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Last Updated  Mar 24, 2017


Coronagraphic imaging with JWST’s Mid-Infrared Instrument (MIRI) provides high-contrast imaging in photometric bands from 10 to 23 μm, using a Lyot stop mask and three four-quadrant phase masks (4QPM).

Introduction

The imaging channel on MIRI is equipped with 4 coronagraphs that provide high-contrast imaging capabilities covering photometric 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 ~3λ/D), MIRI also incorporates the 4-quadrant phase mask coronagraph technology (4QPM; Rouan et al., 2000) to provide the smallest possible IWA of ~1λ/D at 10 to 16 μm. The design offers a very small IWA in a stable environment. These advantages might be used for studying exoplanets and their circumstellar environments.

Each coronagraph is at a fixed position in MIRI's 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 themselves provide a contrast of a few hundred. The image, however, does not indicate what the data will look like after the subtraction of a reference point source. After reference star subtraction, the coronagraphs are expected to yield a contrast of 10-5–10-6.


Coronagraphic filters

4QPMs are not achromatic and have narrow spectral bandpasses. Coronagraphic filters are therefore associated directly with each coronagraph and are not interchangeable. Selecting the filter also selects the coronagraph.

Figure 2. MIRI coronagraph filter bandpasses

MIR coronagraph filter bandpasses

Each MIRI coronagraph filter is associated with a specific mask.
 

Table 1. Filter-coronagraph pair properties

FilterCoronagraphPupil mask transmission (%)1Central wavelength (μm)Bandwidth2 (μm)IWA3 (arcsec)Rejection4 (on-axis)
F1065C4QPM16210.5750.750.33260
F1140C4QPM26211.300.80.36285
F1550C4QPM36215.500.90.49310
F2300CLyot spot57222.755.52.16850

1 Coronagraph filters are paired with pupil masks to remove diffracted light induced by both the telescope pupil and the coronagraphic occulting spot, but at the expense of some loss of total intensity.

2 Bandwidth is defined to extend down to wavelengths that correspond to 5–10% of the transmission efficiency.

3 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 2 intensities (out of mask / on the mask).

5 The spot refers to the occulting mask. 

 

 


Coronagraph exposure specification

MIRI coronographic imaging supports only one detector readout patternsfast mode.

(More information coming soon)



 

Related links

MIRI Overview

MIRI Coronagraph Masks

MIRI Detector Readout Overview

MIRI Coronagraphic Imaging APT Template

MIRI Optics and Focal Plane 

MIRI Filters and Bandpasses

Astronomers Proposal Tool

JWST Exposure Time Calculator, ETC -orig

References

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"

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"The Four-Quadrant Phase-Mask Coronagraph. I. Principle"

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"A new concept of achromatic phase shifter for nulling interferometry"

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