7.6 Sensitivity and Exposure Time Estimate

Plots illustrating the point source sensitivity of the IRS are given in the IRS chapter of the SOM and can also be viewed at

http://ssc.spitzer.caltech.edu/IRS/sens.html.

This information can also be obtained using the Spitzer Spectroscopy Performance Estimation Tool (SPEC-PET) available at

http://ssc.spitzer.caltech.edu/tools/specpet/.

Note, however, that the SPEC-PET only supplies the same information available from the IRS sensitivity plots in the SOM.

These point source sensitivity values can be converted to extended source sensitivities using the provisional relations (which yield results with uncertainty of $\approx20\%$) that are given in the IRS chapter of the SOM. The sensitivity plots show that a single, 60-second exposure will yield signal-to-noise ratio of unity ( ${\rm S/N} \approx 1$) in the SL module for a point source with a flux density of $\approx 0.2 \; {\rm mJy}$ at $10 \; \mu {\rm m}$; this converts to an extended source with a surface brightness of $\approx 0.49 \; {\rm MJy \; sr}^{-1}$ at $10 \; \mu {\rm m}$. Compared to the $12 \; \mu {\rm m}$ surface brightness of the Ring Nebula, we can estimate that an exposure of 60 seconds or 240 seconds would achieve a S/N value of approximately 5 or 10, respectively. (This S/N calculation correctly assumes that the target is in the faint source limit for 60- and 240-second exposures in the SL module, which is true for flux densities $\lesssim 10 \: {\rm mJy}$ or extended source surface brightnesses $\lesssim 25 \; {\rm MJy} \; {\rm sr}^{-1}$. See the IRS Staring Mode chapter of this Cookbook and Section 7.2.1 of the SOM v7.0 for more details.)

Unlike the IRS Staring Mode, in Spectral Mapping Mode only one spectrum (at one slit position) is obtained per exposure. However, the entire mapping cycle can be repeated (up to the 3-hour time limit on individual IRS AORs). For example, repeating the mapping over 2 cycles of 240-second exposures each would increase the S/N of the combined spectrum at each map position to $\gtrsim 14$ (as well as providing redundancy against cosmic ray hits). In general, it is strongly recommended to have at least 2 cycles per position to provide greater redundancy against varying pixels and cosmic rays. Note that the sensitivity values quoted here and in the SOM were calculated assuming the lowest possible IR background level (i.e., at the ecliptic poles). If we wanted to be very conservative, we might consider using medium backgrounds simply because of the ecliptic latitude. However, in this case, the difference between the medium and low background levels is negligible. In general, observations with the IRS, especially those obtained through the long, low resolution slits, will typically have to contend with higher background levels, resulting in reduction of the obtained S/N.