NIRCam Coronagraphic Occulting Masks and Lyot Stops

NIRCam coronagraphy offers 3 round and 2 tapered bar occulting masks paired with apodizing Lyot stops that allow high-contrast imaging with nominal inner working angles (IWAs) ranging from 0.23" to 0.61".

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When centered on a bright point source, an occulting mask blocks the core of the point spread function (PSF). Diffracted light in the wings of the PSF is apodized by a Lyot stop in the pupil plane. Together, the occulting mask and Lyot stop constitute a "Lyot coronagraph," which enables high-contrast imaging (HCI) of faint features near bright point sources.


Occulting masks

NIRCam's occulting masks are binary half-tone patterns composed of 2–4.3 μm squares that approximate greyscale transmission masks. They are lithographically etched from a chrome-on-aluminum coating deposited atop a multilayer, anti-reflection (AR) coated sapphire substrate that is the basis of the coronagraph optical mount (COM). Also located along the edges of the COM substrate are 5″ × 5″ and 2.5″ × 2.5″ squares of nichrome with an optical density of ~3, the larger set of which serve as neutral density (ND) filters for bright target acquisition (Figure 1). The transmission profiles of the 5 occulting masks are shown in Figure 2; the transmission of the COM substrate is shown in Figure 1 of NIRCam Filters for Coronagraphy.

Figure 1. NIRCam coronagraphic occulting masks and neutral density squares


NIRCam module A coronagraph optical mount (COM), which contains 3 round and 2 bar occulting masks for coronagraphic imaging and several neutral density squares for acquisition of bright targets.  The masks are annotated with their names and nominal ranges of wavelength and IWA (6λ/D for round masks; 4λ/D for bar masks), which are delimited by the bandpass filters available for each mask in the primary coronagraphic channels. Also given are the physical IWAs (defined as the half-widths at half-maximum opacity) of the masks. The HWHM IWAs of the bar occulters vary by a factor of ~3 between opposite ends of the bars.  (See Table 1 of NIRCam Coronagraphic Imaging.) Figure adapted from Krist et al. 2010.

 Figure 2. Transmission profiles of NIRCam coronagraphic occulting masks


Transmission profiles of NIRCam's coronagraphic occulting masks as functions of radial distance from the round masks and perpendicular distance from the axes of the bar occulters. The upper two plots show the transmission profiles for the shortest and longest wavelength optical filters that can be used with the SW and LW bar occulters in the primary coronagraphic channels, respectively. The vertical lines mark the 50% transmission points. The data used to create these plots are available here.

Lyot stops

The Lyot stops are metallic patterns deposited photolithographically onto wedge-shaped Si and BaF2 substrates located in the pupil wheels of the LW and SW channels, respectively. Each channel has 2 Lyot stops designed to suppressed diffracted light from the round and bar occulters (Figure 3). When inserted into the optical train, the wedges shift the optical path so that the COM is projected onto the detector arrays (Figure 1 of NIRCam Coronagraphic Imaging).  

The transmission of the Si substrate is > 95%–98% between 3 and 5 µm. The transmission of the BaF2 substrate is more chromatic: ~95% at 1 µm, ~75% at 2.5 µm, and ~80% for most of the coronagraphic filters.   Each Lyot stop has a throughput of ~20%, so the combined loss of light from the coronagraphic optics at distances > 1″ from the centers of the occulters is ~86-90%. The JWST Exposure Time Calculator (ETC) provides a "total system throughput" plot for any coronagraphic configuration.

Figure 3. NIRCam coronagraphic diffraction and Lyot stops

Top row: Simulated images showing diffraction patterns from the (a) round and (b) bar occulters. Bottom row: Illuminated regions of the JWST pupil transmitted through holes in the Lyot stops designed for the (c) round and (d) bar occulters. © Mao et al. 2011.

Focal plane images

Figure 4 shows actual images of direct and coronagraphic coronagraphic point spread functions (PSFs) obtained during JWST Commissioning (Program 1441), using the MASK335R and MASKLWB occulters and their corresponding Lyot stops. The coronagraphic PSFs (right panels) reflect the Fourier transforms of the Lyot stops. 

Figure 4. In-flight NIRCam coronagraph focal plane images using round and bar occulters and Lyot stops

Focal plane images of NIRCam LW PSFs obtained during target acquisition (TA) of the unocculted science target (left panels), during astrometric confirmation of the TA (middle panels), and after positioning the science target behind the occulter (right panels). The top and bottom rows show the F335M image sequences for MASK335R and MASKLWB, respectively. The images were recorded during the JWST commissioning period (Program 1441).


References

Girard, J. H., et al. 2022, Proceedings of the SPIE, 121803Q
JWST/NIRCam Coronagraphy: commissioning and first on-sky results

Krist, J., et al., 2010, SPIE, 7731,3J
The JWST/NIRCam coronagraph flight occulters

Mao, Y., et al., 2011, SPIE, 81500E
NIRCam coronagraphic Lyot stop: design, fabrication, and testing



Notable updates

  • Added text relevant to new dual-channel imaging mode

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    Added focal plane images (on-sky)

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    Added figure: nominal pointing positions

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    Added figures: transmission vs. radius
Originally published