JWST's NIRCam offers Lyot coronagraphy with round and bar-shaped occulting masks, yielding high contrasts at sub-arcsecond inner working angles in the wavelength range 2–5 µm.

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For APT version 2022.7 and above (starting in Cycle 2), NIRCam coronagraphic imaging allows users to save the data seen by both SW and LW channels simultaneously (as for regular imaging but in module A only).

The coronagraphic mask and associated Acq Filter is fixed. One should therefore chose the mask wisely based on which channel (filters, working angle) drives the science case.

Examples:

MASK210R and MASKSWB allows reaching the smallest inner working angles (IWA) but the point spread function (PSF) scales with wavelength. When associated with LW filters, the larger PSF and possible saturation might yield a degraded IWA. For the longest wavelength, the smaller mask will block the core of the PSF which will feature strong satellite spots for an unocculted coronagraphic PSF. This can be checked with the ETC.
MASK430R is very close to the gap between SW detectors and thus will yield a very limited field of view.
MASKLWB only: there is a new rectangle subarray (SUB400X256) to accommodate better all filter-dependent field points along the bar.

NIRCam coronagraphy enables high-contrast imaging (HCI), in which the diffracted light of a bright object is suppressed to reveal much fainter objects nearby. Please refer to JWST High-Contrast Imaging and its child pages for detailed JWST HCI information.

NIRCam offers 5 coronagraphic masks (occulting masks) in the focal plane and two Lyot stops (apodizing masks) in the pupil plane. One Lyot stop is used with the round coronagraphic masks, and the other Lyot stop is used with the bar-shaped coronagraphic masks. The HCI Optics article provides a general summary of JWST's HCI optics.

Neutral density (ND) squares share the focal plane with the coronagraphic masks, and provide ~7.5 magnitudes of attenuation (optical density ~3) for target acquisition of bright objects. Fainter objects (K > 12) can be acquired without using the ND squares.

Coronagraphic masks

NIRCam has 3 round- and two bar-shaped coronagraphic masks for occulting a bright object.

The inner working angle (IWA) is roughly the radial distance from the center of the occulting mask at which the transmission of the mask rises to 50% of its asymptotic value at the largest apparent separations. IWA is commonly taken to mean the smallest apparent separation between a bright and a faint object at which the faint object could be detected.

NIRCam's 3 round coronagraphic masks have IWAs of 0.40″, 0.63″, and 0.81″ (radius), respectively corresponding to 6λ/D at 2.1, 3.35 and 4.1 μm, where λ is the observed wavelength and D is 6.5 m, the nominal diameter of the JWST aperture.

NIRCam's 2 bar coronagraphic masks are tapered, with IWA varying by a factor of 3 along their lengths. Compared to the round masks, the bar masks sacrifice some field of view in the direction along the bar as a function of azimuth around the bright object. During an observation, the bright object is positioned behind the bar at the location where IWA ~ 4λ/D.

Table 1. NIRCam coronagraphic (occulting) masks

Coronagraphic mask	Shape	IWA	Nominal wavelength range
MASK210R	round	0.40″	1.82–2.12 μm
MASK335R	round	0.63″	3.0–3.56 μm
MASK430R	round	0.81″	4.10–4.60 μm
MASKSWB	bar	0.13″–0.40″	1.7–2.2 μm
MASKLWB	bar	0.29″–0.88″	2.4–5.0 μm

NIRCam coronagraphic occulting masks and neutral-density squares for target acquisition — The NIRCam module A coronagraphic substrate, which includes bar and round masks for occulting bright objects and 5" × 5" neutral density squares for target acquisition. The 4 lines of information at the top are (1) nominal wavelength range, (2) nominal wavelength at which IWA = 4 or 6 λ/D, (3) mask name, and (4) inner working angle (IWA) HWHM (half width at half maximum). For acquisition of targets fainter than K = 12, clear squares (ND = 0) are utilized. The clear squares are located between the ND ~ 3 squares. Adapted from Krist et al. 2010, Figure 2.

Filters for NIRCam coronagraphic imaging

Words in bold are GUI menus/
panels or data software packages;
bold italics are buttons in GUI
tools or package parameters.

Only a subset of all NIRCam filters is available for NIRCam coronagraphic imaging. The available subset of filters depends on the selection of the coronagraphic mask, as described in NIRCam Filters for Coronagraphy.

