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.


Introduction

Parent page: MIRI Observing Modes
See also: MIRI Coronagraphic Imaging APT Template, JWST High-Contrast ImagingJWST High-Contrast Imaging 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 ~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 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 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, the coronagraphs are expected to yield a contrast of 10-5– 10-6.


Coronagraphic filters

Main article: MIRI Filters and Dispersers

4QPMs have narrow spectral bandpasses. 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.

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


FilterCoronagraphPupil mask transmission (%)1Central wavelength (μm)Bandwidth2 (μm)IWA3 (arcsec)Rejection4 (on-axis)
F1065C4QPM16210.5750.750.33260
F1140C4QPM26211.400.80.36285
F1550C4QPM36215.500.90.49310
F2300CLyot spot 57222.755.52.16850

1 Coronagraph filters are paired with pupil masks to reduce diffracted light from 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.

3Inner 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 The spot refers to the occulting mask in the Lyot-type coronagraph.



Coronagraph exposure specification

See also: MIRI Detector Readout Overview

MIRI imaging only supports the FAST 1 detector readout pattern.





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
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








Last updated

Published December 22, 2017


The format for 'Last updated' is shown below. Enter updates above this internal"Comment" box. Only enter major updates (not typos or formatting changes).


Updated April 05, 2017

  • Lorem ipsum dolor sit amet, consectetur adipiscing elit. Aliquam fermentum vestibulum est. Cras rhoncus. 
  • Pellentesque habitant morbi tristique senectus et netus et malesuada fames ac turpis egestas. Sed quis tortor. 

Published March 02, 2017