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GNIRS ITC (Spectroscopy only)


  • Observations with Adaptive Optics: Adaptive Optics is not yet available in the GNIRS ITC. When planning an observation of a point-like object using AO in either a long slit or a cross-dispersed configuration, with 0.05" pixels: (1) select a Point source brightness that is 1.4 mag brighter at K or 2.0 magnitude brighter at H than the target's actual values; (2) in the Instrument optical properties section entered the required GNIRS configuration, and (3) in the Analysis method select a small "Software aperture of diameter (or slit length)" (e.g., 0.10 or 0.15 arcsec; do not select "Software aperture that gives 'optimum' S/N ratio").

  • For discussion of the effect of slit width on sensitivity in the JHK bands see here.

  • No choice of well depth is available in the ITC. Currently the ITC selects the well depth depending on the exposure time. For exposure times > 1 second it chooses the shallow well; for exposures of 1 second or less it assumes the deep well. This could result in confusion about when the array saturation level is reached (e.g. on a bright star) if the PI selects a different well depth in the OT than used by the ITC. However, it does not affect S/N calculated by the ITC. It is planned to make the we'll depth selectable in a future version of the GNIRS ITC.

  • In cross dispersed mode the ITC works with the 111 l/mm grating, but currently only if the "Spectrum central wavelength" is set to a K band wavelength. For example, rather than setting that wavelength to 1.65 um (H band, order 4) the user should set it to 4/3 x 1.65 = 2.2 um (K band, order 3), etc.

  • The atmospheric transmission and extinction files used in the ITC only extend down to 0.9 um. The ITC plots therefore artificially cut off at 0.9 um, although the instrument itself is somewhat sensitive to shorter wavelengths (but dropping off rapidly). The Sensitivity Table gives approximate sensitivities for orders 7 (0.93um), 8 (0.83um), and 9 (0.75um) for the 32 and 111 l/mm gratings and short camera only, as those orders are automatically observed in cross-dispersed mode.

  • In the four sections of this form, select the appropriate astronomical source, instrument, telescope, observing condition and observation parameters. Click on the calculate button (calculate button) at the bottom of any section to submit the parameters from all the sections to the server or the reset button (reset button) to reset all parameters to their defaults. The results are reported in a separate web page that can be resized and printed.

    General guidance on use of the Integration Time Calculators (ITCs) is available; specific help on each section of the form is available by following the more info and other (blue highlighted) links. Note: at this time Netscape 6 and 6.01 do not work with web forms. Changes, enhancements and bugs within the ITC are listed chronologically.

    Note that high values of signal-to-noise ratio (exceeding several hundred) are usually not realistic, as the values actually achieved depend on many factors besides those included in the ITC. See here for further information.

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