NIRI f/6 Spectroscopic Sensitivity Estimates

Brightnesses of point and extended sources giving signal-to-noise ratios (S/N) of 5 per spectral pixel in a one-hour integration are provided in the table below for NIRI at f/6. They were estimated using the NIRI Integration Time Calculator (ITC) and are the mean S/N in the wavelength intervals given in the table. These values are for silver coatings on the telescope optics and NIRI on the side port.

Important caveats: The estimates are based on the Integration Time Calculator (ITC) and are eyeball average sensitivities 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. In addition, although airmass, water vapor and transparency are crudely included in the ITC, other sky conditions are not. 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 the results of the ITC with some caution and conservatism.

These sensitivities are for 70%-ile image quality, 50%-ile cloud cover (clear), 50%-ile water vapor column, and 80%-ile sky background at 1-2.5um (50%-ile at 3-5um), and are for an average airmass of <1.2.

Total throughputs for NIRI were estimated by taking into account all optical surfaces in the system, using measured (JHK) or assumed (LM) grism transmittances and and including the expected detector quantum efficiency. Additional thermal background is incident on the NIRI detector when using the current K, L, and M grisms, and this also also has been included. Slit widths are approximately the actual values.

It is assumed that the integration is made up of four 15-minute exposures, ABBA, i.e. two on-source, and two nodded along the slit for point sources or two nodded off-source for extended sources. For the thermal IR, even if the individual exposures are considerably shorter the same sensitivities will be achieved after one hour. In the J, H, and K bands, this will not be the case if the integrations are too short, as read noise will be a significant contributor.

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, Gemini's 70%-ile image quality with tip/tilt correction (e.g., a 50% encircled energy diameter of 0.55 arcsec at K), and an optimized length along the slit (which gives the highest S/N; this is ~1.4 times the EED).

For extended sources nodding to sky (completely off of the source) was assumed and the sensitivies are for 1 square arcsec. If the extended source is sufficiently compact, nodding along the slit will improve the sensitivity by sqrt(2). 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).

Sensitivities for slitless spectroscopy will be much (typically 10 times or more) lower than the lowest values given below.

Last update 2006 September 1; Andrew Stephens, Tom Geballe & Joe Jensen