Observing overheads

Although the overheads in observing have been improved significantly with the removal of the observation log and observing system log from the instrument computer, a significant amount of time is spend on reading out and storing each exposure. Especially for the typical short exposures used in the infrared this overhead is significant. It has been a long standing idea to fold the process of doing (small) telescope offsets (dithering) in between exposures in this ``dead-time''. The only safe way to do this properly required a new sequencer command ``notcam.wait-shutter-closed'' that signals when an exposure stops. Since September this command has been implemented in all standard dither scripts offered for NOTCam. Since late October it is also included in the script generator. Timing tests at the telescope shows that with autoguiding we save on average $\sim$5.6 seconds per exposure, which is exactly as expected from having the readout and the file storage time folded into the time it takes to do a typical telescope offset (10-15 arcsec). The total time spent on a dither pattern has now improved by a factor T_now/T_before = 0.76 for ``exp 3'' and 0.82 for ``exp 10''.

For the ramp sampling mode the command and its benefits still need to be tested.