MIRI Coronagraphic Performance

The performance of the MIRI flight coronagraphs was tested and verified in the flight model (FM) imager prior to its delivery to be integrated with the full MIRI Optics Module (OM).

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MIRI treatment of limiting contrast, Climit(s)

Climit(s) is the limiting contrast, which is the detection limit and the best that can be done. It is the companion-to-host flux ratio of the minimum detectable companion. Climit(s) is a function of essentially everything HCI related—myriad eclectic technical factors and procedures, end-to-end.

Figure 1 shows the best available treatment of various contrasts for MIRI's 4QPM2 coronagraph, as adapted from Boccaletti et al. (2015).

The "technical factors" behind the curves include:

  • 4QPM2 inner working angle (IWA), filter, and nominal wavelength ( 0.49", F1140C, 11.30 μm) 
  • Stellar distance and spectral type of host source (10 pc, M0V)
  • Exposure time (3,600 s)
  • Telescope area and transmission (25 m2, 85%)
  • Detector quantum efficiency and noise (80%, readout 20 e rms, 0.001 flat field stability)
  • Lyot stop transmission (62% for 4QPM)
  • A random positional error of 10 mas and a wavefront error of 10 nm between rolls.
  • Reference star subtraction strategy
  • False -alarm probability of 3 × 10–3 ("three-sigma"), which assumes normally distributed errors with zero mean after reference-star subtraction.
  • Currently available estimates of JWST aberrations.

For the MIRI 4QPM2 coronagraph, the approximate minimum contrast ratio for 3-sigma detection of a faint companion near a bright host, as a function of their apparent separation s in arcseconds.

Boccaletti et al. (2015) also present variants of 3 particular technical assumptions: the 4QPM coronagraph (one of 3), filter, and the spectral type of the host source.

If the user's operating point (s, Cflux) lies above the red-dashed curve, that source is detectable under the technical and procedural assumptions of Boccaletti et al. (2015).

This is the best information on limiting coronagraphic performance for MIRI at the current time. In the future, when better understandings of wavefront errors and other technical factors have improved—or when users become interested in different combinations of technical factors—improved calculations of  Climit(s) will be made available for MIRI coronagraphs.

Meanwhile, users may be able to extrapolated estimates of Climit(s) using the Exposure Time Calculator (ETC) and other proposal tools.

Figure 1. MIRI limiting contrast


Limiting performance of a MIRI 4QPM coronagraph. Adapted from Boccaletti et al. (2015), Figure 10, top-right panel, showing CPSF(s)Craw(s)Climit(s), and Cideal(s) for the Boccaletti et al. set of technical and procedural assumptions.


References

Boccaletti, A. et al. 2015, PASP, 127, 633
The Mid-Infrared Instrument for the James Webb Space Telescope, V: Predicted Performance of the MIRI Coronagraphs
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