# Introduction

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 which includes the following components: (1) background, (2) photon conversion efficiency (PCD), (3) encircled energy, (4) and detector performance.

# Calculation

For the imager, the factor _{bgd }in Equation 1 has a worst case in the F2550W filter, and the “high-background” case is estimated to be around 5,100 ^{–}^{-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 for the relevant encircled energy and sampling factors. The target spectral shape is modeled as a 5,000 K blackbody. The factor **\(
f_{br\_pix}\)**_{ }in Equation 1 describes the fraction of the total signal from a point target that will fall in the . For the imager, based on analysis of model PSFs, **\(
f_{br\_pix\_imager}\)** is set at 0.13 for λ ≤ 8 μm. At longer wavelengths, to allow for image dilution by diffractive broadening, the setting used is **\(
f_{br\_pix\_imager}\)** = 0.13 (8 μm/λ)^{2}.

**MIRI imager bright source limits **

Filter | Bright source limit (mJy) full frame | Bright source limit (mJy) 64 × 64 subarray |
---|

F560W | 13 | 420 |

F770W | 7.4 | 240 |

F1000W | 16 | 520 |

F1130W | 69 | 2250 |

F1280W | 29 | 950 |

F1500W | 37 | 1230 |

F1800W | 66 | 2200 |

F2100W | 66 | 2200 |

F2550W | 192 | 6400 |

## Low-resolution spectrograph (LRS)

For the LRS, where the PSF is co-added along the dispersion direction, **\(
f_{br\_pix\_lrs}\)** = 0.36 (the square root of the value used for the imager) for λ ≤ 8 μm. For λ > 8 μm, **\(
f_{br\_pix\_lrs}\)** = 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.

## Medium-resolution spectrograph (MRS)

For the MRS, **\(
f_{br\_pix\_imager}\)** 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.

# References

Glasse, A. et al. 2015, PASP, 127, 686

The Mid-Infrared Instrument for the James Webb Space Telescope, IX: Predicted Sensitivity

JWST technical documents