MIRI Bright Source Limits

Bright source limits of JWST's Mid-Infrared Instrument (MIRI) are predicted by a saturation model that uses measurements obtained during ground testing.

JWST sensitivity update

Preliminary results from observatory commissioning indicate that system throughput is higher than pre-mission expectations, by amounts that vary with wavelength. Until detailed results are available and updated in Pandeia, results calculated by the ETC and JIST tools will underestimate S/N. An announcement will be made when the tools are updated.

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The performance of MIRI 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, and (4) detector performance  (Pontoppidan 2016).

Users should ultimately use the Exposure Time Calculator (ETC) for all bright limit calculations.

The values in the table and plots below were obtained from ETC 1.5.1. The JWST ETC assumes a safety margin when calculating when saturation occurs to account for pre-flight uncertainties in the overall responsivity and throughput of the observatory and instrument. 

All modes

Figure 1. MIRI bright source limits for point sources (for Ngroups = 5)

MIRI bright source limits for point sources for all modes. The plotted values represent the flux densities at which the signal will reach 70% of full well in Ngroups of 5. All calculations assume FULL frame, except the LRS slitless mode, which exclusively operated in the SLITLESSPRISM subarray. Spectroscopic values are calculated per spectral pixel.


Table 1.MIRI imager bright source limits (for Ngroups = 5)

FilterFull Frame (mJy)SUB64 subarray (mJy)

Table 2.MIRI imager subarray bright source limits (for Ngroups = 5)

SubarrayBrightness limit
in F560W (mJy)
Brightness limit
in F2550W (mJy)











Low-resolution spectrometer (LRS)

Figure 2. Bright source limits for slitted LRS observations of a point source

Bright source, point source limits (calculated for the high background case only) for the MIRI low-resolution spectrometer slit observations. The minimum value is 63 milliJansky at λ = 5.5 μm. © Glasse et al.

Medium-resolution spectrometer (MRS)

Figure 3. Bright source limits for slitted MRS observations of a point source

Bright, point source source limits (calculated for the high background case only) for the MIRI medium-resolution integral field spectrometer. The minimum value is 3.8 Jansky at λ = 8 μm. © Glasse et al.


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 

Latest updates
    Updated plots and values for ETC v1.5.1

Originally published