Observing overheads

In summary: both the telescope dither overhead and the acquisition related overheads (such as file storage time) have increased from 2006 to 2008, but these increases are well understood since they are part of an improved system, more accurate pointing in the first case, and the sequencer in the latter case. The readout overheads on the other hand are not understood. Their cause is believed to be due to an aging ISA PC-board. Nevertheless, using a ``spare'' ISA PC board for testing gave no difference. Then, changing to a faster PLD on a similar PC board seemed to solve the problems related to reading (at least no read errors were reported) on an old data acquisition computer (elizabeth), but when this was tried on the currently used computer (marissa), no improvement was found. More tests are planned.

It should be noted that the current readout overhead problems can add up to 40% additional overhead to NOTCam observations. The amount is quite variable, and this also makes planning the time-line of the observations difficult.

A special exposure command for NOTCam has been suggested in which the dither overhead can be folded into the readout and file storage overhead which would save some time but some development is needed. In any case, the problem with readout overheads is the dominating problem at the moment.

Thomas Augusteijn 2009-05-14