NIRCam Coronagraphic Occulting Masks and Lyot Stops
Each NIRCam module includes a Coronagraphic Optical Mount (COM) that carries three round and two bar-shaped coronagraphic occulting masks in the focal plane.
When centered on a bright point source, an occulting mask blocks the core of the point spread function (PSF). Diffracted light remaining in the PSF wings is blocked by a corresponding Lyot stop in the pupil plane. The pairing of the occulting mask and Lyot stop constitutes a "Lyot coronagraph", which enables high-contrast imaging of faint features near bright point sources.
Three round occulting masks are available in the pupil plane with sizes optimized for observations at different wavelengths. Table 1 gives their radii (in arcseconds) and the wavelengths at which those radii correspond to six resolution elements, or 6 λ/D, where the D is the 6.5 m diameter of the JWST primary mirror. (Resolution at the diffraction limit is measured in units of λ/D.)
Table 1. 6λ/D wavelengths for the round occulting masks
Round coronagraphic mask
|Radius||λ for 6λ/D|
The two bar coronagraphic occulting masks are tapered, with radii varying by a factor of three along their lengths. Table 2 gives the half widths at half maximum (HWHM) at the bar end points and centers. Note these wavelengths extend beyond the observable range (see NIRCam Filters for Coronagraphy).
Table 2. 4λ/D wavelengths for the end points and center (in HWHM) of each bar coronagraphic mask
|Bar coronagraphic mask||HWHM||λ for 4λ/D|
Nominal inner working angles (IWAs) reported in Table 1 & 2 correspond to the half-width at half-maximum (HWHM), the angular distance at which the coronagraph throughput is half. For the round masks it was designed to be at ~6λ/D and for the bar masks it was designed to be ~4λ/D for each given filter's central wavelength to which a special "fiducial" pointing is assigned as shown in Figure 1.
When a bar mask is used with a given filter, the source is placed behind the occulter at a position (shown in Figure 1) that provides HWHM = 4λ/D for the central wavelength λ of the filter.
In each module, NIRCam has two Lyot stops: one designed to suppress diffracted light from the round occulters, and the other to suppress diffracted light from the bar occulters. The final PSF is the Fourier transform of the Lyot stop. The geometrical transmission (Figure 4, c and d) of the Lyot stops is ~20%.
Additional throughput losses: other absorbing materials
The focal plane masks or occulters are physically made with aluminum dots on a a 66 mm by 22 mm by 2 mm sapphire substrate whose transmission also slightly depends on wavelength.
Each of the four Lyot stops (one for the round occulter, one for the bar occulter, two modules) are mounted on a substrate whose wedge shifts the field of view by several tens of arcseconds (with respect to plain imaging) and images the occulters onto the detectors. The wedges' transmission is more chromatic for the SW as the substrate is BaF2 (~95% at 1µm and around ~75% at 2.5µm. around ~80% for most of the coronagraphic filters). For the LW wedges, the substrate material is Si and with > 95-98% transmission between 3 and 5 µm.
Overall the NIRCam coronagraphic optics introduce a pure loss in throughput (> 1" away from the center of each occulter) of the order of a factor ~7 to ~10. In the JWST Exposure Time Calculator (ETC), a "total system throughput" plot is displayed for each given instrument setup.
Krist, J., et al., 2010, SPIE, 77313J
The JWST/NIRCam coronagraph flight occulters
Mao, Y., et al., 2011, SPIE, 81500E
NIRCam coronagraphic Lyot stop: design, fabrication, and testing