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.
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 three 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 four coronagraphs. Selecting the filter also selects the coronagraph, and vice versa. 4QPMs have narrow spectral bandpasses.
|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.
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