JWST High-contrast imaging opens the sky near bright objects to exploration.
IntroductionThe purpose of high-contrast imaging (HCI) is to obtain images of faint sources (“companions”) located near bright, point sources (“hosts”). Typical hosts are stars and quasars. Companions include exoplanets, circumstellar structures of gas and dust, and luminous feeding zones around supermassive black holes. In normal imaging, the image of a faint companion could be swamped and lost in the noise of diffracted light in the wing of the host's image. HCI strategies—comprising special optics, observing procedures, and post processing—are designed to reduce the impact of host light at the position of the companion, in order to make the companion detectable against the residual noise.
In HCI, the reduction of wing light from the host occurs in two steps: optical cancellation and image subtraction. Optical cancellation is achieved by special optics—pairs of masks located on the focal and pupil planes. Image subtraction is performed in post processing by differencing a scaled PSF-reference image and a science image (see PSF-subtraction strategies).
JWST offers 3 HCI designs for optical cancellation—2 types of coronagraph and 1 interferometer (see HCI Optics). Subsets of these designs are implemented in 3 JWST instruments:
- Lyot coronagraph (LYOT; 5 implementations in NIRCam, 1 in MIRI)
- 4-quadrant phase-mask coronagraph (4QPMC; 3 implementations in MIRI)
- aperture-masking interferometer (AMI; 1 implementation in NIRISS)
The instrument-specific modes for HCI, including allowed mask-filter combinations, are described at:
For more detailed information about JWST HCI options, see the references listed below.
High contrast imaging articles
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
Greenbaum, A.Z., Pueyo, L., Sivaramakrishnan, et al. 2015, ApJ, 798, 68
An Image-Plane Algorithm for JWST's Non-Redundant Aperture Mask Data