In JWST high-contrast imaging (HCI), the term contrast refers to the brightness ratio between a faint companion (planet, debris disk, accretion structure, etc.) and its much brighter host (typically a star or quasar).

See also: MIRI Coronagraphic Imaging, NIRCam Coronagraphic Imaging, NIRISS Aperture Masking Interferometry

When developing an HCI investigation, the scientific goal is represented by an operating point (s, C_flux), where

• s is the apparent angular separation between the companion and host, and 
• C_flux = flux_companion/flux_host is the companion-to-host flux ratio at the wavelength of interest.

This operating point is considered feasible if C_flux > C_limit(s), where C_limit(s) is the limiting contrast – the value of C_flux for the minimum detectable companion at separation s, given a specified detection threshold.

C_limit(s) is a function of many interrelated factors, and is, therefore, a challenge to estimate. In particular, C_limit(s) depends on the instrumental configuration, the observing strategy, and the post-observation calibration and processing. C_limit(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.). 

To be valid, the feasibility test C_flux > C_limit(s) assumes that the planned observation has the same technical and procedural factors that produced the calibration of C_limit(s).

C_limit(s), is best understood by disambiguating the term "contrast" (C), which carries several distinct meanings in HCI:

1. C_flux is the term for the companion-to-host flux ratio. C_flux is a property of nature, independent of any instrumental or observational details or considerations.
   
   
2. C_raw(s) is the raw contrast as a function of angular separation s, describing the level of residual starlight suppression prior to post-processing. It is independent of integration time and detector noise, but depends on the instrument configuration and observing conditions.
   
   
3. C_limit(s) is the limiting contrast defined in the previous section. C_limit(s) affirmatively does take into account any and all relevant technical and procedural factors, such as observational strategy, pointing and instrumental errors, detection threshold, and post-observation processing (especially the PSF-subtraction strategy). 
   
   
4. C_ideal(s) is a floor for the limiting contrast C_limit(s), because it makes certain optimistic, simplifying assumptions. For example, C_ideal(s) may assume that pointing errors are zero or that photometric noise is ideal, with photon-counting noise dominating.
   
   

For instrument-specific treatments of C_limit(s) derived from in-flight data, see:

• HCI NIRCam Limiting Contrast
• HCI NIRISS Limiting Contrast
• HCI MIRI Limiting Contrast

Where no published treatment exists for a desired configuration, C_limit(s) can be estimated using the JWST Exposure Time Calculator (ETC) together with STPSF (formerly WebbPSF), PanCAKE, and instrument-specific simulators such as pyNRC for NIRCam.

References

Beichman, C. A., et al. 2010, PASP, 122:162
Imaging Young Giant Planets from Ground and Space

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

Boccaletti, A., et al. 2022, A&A, 667, A165
JWST/MIRI Coronagraphic Performances as Measured On-Sky

Carter, A. L., et al. 2023, ApJL, 951, L20
The JWST Early Release Science Program for Direct Observations of Exoplanetary Systems I: High-Contrast Imaging of the Exoplanet HIP 65426 b from 2 to 16 μm

Greenbaum, A.Z., Pueyo, L., Sivaramakrishnan et al. 2015, ApJ, 798, 68
An Image-Plane Algorithm for JWST's Non-Redundant Aperture Mask Data

Sivaramakrishnan, A., Tuthill, P., Lloyd, J. P., et al. 2023, PASP, 135, 015003
The Near Infrared Imager and Slitless Spectrograph for the James Webb Space Telescope – IV. Aperture Masking Interferometry

Soummer, R., Pueyo, L., and Larkin, J., 2012, ApJ, 755 , L28
Detection and Characterization of Exoplanets and Disks Using Projections on Karhunen-Loeve Eigenimages