IRS: Sensitivity

Experience with IRS data has shown that residual spectral fringes may remain even after flat-fielding during the standard pipeline processing. While this is most likely to occur in SH and LH data, fringes can also be present in LL data.

Signal-to-noise estimates based on the sensitivity curves in the SOM (which are used in the online SPEC-PET tool) assume an "ideal" spectrum that is not affected by signal degradation caused by cosmic ray damage to the detectors, and other factors, such as flat errors and residual fringes in the spectrum. These factors limit the largest S/N reachable in the continuum to ~100 for the low resolution modules and ~50 for the high resolution ones.

The pipeline flat-fielding removes the majority of spectral fringes. However, if periodic features are apparent in some orders of flat-fielded data, then observers searching for faint features should attempt to defringe those orders before measuring line strengths or estimating upper limits.

In all of the figures shown in this section, it is assumed that the source is in the faint source limit (i.e., for a selected exposure time, the target flux is BELOW the relevant line shown in the Figure below). If the source is bright and lies above the line, then the curves shown in the following figures will significantly OVERESTIMATE the expected signal-to-noise ratio. A simple scaling rule is given in the SOM that can be used to convert the expected S/N ratio based on the (faint source) sensitivities to a more realistic value for bright sources.

As part of the normal maintenance plan for the IRS, the detector bias levels will occasionally be adjusted. Each change in the bias level will change the module sensitivities, and will affect the long wavelength modules more than the short wavelength modules. Refer to the this website for the most up-to-date IRS sensitivity information.

In order to reduce the number of unreliable pixels (rogues) in the LH detector, the bias voltage was changed from 2 to 1.6 volts on 10 October 2005. This reduced the sensitivity of the detector by ~30%. On the other hand, the number of rogue pixels was reduced by a factor of 2 to 3, resulting in cleaner spectra. The plots have been updated here with the best knowledge as of 19 Oct 05.

1-sigma continuum sensitivity (in the faint source limit) of all 4 modules in a 512 second integration time at 0 degrees ecliptic latitude.

1-sigma continuum sensitivity (in the faint source limit) of all 4 modules in a 512 second integration time at 40 degrees ecliptic latitude.

Sensitivities at AOT integration times

Extended Sources

Bright Sources

The low resolution modules rapidly reach an asymptotic BSL in a few tens of seconds of integration time, whereas the high resolution modules take longer to reach this plateau. Sources that have fluxes which lie below the curves plotted in the Figure for a given integration time are in the faint source limit (i.e., they are background or read noise limited), whereas sources that have fluxes which place them above the line for a given module have signal-to-noise ratios (S/N) that are dominated by the source flux. Knowledge of the regime in which a source lies is crucial to calculating a realistic S/N estimate needed to plan an IRS observation. Note that for all allowed AOT integration times of the low resolution modules, sources brighter than 130 mJy will always be source- dominated. For all allowed AOT integration times of the high resolution modules, sources brighter than 620 mJy will always be source-dominated.

In the SOM, we provide a simple scaling rule that allows the original estimate of the signal-to-noise ratio for faint sources obtained through the use of the figures above to be corrected for the contribution of the shot noise from a bright source.

Bright Source Limit (BSL) as a function of integration time for each of the IRS modules. Targets with fluxes above the curves for a given integration time are source-dominated; targets with fluxes below the curves are read noise or background limited. The plotted points on the curve for each module indicate the allowed AOT durations for each module.

Peak-Up Imaging

PUI 1-sigma sensitivity (microJy) for 6/14/30 second ramp times

Background levels

Also see Background estimates for use in sensitivity plots, common to all instruments.

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