Treatment of limiting contrast (Climit) is based on current information about telescope aberrations and the expected performance of JWST MIRI.
Limiting contrast (Climit(s)) is the companion-to-host flux ratio of the minimum detectable companion. It is the detection limit and the best that can be done.
The article Contrast Considerations for JWST High-Contrast Imaging provides a general treatment of "contrast" (C), including Climit(s) in particular.
Climit(s) is a function of essentially everything related to high contrast imaging (HCI): myriad eclectic technical factors and procedures, end-to-end.
This treatment of Climit(s) for MIRI is based on Boccaletti, A., et al 2015.
Limiting contrast, Climit(s), for MIRI
Figure 1 shows the best available treatment of various contrasts for a MIRI 4QPM2 coronagraph, as adapted from Figure 10 (upper right) in Boccaletti, A., et al 2015.
The "technical factors" behind the curves include:
Under those assumptions, Figure 1 shows 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 three particular technical assumptions: the 4QPM coronagraph (1 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, A., et al 2015.
This is the best information on limiting coronagraphic performance for MIRI at the current time. In the future, with a better understanding of wavefront errors and other technical factors—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.
Boccaletti, A., et al. 2015, PASP, 127, 633