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 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).
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.
|Filter||Coronagraph||Pupil mask transmission (%)1||Central wavelength (μm)||Bandwidth2 (μm)||IWA3 (arcsec)||Rejection4 (on-axis)|
|F2300C||Lyot spot 5||72||22.75||5.5||2.16||850|
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.
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 two intensities (out of mask / on the mask).
5 The spot refers to the 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