MIRI Sensitivity
The sensitivity model for JWST's Mid-Infrared Instrument (MIRI) incorporates ground testing measurements to model parameters such as background, photon conversion efficiency, encircled energy, and detector performance.
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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). The results of the JWST throughput measurements have been integrated into a sensitivity model that includes the following components: (1) background, (2) photon conversion efficiency (PCE), (3) encircled energy, and (4) detector performance (Pontoppidan 2016).
The values in the table and plots below were obtained from ETC 3.0.
Values
The tables and figures below show point source sensitivity values for a 10 ksec (10,000 s) observation to S/N = 10.
Table 1. Minimum detectable flux densities for a S/N = 10 in 10 ksec for MIRI imaging filters assuming FULL array, assuming an unresolved point source.
| Filter | Low Background Detection Limit (μJy) |
|---|---|
| F560W | 0.12 |
| F770W | 0.24 |
| F1000W | 0.48 |
| F1130W | 1.09 |
| F1280W | 0.88 |
| F1500W | 1.21 |
| F1800W | 2.23 |
| F2100W | 3.99 |
| F2550W | 12.2 |
MIRI point source sensitivity plot for the medium-resolution spectrometer (MRS), low-resolution spectrometer (LRS), and imager (in black circles) configurations. Values plotted represent the minimum detectable signal corresponding to S/N = 10, in 10,000 seconds on-source integration time. For spectroscopic modes the numbers represent continuum sensitivities, with S/N calculated per spectral pixel.
Note: Sensitivity plots are based on in-flight measurements and based on ETC 3.0. The ETC should always be used to obtain the most up-to-date numbers for science programs.
MRS time dependent sensitivity
The MRS photometric calibration is additionally complicated by a time-dependent evolution in the effective count rate registered by the instrument that was observed during the first year of operations (see JWST Observer). The time-dependent evolution is taken into account in JWST Exposure Time Calculator released for Cycle 3. A detailed description of the MRS calibration and improvements in the pipeline to account for the time-dependent evolution is given on the JWST MIRI MRS Pipeline Caveats page.
References
Glasse, A., et al., 2015, PASP, 127, 686
The Mid-Infrared Instrument for the James Webb Space Telescope, IX: Predicted Sensitivity
Pontoppidan, K. 2016, Proc of SPIE, 9910, 16
Pandeia: a multi-mission exposure time calculator for JWST and WFIRST