The performance of JWST has been measured directly at both the component and system level using the fully assembled MIRI flight model in a flight-like radiative environment with a well characterized radiometric source (Glasse et al. 2015). Results of these JWST throughput measurements have been integrated into a sensitivity model, which includes the following components: (1) background, (2) photon conversion efficiency (PCD), (3) encircled energy, (4) and detector performance.
Users should ultimately use the Exposure Time Calculator (ETC) for all saturation/sensitivity calculations. However, the origin of these calculations is described below.
For the imager, the factor ibgd in Equation 1 has a worst case in the F2550W filter, and the “high-background” case is estimated to be around 5,100 e– s-1. The factor g
(the end-to-end gain from astronomical target to photocurrent)
in Equation 1 is calculated as a by-product of the sensitivity model using the photon conversion efficiency (PCE) curves and accounts for the relevant encircled energy and sampling factors. The target spectral shape is modeled as a 5,000 K blackbody.
The factor in Equation 1 describes the fraction of the total signal from a point target that will fall in the brightest pixel. For the imager, based on analysis of model PSFs, is set at 0.13 for λ ≤ 8 μm. At longer wavelengths, to allow for image dilution by diffractive broadening, the setting used is = 0.13 (8 μm/λ)2.
Table 1. MIRI imager bright source limits
|Filter||Bright source limit (mJy) |
|Bright source limit (mJy) |
64 × 64 subarray
Figure 1. MIRI imager bright source limits
MIRI imager bright source limits for both full and SUB64 arrays.
Low-resolution spectrograph (LRS)
For the LRS, where the PSF is co-added along the dispersion direction, = 0.36 (the square root of the value used for the imager) for λ ≤ 8 μm. For λ > 8 μm, = 0.36 (8 μm/λ)2. A plot of the resulting bright limits for LRS are shown in Figure 2 for "FAST" mode full array readout. As for the imager subarrays, if the “SLITLESSPRISM” subarray with its 0.16 s frame time is used, then the limits should be increased by a factor of 17.
Figure 2. LRS bright limits for "Fast" mode full array readout
Bright source limits (calculated for the high background case only) for the MIRI low-resolution spectrometer for "FAST" mode full array readout. The minimum value is 63 mJy at λ = 5.5 μm.
Medium-resolution spectrograph (MRS)
For the MRS, is scaled by the ratio of the solid angle viewed by each MRS pixel to the solid angle viewed by an imager pixel. The resulting bright source limits are plotted in the Figure 3 for "FAST" mode. Note, however, that the nominal MRS readout pattern currently uses "SLOW" mode. If "FAST" mode is not implemented for the MRS on orbit, the MRS bright source limits presented here should be reduced by a factor of ~10.
Figure 3. Bright source limits for the MIRI medium-resolution integral field spectrometer for "FAST" mode
Bright source limits (calculated for the high background case only) for the MIRI medium-resolution integral field spectrometer for "FAST" mode. The minimum value is 3.8 Jy at λ = 8 μm. (Glasse et al. 2015)