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.
A preflight description of the coronagraphs and their predicted performance can be found in Boccaletti. et al. (2015). A summary of the post-commissioning results can be found in Boccaletti et al. (2022).
The imaging channel on MIRI is equipped with 4 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 to investigate the environments near bright point-sources, including exoplanets and other faint circumstellar sources, plus the environments surrounding active galactic nuclei (AGNs).
Each coronagraph is composed of 3 optical elements. The Lyot spot and the 4QPMs are mounted in a support structure (that also holds the LRS slit) at the first MIRI focal plane. Further downstream, collimated light passes through a passband filter/Lyot stop combination mounted in the MIRI filter wheel. After the filter wheel, light is re-imaged onto the focal plane array.
See also: MIRI Filters and Dispersers
Coronagraphic filters are associated directly with each coronagraph and are fixed for each of the 4 coronagraphs. Selecting the filter also selects the coronagraph, and vice versa. 4QPMs have narrow spectral bandpasses.
Table 1. Filter-coronagraph pair properties (Filters and Masks are not interchangeable; selecting the filter automatically selects the mask)
|Filter||Coronagraph||Pupil mask transmission (%)†||Central wavelength (μm)||Bandwidth‡ (μm)||IWA§ (arcsec)||Rejection* (on-axis)|
† 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.
‡ 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.
* 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).
¶ Band pass useful for NH3 and silicates.
◊ The "spot" refers to the circular occulting mask in the Lyot-type coronagraph.
Boccaletti, A. et al. 2022, A&A, in press
JWST/MIRI coronagraphic performances as measured on-sky
Boccaletti, A. et al. 2015, PASP, 127, 633
The Mid-Infrared Instrument for the James Webb Space Telescope, V: Predicted Performance of the MIRI Coronagraph
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