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

 


Calculation

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 Users should ultimately use the Exposure Time Calculator (ETC) for all saturation/sensitivity calculations. However, the origin of these calculations is described below.

 


Imager

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 

LaTeX Math Inline
bodyf_{br\_pix}
 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, 
LaTeX Math Inline
bodyf_{br\_pix\_imager}
 is set at 0.13 for λ ≤ μm. At longer wavelengths, to allow for image dilution by diffractive broadening, the setting used is
LaTeX Math Inline
bodyf_{br\_pix\_imager}
 = 0.13 (μm/λ)2. 

 

Table 1. MIRI imager bright source limits 

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FilterBright source limit (mJy)
full frame
Bright source limit (mJy)
64 × 64 subarray
F560W13420
F770W7.4240
F1000W16520
F1130W692250
F1280W29950
F1500W371230
F1800W662200
F2100W662200
F2550W1926400
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Figure title

Figure 1. MIRI imager bright source limits

MIRI imager bright source limitsImage Modified

Figure caption

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,

LaTeX Math Inline
bodyf_{br\_pix\_lrs}
 = 0.36 (the square root of the value used for the imager) for λ ≤ μm. For λ μm, 
LaTeX Math Inline
bodyf_{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 SLITLESSPRISMsubarray with its 0.16 s frame time is used, then the limits should be increased by a factor of 17. 
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Figure 2. LRS bright limits for "Fast" mode full array readout

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Figure caption

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,

LaTeX Math Inline
bodyf_{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.
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Figure title

Figure 3. Bright source limits for the MIRI medium-resolution integral field spectrometer for "FAST" mode

Image Modified

Figure caption

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) 

 


 

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Related links

JWST User Documentation Home
MIRI Imaging
MIRI Low Resolution Spectroscopy
MIRI Medium-Resolution Spectroscopy 
MIRI Detector Readout Fast
MIRI Detector Readout Slow 
JWST Exposure Time Calculator
JWST ETC website 

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References

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glasse
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Glasse, A. et al. 2015, PASP, 127, 686
The Mid-Infrared Instrument for the James Webb Space Telescope, IX: Predicted Sensitivity

JWST technical documents

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Last updated

Published December 30, 2016


 

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Updated December 30, 2016
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