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T-ReCS Performance and Use

As of September 2006, non-T-ReCS-specific information on mid-IR observing, calibration and data reduction (such as baseline calibrations, standard stars and a proposal/phase II checklist) can be found on the general MIR resources page.

Status and availability:

All modes (Imaging, Lo-Res and Hi-Res Spectroscopy) have been commissioned and are available for use. See the latest Call for Proposals for further details and the associated instrument calendar for target RA constraints.

Modes of operation:

The principal T-ReCS science modes, plus the typical settings of five instrument components in each mode, are listed in the following table. (Note: l/D means wavelength/diameter of primary mirror)

Mode	Window	Slit Width	Filter	Grating	Detector
10 µm Imaging	ZnSe	Open or Occulting Bar	N, Si1-Si6	Imaging flat	High-background
20 µm Imaging	KRS-5	Open or Occultingbar	Qa or Qb	Imaging flat	High-background
10 µm Lo-Res Spectroscopy	ZnSe	10 µm l/D or 2l/D	N	10 µm Lo-Res	Low-background
20 µm Lo-Res Spectroscopy	KRS-5	20 µm l/D or 2l/D	Qbroad	20 µm Lo-Res	Low-background
10 µm Hi-Res Spectroscopy	ZnSe	10 µm l/D or 2l/D	N	10 µm Hi-Res	Low-background

T-ReCS Components:

Details of the optical elements are listed on the T-ReCS Components page (also linked from the T-ReCS menu).

Operational use:

Like other mid-IR imagers and low resolution spectrographs, accurate cancellation of the sky and telescope background is most readily achieved by the technique of chopping and nodding. Currently the maximum chopper throw allowed on Gemini is 15 arcsec, which is considerably smaller than the T-ReCS field of view. See the telescope characteristics for more details of the chopping performance.

Sensitivity:

The Integration Time Calculator can provide estimates of required integration times for the available observing modes. A guideline sensitivity table derived from real on-sky observations is also available.

The estimated image quality delivered to the instrument is given as part of the observing condition constraints.

Observing overheads:

	Initial Setup	Observing efficiency
Imaging	10 mins	30%
Spectroscopy (chop-nod mode)	20 mins	24%

Mid-IR observing overheads can be condsiderable 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 10 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 10 minute set-up).

It may be 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. Unlike Michelle, T-ReCS set-up involves taking a short image of the source through the slit, and a short image of the field with the slit removed. The first image shows exactly how well the target was centered in the slit, and the second shows exactly where the slit was set up on the field, which is useful if the source is extended or if there are mulitple sources on the field.

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.

Calibration:

A baseline calibration set will be taken for each observation, queue or classical. For imaging, the PI must specify these calibrations from a set of photometric standards that is available. For spectroscopy, the PI must specify the telluric standard stars to be observed.

Observing Strategies:

See the Observing Strategies page for guidance and special considerations when observing with T-ReCS. When you're done, don't forget to consult the proposal and phase II Checklist.

Target acquisition:

See above; typical acquisition takes about 10 minutes. If acquisition is to be done in a different filter than the observations (e.g. using a narrow band filter to set-up on an emission line peak before taking a spectrum) a special acquisition will be needed. For this, please inform your Contact Scientist via a note in the Phase II Science Program.

Last update 2007 May 27; Jim De Buizer
Previous versions by Jim de Buizer, Kevin Volk, and Rachel Mason