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GNIRS Integration Time Calculator (Spectroscopy only)

Home » Sciops » Instruments » GNIRS » ITC, Sensitivity and Overheads

IMPORTANT NOTES:

  • 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").

  • Wide slit spectroscopy at JHK: The ITC currently does not properly take into account the widening of unresolved sky emission lines in GNIRS spectra when slits are wider than 2 pixels (0.30 arcsec with the short blue camera and 0.10 arcsec with the long blue camera). This results in the S/N calculated by the ITC being too high at pixels adjacent in wavelength to OH lines, which are prevalent in the J and H bands and in the K band out to ~2.20µm. There is no simple prescription for accounting for this. For observations with the short blue camera with either the 0.675 or 1.0 arcsec slit, or with the long blue camera with either the 0.20 or 0.30 arc sec slit we advise that one double the total exposure time (i.e., the integration time) that the ITC gives to produce the required S/N.

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

  • Currently the read mode buttons in the ITC don't do anything; instead the read mode is selected according to the exposure time. For exposure times of 1 sec or less the ITC uses the "high background" read mode, exposures of 1- 20 seconds assume the "medium background" read mode, those of 20 - 60 seconds assume the "low background" read mode and exposures of more than 60 seconds assume the "very low background" mode. For more information see the GNIRS read modes table.

  • The ITC does not yet work with either the 10 l/mm grating and cross dispersion or the 111 l/mm grating and cross dispersion. If you intend to use cross dispersion with either of these combinations, enter "no" in the cross dispersion box and select the appropriate slit width and central wavelength in each of the J,H, and K bands separately. Determine the exposure time needed to reach the desired S/N in each band and from that, the time required to obtain the cross dispersed spectrum at the desired S/N (which should be the time for the band with the longest individual time estimate).



    • 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.

    If you can read this but the form below does not appear then you may need to check with your system administrator about allowing communication with http://itc.gemini.edu:9080 through your firewall.