MIRI Coronagraph Masks
MIRI offers coronagraphic imaging with four individual coronagraph occulters, located with the LRSslit on a mounting bracket placed at the focal plane. One of the coronagraphs is based on the classic Lyot design, while the other three incorporate 4-quadrant phase masks (4QPMs). (Please refer to the JWST High-Contrast Imaging article for additional information about 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 was developed as described 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 re-imaged 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 two 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.
4-quadrant phase mask (4QPMs) coronagraphs
Lyot coronagraphs provide excellent contrast outside the occulter, but their IWA is limited to the projected radius of this occulter (≥3λ/D). Phase mask coronagraphs reduce the IWA to near 1 λ/D by replacing the occulting spot with a transparent mask that imparts phase differences throughout the focal plane so that the light interferes destructively for a point source placed at the apex of the four quadrants. The 4QPMs are useful for investigating objects, structures and diffuse emission very close to bright point sources, such as the inner regions of debris disks and exoplanets orbiting close to a star, very tight binary star systems, and the near-nuclear environments 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 two 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 four quadrants. The disadvantage of a 4QPM is that it only operates 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 four edges of the mask.
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
Kenworthy, Matthew, 2017, "Apodizing Phase Plate Coronagraph," Leiden Observatory website [Updated 2017/09/13]
Rouan, D. et al. 2000, PASP, 112, 1479
The Four-Quadrant Phase-Mask Coronagraph. I. Principle