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Observing Overheads

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  Initial Setup Observing efficiency
Imaging 6 mins 30%
Spectroscopy (chop-nod mode) 20 mins 24%

Mid-IR observing overheads can be considerable and must be taken into account when proposing for time on T-ReCS.

Experience with T-ReCS indicates that each new imaging target will incur a configuration overhead of 6 minutes (e.g. to set the peripheral wavefront sensor(s) for aO and tip-tilt correction).

Once on target, a typical imaging observation with chopping and nodding means that about 30% of the time is spent observing the source (this includes the chop and nod overheads with the nominal 71% chop duty cycle and 15 arcsec chopping at 3 Hz). Thus an on-source integration of 30 minutes takes about 1.8 hours (including the 6 minute set-up).

It is possible to chop on-chip, but currently the "off source" part of the chop/nod cycle is unguided and thus is unsuitable for imaging programs.

The overheads for spectroscopy observations are larger than those for imaging observations. For each wavelength range (N or Q) and each slit position, in addition to an initial 10 minute overhead to slew to the target and set the peripheral wavefront sensor(s) there is roughly a 10 minute overhead for centering the target in the slit. See the spectroscopy target acquisition page for more details about the target centering procedure.

Once on target, experience has shown that the on-target efficiency of spectroscopic observations is lower than for imaging, about 24%, so a spectroscopic observation of 30 minutes on-source time takes about 2.4 hours total time. If another slit orientation is requested or one switches from lowres10 to lowres20 modes additonal 10 minute overhead will be incurred.

In highres10 mode there is no significant overhead for changing grating wavelengths as long as the target does not need to be re-centered in the slit. However it is more efficient to carry out a series of observations going from long wavelengths to short wavelengths rather than the reverse, so sequences should be created ordering the wavelengths from longer wavelength to shorter wavelength.

If one requires high astrometric accuracy in an imaging observation there are additional overheads involved. See the mid-IR astrometry page.

Gemini Observatory Participants