NIRI Imaging Sensitivity Estimates

Using the NIRI Integration Time Calculator (ITC) we have estimated the brightnesses of both point and uniform surface brightness sources that give a signal-to-noise ratio (S/N) of 5 in a one-hour integration.

Important caveat: These estimates make assumptions about the telescope, camera, and detector performance. Airmass, water vapor and transparency are crudely included in the ITC, but 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.

The details of how the ITC calculates limiting magnitudes is described in the "more info" links accessible from the ITC. ITC help pages include information on the filter zero points, camera and detector properties, and calculation methods.

The ITC includes adjustments for observing conditions in its calculations. For the tables presented here, we assumed 70-percentile image quality for all categories. The meanings of the observing condition criteria are explained in detail in the Observing Conditions ITC help pages. We also assumed photometric conditions (50-percentile cloud cover) an airmass of less than 1.2 for the sensitivites presented here. At thermal wavelengths (3 to 5 microns) we adopted median water vapor and background conditions; for shorter wavelengths we used "any" and 80-percentile for water vapor and background conditions because they have very little impact on the near-IR sensitivity.

Regarding readout noise and detector well depth, note that:

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

The estimates below are for exposures that are background-limited in all cases. As a result, 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 additional adaptive optics correction. Because of variations in the background levels in the thermal IR with weather and airmass, observations at L band and M band use the f/32 camera to prevent saturating on the sky background.

All magnitudes listed below result in a S/N of 5 in a total integration time of 1 hour. Individual exposures fill approximately 50% of the full well (180 sec in the H2 1-0 narrow band filter, 120 sec at J, 60 sec at H and K, and 1 sec at L' and M'). Observations are assumed to be dithered (both for pointlike and extended sources) and sky subtraction noise is included. All individual exposures include the source.

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. Note that for accurate photometry much larger apertures usually must be used, and sensitivities are reduced, probably by a factor of 2 or more.

Approximate total throughput values in the table are as measured for the entire system, including the telescope, instrument, and detector. These values are for silver coatings on the telescope optics and NIRI on the side port. The ITC now defaults to the current silver mirror coatings.

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