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).

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


The values in the table and plots below were obtained from ETC 4.0.


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.

FilterLow Background Detection
Limit (μJy)
F560W0.13
F770W0.24
F1000W0.46
F1130W1.02
F1280W0.83
F1500W1.18
F1800W2.42
F2100W4.70
F2550W15.3

Figure 1. Expected point source sensitivity for the various MIRI instrument modes

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 on ETC 4.0. 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 MIRI MRS Calibration Status page.


Imager time dependent sensitivity

The MIRI imager experiences a time-dependent reduction in observed count rates at wavelengths longward of F1280W; this count rate loss is similar to the MRS, but of a significantly smaller magnitude. More details on this can be found on the MIRI Imaging Calibration Status page. This count rate loss is included in ETC 4.0.


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 

 


Notable updates
  •  
    Updated table and plot based on ETC 4.0

    • Added section on imaging time dependent sensitivity

  •  
    Updated based on in-flight measurements
Originally published