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Overview: A "Spectroscopy Performance Estimation Tool" (SPEC-PET). For user configured Spitzer/IRS + MIPS SED observing parameters, the SPEC-PET returns an estimate of the instrument sensitivity and an estimate of the signal-to-noise (for IRS only) for user supplied source continuum flux density, wavelength.
Input: choose an IRS mode, configure observing parameters and select wavelength for which you require a sensitivity estimate. Specify a source continuum flux density at that fiducial wavelength. For MIPS SED, select exposure time and number of cycles.
Output: sensitivity for the selected observing mode and an estimate of the signal-to-noise (for IRS only) for user supplied source continuum flux density, wavelength. For MIPS SED, the output is estimated sensitivity at 3 wavelengths, spanning the SED passband: 60, 75, and 90 microns.
IRS: Choice of modes, allowed ramp duration and number of cycles. For detailed information, see the IRS instrument description webpage, the IRS AOTs overview, and the Spitzer Observer's Manual:
User input: background level, mode, ramp duration and number of repeats. Also wavelength for sensitivity estimate, and point source continuum flux density (at the wavelength specified) for the signal-to-noise estimate.
MIPS SED: select exposure time, and number of cycles
IRS SENSITIVITY at selected wavelength: The instrument sensitivities have been pre-calculated as a function of mode and ramp duration, for a low and high zodical light background levels. Details are given on the IRS sensitivity webpage: http://ssc.spitzer.caltech.edu/irs/sens.html. This tool reports the sensitivities, given graphically, in the Spitzer Observer's Manual (SOM) and the instrument webpages. See Section 7.2.1.2 of the SOM v8.0.
For the high resolution modules, we report the point source sensitivity as the 1-sigma staring point source continuum sensitivity (PSSC), in mJy, at the full resolution of the spectrograph. We also report the 1-sigma point source sensitivity (PSSL), in W/m^2, to an unresolved emission line.
For the low resolution modules, we report the sensitivity as the 1-sigma staring point source continuum sensitivity (PSSCS), in mJy, after smoothing to a resolution of R=50.
For the 1-sigma staring extended source sensitivities per pixel for continuum and line emission (ESSC, ESSL), we use the rough conversion from point source sensitivities, as described in SOM v8.0, section 7.2.1.2.1.
Signal-to-Noise Estimate: The signal-to-noise calculation applies the shot noise correction, described in Section 7.2.1.2.2 and Figure 7.24 of the Spitzer Observer's Manual, v8.0, as follows: in the faint-source limit, the signal-to-noise tends to: S/N_faint = signal / sensitivity. To correct for shot noise, this is corrected to S/N = S/N_faint / sqrt(1 + source flux/bright source limit), where the bright source limit is given in the SOM v8.0 Figure 7.24 for low background. This correction is applied to all sources, regardless of the source flux density.
Caveats with sensitivity and signal-to-noise estimates: Sensitivity and signal-to-noise estimates based on the sensitivity curves in the SOM (and used in the SPEC-PET tool) assume an "ideal" spectrum that is not affected by signal degradation caused by extraction, cosmic ray damage to the detectors, and other factors, such as fringes in the spectrum. Observationally, the largest continuum signal-to-noise reachable with the low-resolution modules is ~100; for the high-resolution modules it is ~50. Higher limits may be possible if the observer corrects the spectra for fringing.
Warning: For a single cycle of the IRS Staring mode AOT, two exposures are obtained, with the source dithered along the slit. The sensitivity quoted here 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 1/(square root of the number of cycles). These scalings assume optimal co-addition of the images. n.b. The sensitivities given in the SOM (section 7.2.1.2) are for a single exposure.
| Mode | Definition |
|---|---|
| SH ("Short-High") | The IRS Hi Resolution 10.0-19.5 micron module. |
| LH ("Long-High") | The IRS Hi Resolution 19.3-36.9 micron module. |
| SL2 ("Short-Low 2nd order") | The IRS Low Res 5.2-8.7 micron 2nd order module. |
| SL1 ("Short-Low 1st order") | The IRS Low Res 7.4-14.0 micron 1st order module. |
| LL2 ("Long-Low 2nd order") | The IRS Low Res 20.6-40.0 micron 2nd order module. |
| LL1 ("Long-Low 1st order") | The IRS Low Res 14.0-21.3 micron 1st order module. |
| Sensitivity | Definition |
| PSSC | The 1-sigma staring point source continuum sensitivity at full resolution of the spectrograph. [PSSC] = mJy. |
| PSSCS | The 1-sigma staring point source continuum sensitivity after smoothing to a resolution of R=50. [PSSCS] = mJy. |
| PSSL | The 1-sigma point source sensitivity to an unresolved emission line. [PSSL] = W/m2. |
| ESSC | The 1-sigma staring extended source sensitivity per pixel for continuum. [ESSC] = MJy/sr. |
| ESSL | The 1-sigma staring extended source sensitivity per pixel for line. [ESSC] = W/m2/sr. |
Over most of its range, the SED mode will achieve in 500 seconds of integration a 5-sigma detection of about 82 mJy at 60 microns, about 201 mJy at 75 microns, and about 447 mJy at 90 microns. The SENS-PET returns sensitivities at wavelengths of 60, 75 and 90 microns.