Detector

During the last observing run we have had intermittent problems with one of the CCDs showing only 0's when the instrument was at certain orientations with respect to the telescope. The same problem has occurred in the past. On that occasion we decided to open the cryostat to make an inspection of a suspected wiring fault to the detector, but nothing was found. After mounting the instrument back on the telescope it was found that the affected CCD worked properly, but another CCD did not work at all and was later found to be effectively dead. We have done several tests on and off the telescope which shows that the particular CCD only shows 0's at particular orientations as before. Given our past experience we have decided not to do anything until after the next observing run which starts at the end of April and is the last observing run with MOSCA in this semester. Normal observations should be possible as long as we limit the orientation of the rotator which should not seriously affect observations, and we will review the situation after the observing run.

Another issue is that of including over-scan pixels for the detectors to be able to correct more precisely for the varying biases level. In the current controller there is not enough memory on the detector controller interface `PC-board' to read the MOSCA array in full resolution (no binning) and to have an over-scan. It turns out that larger memory can be used and new 128MB memory was bought and installed on a spare PC-board and tested on an old instrumentation computer, but due to a design fault on the board it only finds the first 32MB. Further investigation found the other memory appeared at high addresses but with gaps at intervals of 32MB. This needs to be investigated in more detailed, but in principle it appears to be possible to modify the controlled as as to be able to read the full array plus over-scan. An increase in memory would also be beneficial to NOTCam as this would allow the `frame' readout mode to be expanded to allow more sub-reads in a single exposure.