Last Updated Mar 24, 2017
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).
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.
|Filter||Coronagraph||Pupil mask transmission (%)1||Central wavelength (μm)||Bandwidth2 (μm)||IWA3 (arcsec)||Rejection4 (on-axis)|
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.
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"
"The Four-Quadrant Phase-Mask Coronagraph. I. Principle"
"A new concept of achromatic phase shifter for nulling interferometry"
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