MIRI Coronagraphic Imaging

Coronagraphic imaging with JWST’s Mid-Infrared Instrument (MIRI) provides high-contrast imaging in wavelength bands from 10 to 23 μm, using one Lyot-type coronagraph and three 4-quadrant phase-mask (4QPM) coronagraphs.

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See also: MIRI Coronagraphic Imaging APT Template, JWST High-Contrast ImagingHCI Optics

The imaging channel on MIRI is equipped with four coronagraphs that provide high-contrast imaging (HCI), covering wavelength bands from 10 to 23 μm (Boccaletti et al. 2015). 

In addition to the classical Lyot coronagraph (which provides an inner working angle (IWA) of ~3.3λ/D), MIRI also incorporates the 4-quadrant phase-mask coronagraph technology (4QPM; Rouan et al., 2000) to provide the smallest possible IWA of ~1 λ/D at 10 to 16 μm. These advantages might be used for studying exoplanets and other faint circumstellar sources.

Each coronagraph is at a fixed position in MIRI's first focal plane so that no mechanisms are used. The observer will have control over two primary variables for MIRI coronagraphy: (1) fixed filter-coronagraph pairs and (2) exposure time (via the number of frames and integrations). 

Figure 1. The MIRI coronagraphic imaging FOV

The MIRI coronagraphic imaging FOV

In the imager focal plane, the coronagraph sky views are positioned at the left side from top to bottom: the classical Lyot coronagraph and the three 4-quadrant phase masks coronagraphs. This image includes simulated point spread functions (PSFs) for a point source behind each phase mask. The coronagraphs provide a raw contrast of ~0.01. The image, however, does not indicate what the data will look like after the subtraction of a PSF reference star. After reference star subtraction (and additional processing), the MIRI coronagraphs are expected to yield contrast of 10-5– 10-6.

Coronagraph filters

Main article: MIRI Filters and Dispersers

Coronagraphic filters are associated directly with each coronagraph and are fixed for each of the four coronagraphs. Selecting the filter also selects the coronagraph, and vice versa. 4QPMs have narrow spectral bandpasses. 

Figure 2. MIRI coronagraph filter bandpasses

MIR coronagraph filter bandpasses

Each MIRI coronagraph filter is associated with a specific coronagraph.
Table 1. Filter-coronagraph pair properties (Filters and Masks are not interchangeable; selecting the filter automatically selects the mask)

FilterCoronagraphPupil mask transmission (%)1Central wavelength (μm)Bandwidth2 (μm)IWA3 (arcsec)Rejection4 (on-axis)
F2300CLyot spot67222.755.52.16850

1 Coronagraph filters are paired with pupil masks to reduce diffracted light from both the telescope pupil and the coronagraph, but at the expense of some loss of total intensity.

2 Bandwidth is defined to extend to wavelengths on either side of the central wavelength that correspond to 5%–10% of the transmission efficiency.

Inner working angle (IWA) is defined as the 50% transmission radius.

4 Rejection is the total flux attenuation of a star when centered onto the coronagraph. The term is unitless since it is a ratio of two intensities (out of mask / on the mask).

5 Band pass useful for NH3 and silicates.

6 The "spot" refers to the circular occulting mask in the Lyot-type coronagraph.

Coronagraph exposure specification

See also: MIRI Detector Readout Overview

MIRI coronagraphic imaging only supports the FAST 1 detector readout pattern.

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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
Updated version 

Rouan, D. et al. 2000, PASP, 112, 1479
The Four-Quadrant Phase-Mask Coronagraph. I. Principle

Rouan et al. 2007, Proc. of SPIE, 6693, 16
A new concept of achromatic phase shifter for nulling interferometry



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