IRAC: Sensitivity

Each background estimate is a combination of the zodiacal light, interstellar medium, and cosmic background radiation. A short integration is read-noise limited, so it is essentially independent of background brightness. For estimating the sensitivity of your observations, choose the background most appropriate. The background at IRAC wavelengths is dominated by zodiacal light. So choose low background if your observation is near the ecliptic poles (absolute ecliptic latitude greater than 60), choose high background if it is in the ecliptic plane (absolute ecliptic latitude less than 30), and for all others, choose medium background.

We do not include "confusion noise" (due to overlapping images of distant galaxies or other sources of background structure) in the tables of sensitivity estimates. The confusion limit, based on the source counts from Franceschini et al. (1991), gives the level of fluctuations due to overlapping images of faint galaxies. It is shown in the figures as a dashed horizontal line.

The detectors are assumed to perform according to the IRAC detector measurements of read noise, dark current, and quantum efficiency. The first 5 rows in each table show the sensitivity for full-array readouts, and the last three rows show the sensitivity for subarray readouts.

Useful additional information

• Memo on IRAC signal-to-noise ratio calculation
• Sensitivity Performance Estimator Tool for IRAC and MIPS
• Background estimates for use in sensitivity plots, common to all instruments.
• Spitzer Observer's Manual (SOM), specifically the IRAC chapter.

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