Estimating the coronagraphic point-spread function (reference PSF) is a critical step in the post-processing of coronagraphic observations using NIRCam. First, the scaled wing of the reference PSF is shifted to align with the wing of the image of the central, bright point source, in the science image. The wing of the reference PSF is then subtracted from the wing of the science image. Limiting contrast is defined by the best achievable match between the reference and science images, measured by the lowest possible residuals after the subtraction.
The purpose of PSF subtraction is to achieve limiting contrast between a bright host and the faintest detectable "companion," which is the main source of scientific interest. See here for a NIRCam-specific discussion of limiting contrast.
The companion may be an extended source, such as a circumstellar disk, 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.
NIRCam PSF subtraction strategies
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).
See here for a general discussion of PSF-subtraction strategies.
Currently, NIRCam supports three PSF subtraction strategies, each with its own pointing operations, which vary in complexity and performance.
First, in the "reference-star strategy" (RSS), 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. The RSS is operationally simple, requiring only the 2 target acquisitions (TAs) described in the previous section.
Second, the "angular differential imaging strategy" (ADIS) 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 two positions of the companion-feature image. Because the solar-power budget of the observatory is sensitive to spacecraft roll, and because the restrictions on off-nominal roll are complicated—dependent, for example, on the solar longitude—the ADIS roll maneuver involves complex planning, which is supported by a special "visibility" tool.
Third, the "small grid dither strategy" (SGDS) involves a library of reference-target images, obtained in a pattern of small, highly accurate, sub-pixel 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 LOCI or KLIP.
JWST User Documentation Home
NIRCam Coronagraphic Imaging
NIRCam Coronagraphic Occulting Masks and Lyot Stops
NIRCam Filters for Coronagraphy
JWST High Contrast Imaging Overview
JWST High Contrast Imaging Optics
JWST High Contrast Imaging Inner Working Angle
Contrast Considerations for JWST High-Contrast Imaging
NIRCam-specific treatment of limiting contrast
JWST Coronagraphic Observation Planning
JWST Coronagraphic Sequences
JWST High Contrast Imaging in ETC
JWST High Contrast Imaging in APT
Beichman, C. A., et al. 2010, PASP, 122:162
Imaging Young Giant Planets from Ground and Space
Perrin, M., Stansberry, J., Beck, T., Hines, D., and Soummer, R., 2013, JWST-STScI-003472
Sample Target Acquisition Scenarios for JWST
Stark, C., Van Gorkom, K., & Pueyo, L., 2016, JWST-STScI-004707, SM-12
How to Implement a JWST Coronagraphic Observation Sequence in APT