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Mid-IR observing overheads can be significant and must be taken into account when proposing for time on Michelle. The following table summarises the overheads and efficiency, but please be sure to read the detailed notes below.
|Initial Setup||Observing efficiency|
|Imaging||6 mins||25% (21% in the Qa filter)|
|Imaging polarimetry||6 mins||7.5% (6% in the Qa filter)|
|Chop/nod mode spectroscopy||15 mins||25%|
|Stare mode spectroscopy||15 mins||67%|
Experience indicates that each new imaging target will incur a configuration overhead (slewing, centering, setting the peripheral wavefront sensor for tip-tilt AO correction on a guide star) of 10 minutes or less, depending on the distance of the target from the previous one, and the guide star magnitude. The average value is about 6 minutes as noted in the table above. For setting up a spectroscopic measurement an additional 5 to 10 minutes should be allowed for accurate centering on the slit and for switching between imaging and spectroscopic modes, hence the overhead averages to 15 minutes. Note that these are average values, for programs with faint targets or special acquisition requirements the overheads could be longer. If in doubt please check with the Michelle support astronomers.
Imaging and Low-Resolution Spectroscopy
Once a typical observation with chopping and nodding (i.e, imaging and low-resolution spectroscopy) begins, the time spent actually integrating on the source - the value entered in the observing tool as the "total on-source time" - will be about 25% of the elapsed time. Thus, you specify the actual on-source time for Nod A Chop A + Nod B Chop B (i.e., one beam for each nod position). If chopping on-chip you in fact get double the on-source time that you specified but, because guiding is only possible in one of the chop beams (see here), the quality of one of the two images will be somewhat degraded and unlikely to be useful for science. The factor of two on-source overheads are due to the chop duty cycle and the time required for the telescope to nod and then stabilise. We hope that this eventually can be reduced to a factor of ~1.5.
In early 2007A a new observing mode (longer frame time and fewer read-resets) for the Qa filter was tested which increased the observing efficiency in this filter by about 40%.
The Echelle and Medium-Resolution Gratings
When observing with the echelle and medium resolution gratings, done in stare/nod (as opposed to chop/nod) mode, the observing overhead, after the initial 15 minutes of setup time, is a factor of about 1.5.
In the case of imaging polarimetry the time requested in the observing tool (and the time that should be specified in the integration time calculator) is the total on-source time (this is a change from 2007A and 2006B where the time per wave-plate position was used). The exposure time in any individual wave-plate position is therefore 1/4 the total on-source time. The overhead involved in moving the half-wave plate during the observations means that the overall efficiency is generally of order 7.5%, rather than 25% as in regular imaging mode. Thus for a 2 minute on-source time specified in the OT, the actual time spent per wave-plate position will be 0.5 minutes; coupled with the 7.5% efficiency, the total time needed to take the observation will be about 2 minutes/0.075 = 27 minutes for the filters in the N-band window (after acquisition).