LRS Response Function
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Relative Spectral Response Functions (RSRFs) will be ap- plied to LRS observations to spectrophotometrically calibrate the data. The absolute calibration of the MIRI (and the other JWST instruments) will be based on astronomical observations of very hot (white dwarfs and OB stars), hot (A-type), and warm (G-type [solar-like] stars) to quantify differences between stellar and model spectra and to control for systematic uncertainties in stellar atmosphere modeling. All of the proposed JWST calibra- tors have been observed using the Hubble and Spitzer Space Telescopes with high signal-to-noise ratios (S=N > 100). For a subset of these calibrators, the existing Hubble and Spitzer calibrations are consistent to within 2% (Bohlin et al. 2011). The predicted spectra for these calibrators are drawn from the CALSPEC database14 using models for White Dwarfs (Bohlin 1995) and measurements for A- and G-type stars (Rieke et al. 2008) at wavelengths shorter than 2:5 μm and models at longer wavelengths. From an intercomparison of the measurements, it will be possible to test and refine the RSRFs.
Although the optimal RSRFs can only be determined in orbit using astronomical standards, on-ground RSRFs have been de- termined using the MTS. This simulator incorporates a black- body source with adjustable temperature to produce different flux levels, as well as provision for both point and extended sources to be fed to the instrument. During the MIRI test cam- paign, LRS point source measurements were taken at different black body temperatures between 300 and 800 K using a 100 μm diameter pinhole (1.428 pixels on the detector). As ex- pected, the temperature dependence of the RSRF is negligible. After background subtraction, the LRS spectra were extracted from these slope images using aperture photometry, and the wavelength calibration applied (see § 3.4). Both slit and slitless spectroscopy modes were tested.
Figure 6 shows the RSRF for the 800 K measurement, rep- resenting the Spectral Response Function (SRF) normalized to 7 μm. The SRF is the ratio between the measured point source flux and the flux reaching the MIRI imager entrance focal plane. The latter is calculated with MTSSim, a software tool used to simulate the flux output of the MTS. The blue line in Figure 6 shows the RSRF for slit spectroscopy, and the green line shows the slitless spectroscopy measurement. The SRF values at 7 μm are approximately 5:5 × 104ðDN=sÞ=Jy for slit and 9 × 104ðDN=sÞ=Jy where 1 DN 1⁄4 5:5 e (Paper VIII). As the pinhole used for these measurements was not a perfect point source, the flux losses within slit measurements were larger than expected for a point source. Our slit measurements are therefore not representative for in-orbit point source slit spectroscopy. To get a proper SRF estimate for LRS slit observations we used the slitless measurements, corrected those using the LRS slit mask transmission profile and applied the expected slit throughput for a perfect JWST PSF (Paper IX). Note that the drop in RSRF around 8 μm is due to a discrepancy between the MTS model and true output; this will not be present in the in-orbit SRF. In addition, an empirical correction factor of 1/0.55 was applied to properly represent the real MTS flux output (Paper IX). This factor has been taken into account for both the RSRF and Pho- ton Conversion (see § 4.3) Efficiency calculations.