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GNIRS Performance and Use

Information on near-IR calibrations (baseline calibrations, standard stars, etc.) can now be found on the general NIR resources page.

Status and availability:	GNIRS began regular science use in semester 2004B. With the exception of polarimetry, all of the primary modes of the instrument are available to the community in queue and classical modes (see next section). During 3rd quarter 2005, GNIRS underwent major servicing, including replacement of the thorium-coated lenses on the short camera that produced radiation events on the detector, installation of a broader and more sensitive cross-dispersed filter and two additional narrowband filters for acquisition.
Modes of operation:	GNIRS has many possible configurations. They can be grouped into seven basic spectroscopic modes: Long slit, 0.9-2.5um, R>5000 (low background) ... R=5900 and 18000 Long slit, 1.1-2.5um, R<5000 (OH limited) ... R=1700; wavelengths < 1.15um not recommended Long slit, 2.9-5.5um, all resolutions (high thermal background) ... R=1700, 5900 and 18000 Cross-dispersed, 0.9-2.5um simultaneous ... R=1700 and 5900 (partial coverage) IFU, 0.9-2.5um (low background) ... R=1700 and 5900 IFU, 2.5-5.5um (high background) ... not fully commissioned, please inquire if interested Polarimetry ... not available (requires an external polarization unit) See the gratings section of the GNIRS components pages for full details of operational constraints e.g. order overlap. The only imaging modes of GNIRS are for target acquisition and pupil viewing.
GNIRS Components:	GNIRS has multiple slits, 3 gratings, 3 prisms, 4 cameras, 2 filter wheels, and an IFU. The detector is an InSb Aladdin III array. See the GNIRS components page for details.
Sensitivity:	The Integration Time Calculator can be used to determine limiting magnitudes, exposure times, S/N ratios, background levels, etc. for a wide range of source properties, observing conditions and GNIRS configurations. The ITC was used to generate a table of spectroscopic sensitivity estimates. The agreement between ITC predictions and commissioning results on the telescope is good to within 15% from 0.9-3.5um and TBD at longer wavelengths. As of June 2005, the ITC uses atmospheric transmission and emission spectra at the appropriate resolutions for GNIRS (including R=18000). The estimated image quality delivered to the instrument is given as part of the observing condition constraints .
Observing Strategies:	See the Observing Strategies page for guidance and special considerations for each observing mode. When you're done, don't forget to check the Checklist.
Phase II Preparations:	All Phase II preparations are done with the Observing Tool (OT). GNIRS-specific help, as well as sample observations, can be found on the GNIRS OT page.
Observing Overheads:	Acquisition onto the slit (comprising telescope slew, target acquisition and instrument reconfiguration) takes 15 minutes or less for bright targets (point source H < 15 mag) and ~20min for faint ones (point source H > 15 mag). In principle, "blind" acquisitions using a reference source (for H > 18.5 mag targets) take the same time as a normal acquisition, however as with all acquisitions, actual time depends on the complexity and the quality of the Phase II preparation (e.g., finding charts, helpful notes). The observing overhead (readout + telescope offsetting) is about 1 minute per nod (ABBA) cycle i.e. an overhead of 15 sec per position. Typical exposure times are a few to 20 minutes at shorter wavelengths or at high spectral resolution. For observation of bright targets and at thermal wavelengths, multiple exposures can be coadded at a given telescope position. See Observing Strategies for more on overheads. Readout overhead for different readmodes is given on the science detector page. These overheads are most significant for M-band observations, where readout is ~40% of maximum exposure time.
Target Acquisition:	GNIRS has a "flip-in" acquisition mirror to allow precise positioning of objects on the slit without moving the grating, prism or camera. The choice of acquisition filter is independent of the wavelength of the science observations. PIs must specify the filter in their acquisition observations. See the OT help for more information on defining GNIRS acquisition observations. Objects fainter than ~18.5 at H require "blind" acquisition. The PI must provide IR-visible reference star(s), one or two, that can be acquired with the same guide star as the science target, and with the same coordinate system as the science target. See "blind or offset acquisition" instructions. Acquisition observations must be defined by the PI in the Phase II definition. PIs should specify target brightness (and finding charts if needed) in the filter specified for acquisition, if possible.
Calibration:	Flat-fielding and wavelength calibration observations of GNIRS are done with the facility calibration unit. Basic calibrations for GNIRS (i.e. 1-3 or 3-5 um spectroscopic calibrations) are needed for all observations, queue or classical. These "baseline" calibrations are summarized in the context of all Gemini Near-IR instrument calibrations (i.e. 1-5 um imaging and spectroscopy). Flats, arcs and pinhole calibrations for many of GNIRS science modes can be carried out during morning or evening twilight. Exceptions include GNIRS IFU and R=18,000 (long slit with long camera) modes, which must be executed right before or after the science observation. In the case where twilight calibrations are taken, we reccommend that an arc lamp be taken at the time of the science data, to account for any small shifts caused when re-positioning the grating, though tests show that the grating can be re-positioned to within a few pixels in these modes. Whether these calibrations are completed during the night or during twilight, they will not be charged to the program, though nightime calibrations will be charged to the partner country. Check here for more details. Additional calibrations beyond the baseline set are charged to the program, except for twilight flats when needed in rare cases. Recommended flatfield and arc lamp configurations and exposure times are available by downloading OT library templates for your phase II. Additional links to photometric and spectroscopic standards along with wavelength calibration are available for planning. All calibrations, including baseline tellurics, flatfields and arcs, must be specified in the Phase II definition.
Data processing and software:	The GNIRS sub-package of the Gemini IRAF package is available. The latest release, v1.8, is a significant improvement over the initial release; GNIRS users are highly recommended to upgrade. This package is still under development; please report any problems to the Gemini helpdesk.

Last update July 5, 2006; G. Doppmann