the whole interior of NOTCam was vacuum cleaned. The new cold pixel mask obtained after NOTCam was closed again showed that the cleaning helped somewhat, where the number of dust particles on the detector (seen as cold pixels, or pixels with low response) has diminished, though still not as low as it was in early 2008.

We further looked at the increased background that is measured in dark exposures when the instrument focus mechanism is at low values. The structure in the darks seem to indicate a shading, and a rough estimate pointed to the focus mechanism as a possible origin of the extra dark emission. However, disconnecting power to the mechanism showed no change in count level. The only thing that came to mind is radioactive materials possibly used in that mechanism. When NOTCam was opened we looked at ways to shield the mechanism, but there is no space to mount anything. The effect is being investigated and is briefly described in the NOTCam User's Guide.