In JWST high-contrast imaging (HCI), the term "contrast" means the companion-to-host flux ratio.
When developing an HCI investigation, the user's scientific goal is represented by an "operating point," (s, Cflux), where
This operating point is estimated to be feasible if Cflux > Climit(s), where Climit(s) is the "limiting contrast."
Climit(s) is a function of many interrelated factors, and is, therefore, a challenge to estimate. In particular, Climit(s) depends on the instrumental configuration, the observing strategy, and the post-observation processing calibrations and processes. Climit(s) is especially sensitive to the step where a scaled reference PSF is subtracted from a science image. PSF subtraction is relied upon to extend the grasp of the investigation deep into the systematic noise (see Soummer et al.) . In the absence of data, it is hard to say how well PSF subtraction will perform for JWST.
To be valid, the feasibility test Cflux > Climit(s) assumes that the planned observation has the same technical and procedural factors that produced the calibration of Climit(s).
To gain a better understanding of Climit(s), look more closely at the term "contrast" (C). Although C been widely adopted as a metric of HCI performance, its meaning is sometimes ambiguous in the context of where it appears. Therefore, the various possible meanings of "contrast" are disambiguated as follows:
Q(s) = Cflux×Craw(s) is an auxiliary metric sometimes used to gauge the systematic errors in Craw(s) due to aberrations and their speckles. Q(s) is the wing-to-center surface-brightness ratio of the host-companion pair of sources, which has been called the "instantaneous signal-to-noise ratio." Q depends only on the instrument and on nature, but not on any observational factors, such as exposure time or strategy for PSF subtraction.
At the current time, few treatments of Climit(s) are available for the JWST HCI modes. For now, users must extrapolate Climit(s) from published treatments of Climit(s):
Beichman, C. A., et al. 2010, PASP, 122:162
Boccaletti, A., et al. 2015, PASP, 127, 633
Greenbaum, A.Z., Pueyo, L., Sivaramakrishnan et al. 2015, ApJ, 798, 68
Soummer, R., Pueyo, L., and Larkin, J., 2012, ApJ 755 , L28