MIRI offers coronagraphic imaging with 4 individual coronagraph occulters located with the LRS slit on a mounting bracket placed at the focal plane. One of the coronagraphs is based on the classic Lyot design, while the other 3 incorporate 4-quadrant phase masks (4QPMs). (See here for an overview of JWST's high-contrast imaging (HCI) optics.)
The classical Lyot coronagraph places an occulting spot in the focal plane to block light from a bright point source from reaching the detector. For a typical Lyot coronagraph, this spot is of order 3–6 λ/D in radius so that it blocks the majority of the light from the Airy core, including a few bright rings. Although classical Lyot coronagraphs can provide excellent contrast outside the area blocked by the occulting spot, they are limited with regard to their inner-working angle (IWA) to the projected radius of this spot.
Phase mask coronagraphs are designed to decrease the IWA to near 1 λ/D by replacing the occulting spot with a transparent mask that imparts phase differences across the focal plane so that when the reimaged pupil is formed, the light interferes more destructively than with a Lyot mask, hence rejecting the starlight outside the geometrical pupil. The theory of 4QPMs is developed in Rouan et al. (2000) and Rouan et al. (2007).
In addition to the focal plane masks, Lyot stops sandwiched with the passband filters are located at a reimaged pupil to attenuate the residual light from the diffraction pattern associated with the telescope aperture and a particular coronagraph design, and from phase and amplitude aberrations of the wavefront in the optical train.
The Lyot coronagraph is a traditional design that incorporates a Lyot spot mask of radius of 2".16, which is 3 λ/D in radius at 23 µm. The spot is suspended in the focal plane by 2 supporting struts in the mounting bracket, which themselves block light in the FOV. MIRI's Lyot coronagraph is offered at 23 μm for two reasons: (1) a lack of suitable transmissive optical materials to fabricate more advanced 4QPM masks at these wavelengths, and (2) to provide a broad spectral band to maximize the sensitivity on planetary debris disks (4QPMs only operate over a narrow passband). The Lyot mask is useful for investigating objects, structures and diffuse emission near bright sources such as the outer regions of protoplanetary and debris disks, extended structures around post-AGB stars, and the host galaxies and scattering/ionization “cones” of AGN.
While there are several implementations of phase mask coronagraphs, 4QPMs have been chosen for MIRI (Rouan et al. 2000, 2007). 4QPMs work by introducing a 180° phase shift of the light transmitted through 2 of the quadrants on the diagonal. This cancels the signal from the central point source via destructive interference of light that lands equally in the 4 quadrants. The disadvantage of a 4QPM is that it operates only over a narrow wavelength range. Therefore, maximum cancellation takes effect only when the appropriate filter (and Lyot stop) is in position. Furthermore, the linear boundaries between adjacent quadrants will attenuate light. Therefore, the 4QPM has reduced sensitivity (as low as 10%) in the field along the 4 edges of the mask.
MIRI Low Resolution Spectroscopy
MIRI Optics and Focal Plane
MIRI Spectroscopic Elements
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
Rouan, D. et al. 2000, PASP, 112, 1479
The Four-Quadrant Phase-Mask Coronagraph. I. Principle
Published January 2, 2017