Detector controller

Since NOTCam's arrival in 2001 we have been waiting for the new controller. For many applications NOTCam has given useful data, but because of the large overheads, the typical IR observing modes for broad band imaging are still not feasible. In addition, the measured dark current is too high and also variable. A dead column in the centre of the array, introduced by the controller, is a disturbing feature. The dc-gradient (or ``bias tilt'') across each quadrant is not a smooth gradient, but has several jumps that are not always subtracting out well, producing stripes or bands in the images. And finally, it is not possible to abort an ongoing integration. None of these problems were fixed while waiting for the new controller.

It was discovered that there is a well defined relation between the detector temperature and the reset level. This explains the observe variation in this value which sometimes requires the voltage levels to be adjusted to avoid the count rates getting below zero. The reset level voltages were set such that negative values are not expected to occur independent of detector temperature. The values will still be monitored.

On a few occasions the actual exposure time provided by the array controller were found to be only a fraction of the requested exposure time though the latter appears in the header. For some unexplained reason the array controller decides to read out before it should. Switching the detector controller off and on solves this problem, but if the requested exposure time is short it is easy to miss this and we are looking for some way to detect this when it happens and warn the observer. A daytime test will also be added in which it is checked if the exposure time is not affected, and provide a warning and instructions if there is a problem.