Detector

The problem of one of the MOSCA CCDs failing at certain telescope pointings returned. An inspection of the instrument was made in the clean-room and during tests it was found that simply tightening down the front window flange caused the signal to disappear, suggesting a possible shorting between two items could be causing the problem. With the front plate off and monitoring the output of the CCD with an oscilloscope it was found that it was possible to reproduce the fault by moving the constantan wires that pass the signals from the dewar connector to the actual CCD.

Inspecting the wires under a microscope it was possible to see what could be some minor damage on a couple of the wires, but no obvious place where you could conclusively say was the cause of any fault. All the wires were moved so that they could not touch any part of the dewar walls. After reassembly the instrument it continued to work properly and has been used without any faults for one run since.

We are currently considering if we should replace the affected wire bundle or not. There is a certain risk in replacing the wires (the CCD is not protected electrically when the wires are not connected) but there is also a change that the error returns as has happened in the past. At least one difference is that now we know where the error is.