6.7 Sensitivity and Exposure Time Estimate

The sensitivity of the IRS is a function of the background which at these wavelengths is primarily from the cirrus and zodiacal light. The background for any target can be obtained as a function of observation date in Spot, by entering the target and then clicking ``Background'' in the target window. Plots illustrating the sensitivity of the IRS for high, medium and low background are given in the IRS chapter of the SOM and can also be viewed at

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

High background corresponds to positions close to the ecliptic plane; low background, to positions close to the ecliptic pole; and medium background, to intermediate ecliptic latitudes. See also

http://ssc.spitzer.caltech.edu/obs/bg.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. Note also the following: For each cycle of the IRS Staring mode AOT, two exposures are obtained. The sensitivity quoted by the SPEC-PET assumes that both exposures have been combined, and that the sensitivity of the combined image has scaled as $1/\sqrt{2}$ times the sensitivity of the individual images. Similarly, for multiple cycles, the sensitivity quoted assumes that the sensitivity of the combined image has scaled as the 1/(square root of the number of cycles). The sensitivities given in the SOM (section 7.2.1.2) are for a single exposure. The sensitivity plots in the SOM show that a single, 120-second exposure will yield signal-to-noise ratio of unity ( ${\rm S/N} \approx 1$) in the SH module for an unresolved emission line having flux of approximately $6.0 \times 10^{-19} \; {\rm W \; m}^{-2}$ at $15 \; \mu {\rm m}$. Similarly, a 240 sec exposure will provide ${\rm S/N} \approx 1$ in the LH module for an unresolved emission line having flux of approximately $7.0 \times 10^{-19} \; {\rm W \; m}^{-2}$ at $25 \; \mu {\rm m}$. By comparison with observations of Seyfert galaxies, the emission features in our target are expected to have fluxes of $\approx 5 \times 10^{-18} \; {\rm W \; m}^{-2}$ each. Using exposures of 1 $\times$ 120 seconds and 1 $\times$ 240 seconds for the SH and LH slits, respectively, and recalling that each cycle in staring mode automatically obtains two spectra with the requested exposure time (per slit at two slit positions), results in S/N$\approx$12 in each of the combined spectra from SH and LH for the target line fluxes. Despite the adequate S/N of 1 cycle, we strongly recommend decreasing the ramp time and having a minimum of two cycles for each slit. This provides greater redundancy against cosmic rays and variability in the pixels. Strictly speaking, these signal-to-noise estimates, like the IRS sensitivity curves in the SOM, assume that the target is in the faint source limit (i.e., the shot noise from the target is negligible compared to other sources of noise). For a target in the Bright Source Limit (BSL), we would have to also consider the shot noise of the target itself. In this example, however, the expected line fluxes are well below the BSL for the IRS high resolution modules (see the SOM for IRS sensitivity curves and examples of calculating S/N for faint and bright targets). Observers should also be careful that the possibility of anomalously high dark values in the first 100-200s of LH exposure do not affect the quality of their data. This is described in further detail on

http://ssc.spitzer.caltech.edu/irs/timedepdark.html.