Table 2. NIRCam filters permitted for coronagraphic imaging

Coronagraphic mask	MASK210R	MASK335R	MASK430R	MASKSWB	MASKLWB
Description	Small round	Medium round	Large round	Narrow bar	Wide bar
Nominal wavelength(s)	2.10 µm	3.35 µm	4.30 µm	2.1 µm (center) 1.7–2.2 µm	4.6 µm (center) 2.4–5.0 µm
IWA Prime channel**	6 λ/D 0.40″ (2.1 µm)	6 λ/D 0.57″ (3.35 µm)	6 λ/D 0.87″ (4.30 µm)	4 λ/D 0.23″ (1.82 µm) 0.25″ (2.00 µm) 0.27″ (2.12 µm)	4 λ/D 0.32″ (2.5 µm) 0.46″ (3.6 µm) 0.61″ (4.8 µm)
Permitted filters Prime channel**	F182M* F187N* F200W* F210M F212N	F250M F300M F322W2 F335M F356W F360M F410M F430M F444W F460M F480M	F250M F300M F322W2 F335M F356W F360M F410M F430M F444W F460M F480M	F182M* F187N* F200W* F210M F212N	F250M F277W F300M F335M F356W F360M F410M F430M F444W F460M F480M
Permitted filters Secondary channel**	F250M F277W F300M F335M F356W F360M F410M F430M F444W F460M F480M	F182M* F187N* F200W* F210M F212N	F182M* F187N* F200W* F210M F212N	F250M F277W F300M F335M F356W F360M F410M F430M F444W F460M F480M	F182M* F187N* F200W* F210M* F212N*

Table notes:

* For filters in orange, the transmission of the coronagraph optical mount (COM) can have a substantial impact on the effective wavelength of the observations. For example, the COM transmission increases from 48% at 1.8 µm to 88% at 1.9 µm.
** Starting with APT 2022.7 (Cycle 2), when a bar mask is used, the prime channel filter will determine the fiducial pointing along the bar.

Throughputs of all filters are available at NIRCam Filters. The coronagraphic optical mount (COM) transmission is available at NIRCam Filters for Coronagraphy.

Field of view

For each choice of a coronagraphic mask, the prime channel subarray field of view at the detector is a 20″ × 20″ square approximately centered on the image of the coronagraphic mask. From Cycle 2 and onward, when the LW bar mask (MASKLWB) is selected, this prime channel subarray is a rectangle as described in the NIRCam Coronagraphic Imaging APT Template article, Table 1. The secondary channel field of view depends on which channel is selected as primary because the geometry and number of pixels are conserved. Hence, if SW is primary, the LW field of view will automatically be doubled. On the other hand, if LW is primary, the SW field of view will automatically be halved.

Expected performance versus on-sky performances

The NIRCam Lyot coronagraphs are expected to detect sufficiently warm Jupiter-type exoplanets (possibly ~10 times less massive that what has been dones so far, mainly from the ground), as well as protostellar, protoplanetary, and debris disks around bright stars. Detectability depends primarily on the contrast (flux ratio) and apparent separation between the bright host and faint companion (more information is available at HCI Contrast Considerations). Higher contrast sources are detectable at larger apparent separations. Detections are improved by observing strategies, such as obtaining multiple observations at different roll angles, and by data analysis techniques.

Approximate limiting contrast ratio, required for 5-σ detection of a faint companion versus apparent separation from the nearby bright host. Expectations are shown for each round and bar occulter given subtraction of 2 images obtained at different roll angles (+5° and -5°) for speckle suppression. A position uncertainty of 10 mas and wavefront error of 10 nm between rolls were assumed. NIRCam should achieve almost 12 (18) magnitudes of suppression 1" (4") from the central bright object.
Top left: adapted from Beichman et al. 2010, Figure 6, somewhat obselete (at large separations in the background limited regime).
Bottom left: from Perrin et al. 2018 (MASK335R/F335M, 3,600 s on source). The predicted contrasts are very close to that measured on-sky (with 3,300 s, right) but at separations larger than 1", the contrast seems to flatten along the fundamental noise floor limit.
Right: on-sky contrast from Girard et al. 2022 (MASK335R/F335M, 3,300 s on source for each roll, 3 reference stars). The dashed green curves shows the discrepancy with Beichman et al. 2010 contrasts beyond ~1".
Exposure Time Calculator (ETC) estimates are adequate to prepare proposals in the vast majority of cases.

References

Beichman, C. et al. 2010, PASP, 122, 888
Imaging Young Giant Planets From Ground and Space

Carter, A., et al. 2022, AAS Journals (submitted)
The JWST Early Release Science Program for Direct Observations of Exoplanetary Systems I: High Contrast Imaging of the Exoplanet HIP 65426 b from 2-16 μm

Girard, J. H., et al., 2022 (Commissioning, # 1441), SPIE, 121803Q
JWST/NIRCam Coronagraphy: commissioning and first on-sky results

Green, J. et al. 2005, Proc. SPIE 5905, 0L
High contrast imaging with the JWST NIRCAM coronagraph

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

Krist, J. et al. 2009, Proc. SPIE 7440, 0W
The JWST/NIRCam coronagraph: mask design and fabrication

Krist, J. et al. 2007, Proc. SPIE 6693, 0H
Hunting Planets and Observing Disks with the JWST NIRCam Coronagraph

Perrin, M., et al., 2018, SPIE, 1069809
Updated Optical Modeling of JWST Coronagraph Performance, Stability, and Strategies

Latest updates	02 Dec 2022 Added information on FoV considerations and a note about MASK430R. Changed contrast curves, added information about SW+LW 25 May 2017 New Figure 2
Originally published	11 Feb 2017