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.

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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. 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.

Bold italics style indicates words that are also parameters or buttons in software tools (like the APT and ETC). Similarly, a bold style represents menu items and panels.


Imager


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

FilterFull Frame (mJy)SUB64 subarray (mJy)
F560W4.07133.08
F770W3.58116.99
F1000W7.83255.69
F1130W33.121081.27
F1280W13.67446.53
F1500W17.59574.27
F1800W31.801038.17
F2100W31.881041.11
F2550W95.493117.56

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

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

FULL

4.0795.49

BRIGHTSKY

13.07306.35

SUB256

37.70883.31

SUB128

95.062226.83

SUB64

133.083117.56

Low-Resolution Spectrometer (LRS)



Medium-Resolution Spectrometer (MRS)


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 




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