Estimating the coronagraphic point spread function (reference PSF) is essential to achieving the deepest possible contrast and recovering faint astrophysical signals in the vicinity of a bright central object.
The purpose of PSF subtraction is to achieve limiting contrast between a bright host (star, AGN, etc.) and reveal the faintest detectable astrophysical signal or "companion" in its vicinity. For more information, please refer to NIRCam-Specific Treatment of Limiting Contrast.
The companion may be an extended source, such as a circumstellar disk, a jet, or a point source, such as an exoplanet. The PSF reference image may be a composite of multiple images obtained after pointing changes—either a roll or an offset. To perform PSF subtraction, the reference image is scaled and subtracted from science images.
The idea is to subtract the closest 3-dimensional PSF (X, Y and intensity) profile of an unresolved star to a given scientific observation.
NIRCam PSF subtraction strategies
See also: JWST Coronagraphy in ETC
The most critical and consequential step in the post-processing of NIRCam coronagraphic images is subtracting the estimated wing of the point spread function (PSF) of a target centered on a coronagraphic mask (occulter).
Currently, NIRCam supports three PSF subtraction strategies, each with its own pointing operations, which vary in complexity and performance.
Beichman, C. A., et al. 2010, PASP, 122:162
Perrin, M., Stansberry, J., Beck, T., Hines, D., and Soummer, R., 2013, JWST-STScI-003472
Stark, C., Van Gorkom, K., & Pueyo, L., 2016, JWST-STScI-004707, SM-12