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James Webb Space Telescope
MIRI Coronagraph

The MIRI Imager has coronagraphic capability at four wavelengths using two types of coronagraphic techniques: Lyot coronagraphy and four-quadrant phase mask coronagraphy. The coronagraphic masks, which are located along one edge of the focal plane aperture, include three four-quadrant phase masks (4QPM) and a focal plane mask for a Lyot coronagraph. The 4QPM are rotated slightly (~5 degrees) around their center positions compared to what is shown in the figure below.

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The coronagraphic masks each have a square field of view of ~24" x 24" for the phase masks and ~30" x 30" for the Lyot coronagraph. They are optimized for particular wavelengths (10.65, 11.4, 15.5 and 23 μm) with each coronagraph uniquely associated with a specific filter and commonly referred to by its associated filter. The F2300C coronagraph is the Lyot coronagraph and the other three coronagraphs are 4QPM coronagraphs(F1065C, F1140C and F1550C). In addition to the dedicated filter each mask must be used in conjunction with the pupil mask which is mounted in the filter wheel and whose job is to both restrict the spectral passband and to eliminate straylight.

They coronagraphic apertures share the same focal plane scale (0.11 arcsec per pixel) as the imager

Operations of the MIRI coronagraphs need to be carefully planned because they involve the detection of faint sources near very bright sources. The MIRI detectors descend from the Spitzer IRAC detector heritage that have known persistence problems after exposure to bright sources. Three procedures have been developed to avoid saturating the detector during coronagraphic target acquisition, one for the Lyot mask, one for bright stars with the 4QPM and one for faint stars with the 4QPM. They make use of the neutral density filter (FND) and the MIRI Contamination Control Cover to avoid the bright star being seen by the detector.