High-contrast imaging (HCI) optics for 3 JWST instruments have been designed to enhance the detectability of faint companions located near bright hosts.

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

Three JWST instruments incorporate high-contrast imaging (HCI) optics that enhance the detectability of faint companion sources through either suppression of host-star diffraction or interferometric calibration of a tailored diffraction pattern.

1. NIRCam coronagraphic imaging: 
   • Five Lyot-type coronagraphs (three round, two bars) are available.
   • All allowed NIRCam coronagraphic configurations are listed in the NIRCam Filters for Coronagraphy article.
   • NIRCam coronagraphs with round coronagraphic masks (occulters) work best in narrow- and medium-band filters at λ ~ 2.1, 3.35, and 4.3 µm.
   • Coronagraphs with bar-shaped occulters work best in narrow and medium bands in the ranges 1.7–2.2 µm and 2.4–5 µm. 
      
2. MIRI coronagraphic imaging:
   • One Lyot-type coronagraph and three 4-quadrant phase mask coronagraphs (4QPMs) are available.
   • MIRI 4QPMs work only in narrow-band filters centered at 10.575, 11.30, and 15.50 µm.
   • MIRI's Lyot-type coronagraph works only in a broadband filter centered at 22.75 µm.
      
3. NIRISS aperture masking Interferometry: one aperture masking interferometer (AMI).
   • NIRISS AMI works best with 3 medium-band filters at 3.8, 4.3, and 4.8 μm.
   • NIRISS’s 65 mas pixels satisfy the Nyquist criterion at 4 µm and performance at shorter wavelengths is reduced. Nevertheless, because the wide filter centered at 2.77 µm spans a deep absorption feature of water, its use may be particularly relevant for exoplanetary research.

In addition, HCI can be carried out using basic imaging modes of the observatory (Rajan et al., 2015; Durcan, Janson, and Carson, 2016), as well as using IFU strategies similar to Konopacky et al. (2013). These modes are not yet covered in the documentation. 

Lyot-type coronagraph

See also: NIRCam Coronagraphic Occulting Masks and Lyot Stops, NIRCam Filters for Coronagraphy

The HCI optics of Lyot-type coronagraphs consist of 2 masks: an occulter and a Lyot stop. The first mask—the occulter or "coronagraphic mask"—lies in the first focal plane of the imaging instrument, where it blocks light from the on-axis star while allowing light from other sources to pass. The second mask—the Lyot stop—lies in the plane of the re-imaged pupil, where it blocks starlight diffracted by the edges of the primary mirror segments, the secondary mirror support structure, and the occulting mask itself. In other words, the Lyot stop suppresses the diffraction spikes and rings that would otherwise dominate direct images of bright stars. The STPSF (formerly WebbPSF) software package can be used to explore the detailed morphology of coronagraphic point spread functions.

If the apparent separation between the feature of interest and the host star is greater than the inner working angle (IWA), companion light passes the occulter and—after some throughput loss at the Lyot stop—reaches the detector. On JWST, NIRCam has 5 sets of Lyot-type coronagraphic optics (3 with round and 2 with bar-shaped occulters), and MIRI has one Lyot-type coronagraph, with a round occulter. (See Figure 2.)

Four-quadrant phase-mask coronagraph (4QPMC)

See also: MIRI Coronagraphs

The HCI optics of 4QPMs consist of a phase mask in the focal plane and a Lyot stop in the pupil plane. The phase mask, which is transparent at the operating wavelengths, imparts a 180° phase shift to light passing through 2 diagonally opposing quadrants. Light from a source centered on the common vertex of the 4 quadrants undergoes destructive interference in the downstream pupil plane. A Lyot stop at the re-imaged pupil plane attenuates residual starlight from the diffraction pattern of the telescope aperture, support structures, and coronagraph design itself, as well as from phase and amplitude aberrations on the wavefront. The optical advantage of a 4QPM is a very small IWA in terms of λ/D (~1 λ/D across the 3 MIRI 4QPM filters, λ = 10–16 μm), which somewhat compensates for the lower diffraction-limited spatial resolution at MIRI's long wavelengths. The trade-off is a strong sensitivity to optical aberrations and source misalignments.

Aperture masking interferometry (AMI)

See also: NIRISS Non-Redundant Mask

The HCI optic of an AMI is a non-redundant mask (NRM) in the pupil plane. The mask is opaque except for holes, in the case of NIRISS/AMI, 7 holes. The number of unique (i.e., non-redundant) baselines between pairs of holes in the NIRISS/AMI NRM is N × (N − 1)/2, or 21, where N is the number of holes in the NRM. Each baseline creates a single fringe pattern at focus, and the 21 fringes interfere to create an interferogram, which is actually just a PSF, The PSF's fine structure is more than twice as sharp as the corresponding full aperture PSF, but with much wider wings.

An NRM interferogram possesses certain observables—closure phases and fringe amplitudes—that can be calibrated using a PSF reference observation to remove many instrumental effects. For higher contrast observations, the reference star is expected to be placed within a few to 10 mas of the target star when both are commanded to the center of the pixel. Such repeated placement is aimed at mitigating residual pixel-to-pixel variations that remain after flat fielding and other routine image calibrations are performed. These observables allow the fitting of basic models, such as binary or triple point sources, and simple extended structures. Binary point source flux ratios of up to about 10 stellar magnitudes should be achievable. The search space of an NRM extends inwards to a separation of λ/2B, where B is the hole-to-hole length of the longest baseline. For fitting binary models as well as for true imaging—that is, not using closure relations or model-fitting—the NRM on NIRISS has an inner working angle (IWA) of about 70 mas. Beyond half an arcsecond, NIRCam's coronagraphs provide better contrast.

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

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

Rajan, A., et al. 2015, ApJ, 809, L33
Characterizing the Atmospheres of the HR8799 Planets with HST/WFC3 

Durcan, S., Janson, M., & Carson, J. 2016, ApJ, 824, 58
High Contrast Imaging with Spitzer: Constraining the Frequency of Giant Planets out to 1000 AU separations 

Konopacky, Q. M., Barman, T. S., Macintosh, B. A., Marois, C., 2013, Science, 339, 1398 (Science link)
Detection of carbon monoxide and water absorption lines in an exoplanet atmosphere