NIRCam Coronagraphic PSF Estimation

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. 

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See also: NIRCam Coronagraphic Imaging , JWST High-Contrast Imaging

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 HCI NIRCam 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: HCI ETC Instructions

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 3 PSF subtraction strategies, each with its own pointing operations, which vary in complexity and performance.

  1. In the simple "reference star differential imaging" (RDI) strategy, coronagraphic images are obtained for both the host and reference targets. The reference image is to be scaled and subtracted from the host image to reveal any companion features, which are now as free as possible of residual light from the host. 

  2. The "angular differential imaging" (ADI) or "roll subtraction" strategy is self-referenced, involving only the host target. Two coronagraphic images are obtained that differ only in a small (5°–10°) roll maneuver, which must be sufficient to fully separate the 2 positions of the companion-feature image. Observatory roll is highly restricted, to ±5°, and observation planning is supported by the JWST Coronagraphic Visibility Tool.

  3. The "small grid dither" (SGD) RDI strategy involves a mini-library of reference-target images, obtained in a pattern of small, highly accurate, subpixel offsets (dithers). The library comprises samples of the slight variations in the PSF wing as a function of position relative to the occulter. Because the range of the SGDS pattern is larger than the expected error in TA, the optimal reference PSF for subtraction is a careful interpolation in the SGDS library. A variety of algorithms are available to perform the optimal subtraction, such as KLIP (PCA: Principal Component Analysis as done by the level 3 pipeline) or LOCI.


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Imaging Young Giant Planets from Ground and Space

Carter, A., et al. 2022, AAS Journals (submitted)
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Sample Target Acquisition Scenarios for JWST

Soummer, R. et al. 2012, SPIE, 91433V 
Small-grid dithering strategy for improved coronagraphic performance with JWST

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How to Implement a JWST Coronagraphic Observation Sequence in APT

Latest updates

  • Updated for Cycle 2
Originally published