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NIRI Sensitivity Tables (no AO)

The values in these tables are based on results from the NIRI Integration Time Calculator. They assume 70-percentile (IQ70, ~average) image quality for all categories, photometric conditions (CC50), and an airmass of less than 1.2 for the sensitivites presented here. At 3 to 5 microns median water vapor (WV50) and shorter wavelengths "any" water vapor were assumed; water vapor has little impact on the near-IR imaging sensitivity, or on spectroscopic sensitivity in the middles of the J, H, and K windows, but does significantly effect the edges of the JHK windows where absorption by telluric water vapor is strong. The estimates also use the measured telescope, camera, and detector performances. Note that some sky conditions vary outside of those stated above. For example, OH emission, although scaled by airmass, can intrinsically vary by over a factor of two from night to night and over much shorter timescales - which can alter S/N and sensitivity by +/- sqrt2 in the J, H, and K bands. Thus observers should use both the table and the ITC with some caution and conservatism.

Regarding readout noise and detector well depth, note that:

  • 1-2.5um narrow band faint source imaging and spectroscopy used the shallow well and low read noise mode
  • JHKsK'K imaging used the shallow well and medium read noise mode
  • All thermal IR (3-5um) imaging and spectroscopy used the deep well and high read noise mode

Filter Center wavelength (µm) Point Sources Extended Sources Individual Exposure (sec) Approx. Total Throughput
(mag) (mJy) (mag/arcsec2) (mJy/arcsec2)
J 1.25 23.5 7E-04 23.6 6E-04 600 29%
Jcont 1.207 22.2 2.2E-03 22.5 2.0E-03 600  
H 1.65 22.5 1.1-03 22.6 1.0E-03 60 37%
[FeII] 1.644 21.1 4E-03 21.2 3.5E-03 600  
K 2.20 22.6 0.6E-03 22.7 0.6E-04 120 43%
H2 1-0 S(1) 2.122 21.5 1.7E-03 21.6 1.5-03 600  
L' 3.76 17.0 3.7E-02 17.1 3.3E-02 2.0 28
M' 4.68 14.4 2.9E-01 14.3 2.6E-01 2.0 23%

Imaging Notes

  • The estimates in the table are for exposures that are background-limited in all cases. As a result, in imaging mode there is very little difference in achieved sensitivities between the different cameras. The f/6 camera provides the best sampling and field-of-view for tip-tilt corrected images without adaptive optics correction. Because of the high and varying (with weather and airmass) background levels in the thermal IR , observations at L band and M band use the f/32 camera to prevent saturating on the sky background.
  • For imaging, the ITC calculates the S/N in an aperture that maximizes S/N, given the predicted image quality for the observing conditions and wavelength requested. The sensitivity values in the table below use this optimum ITC aperture. For accurate photometry much larger apertures usually must be used, and sensitivities are reduced, typically by a factor of 2 or more.
  • Approximate total throughput values in the imaging table are as measured for the entire system, including the telescope, instrument, and detector.

Band Wavelength Interval (um) slit width (arcsec) Point Sources: Sens/pix Extended Sources: Sens/pix
(mag) (mJy) (W/m2) (mag/arcsec2) (mJy/arcsec2) (W/m2/arcsec2)
J 1.15-1.25 0.23 18.1 0.09 4E-19 18.8 0.05 1.6E-19
0.46 18.7 0.05 2.3E-19 19.1 0.04 1.6E-19
0.70 19.0 0.04 2.3E-19 19.2 0.034 1.9E-19
H 1.6-1.7 0.23 18.1 0.06 7E-20 18.9 0.029 3.2E-20
0.46 18.6 0.04 8E-20 18.9 0.029 6E-20
0.70 18.7 0.035 1.2E-19 18.9 0.029 1.0E-19
K 2.15-2.25 0.23 17.4 0.07 8E-20 18.1 0.04 4E-20
0.46 17.8 0.05 9E-20 18.2 0.034 6E-20
0.70 18.0 0.04 1.1E-19 18.1 0.04 1.0E-19
L' 3.6-3.8 0.23 11.9 4 3.1E-18 12.4 2.7 2.0E-18
0.46 12.3 3.0 1.8E-18 12.4 3.5 3.2E-18
0.70 13.0 1.6 2.7E-18 12.4 2.7 5E-18
M 4.6-4.8 0.23 10.1 15 9E-18 10.5 10 6E-18
0.46 10.4 11 9E-18 10.5 10 9E-18
0.70 10.6 9 1.3-17 10.6 9 1.3E-17

Spectroscopy Notes

  • Estimated sensitivities are eyeball averages over the specified wavelength intervals. Within each IR band (and indeed within each of these narrow intervals) the sensitivity is a very strong and rapid function of wavelength. The ITC should be used to estimate the signal-to-noise ratio at the precise wavelengths of interest and/or over the entire band of interest.
  • The sensitivity per resolution element (which is 2 or more pixels depending on the grism and slit width) is better than the sensitivities in this table by the square root of the number of pixels per resolution element.
  • Brightnesses for continuum sources are given in broad band magnitudes and in flux densities (mJy). Fluxes for unresolved lines are given in W/m2; for resolved lines these are fluxes per resolution element.
  • For point sources, the calculation assumes nodding along the slit and an optimized length along the slit (which gives the highest S/N in the extracted spectrum. For extended sources nodding to sky (completely off of the source) was assumed and the sensitivies are for 1 square arcsec. Thus, if S/N=5 in a smaller aperture than 1 square arcsec is needed, a longer integration time is required (e.g., four times as long for S/N=5 in 1/4 square arcsec).If the extended source is sufficiently compact, nodding along the slit will improve the sensitivity by sqrt(2).
  • Sensitivities for slitless spectroscopy will be much (typically 10 times or more) worse than the values given in the table.