NIRCam Coronagraphic Imaging

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

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See also: NIRCam Coronagraphic Imaging APT Template

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 five 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 JWST High-Contrast Imaging 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 three 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 three 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 two bar coronagraphic masks are tapered, with IWA varying by a factor of three 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
Wavelength range
MASK210Rround0.40″1.82–2.12 μm
MASK335Rround0.63″3.0–3.56 μm
MASK430Rround0.81″4.10–4.60 μm
MASKSWBbar0.13–0.40″1.7–2.2 μm
MASKLWBbar0.29–0.88″2.4–5.0 μm


Figure 1.  NIRCam coronagraphic occulting masks and neutral density squares for target acquisition
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 four 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

Only a subset of alNIRCam 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 maskMASK210R1MASK335RMASK430RMASKSWBMASKLWB
DescriptionSmall roundMedium round Large roundNarrow barWide bar
Nominal wavelength(s)2.10 µm3.35 µm4.30 µm2.1 µm (center)
1.7–2.2 µm 
4.6 µm (center)
2.4–5.0 µm 
IWA6 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

F182M
F187N
F200W
F210M
F212N

F250M

F300M
F322W2
F335M
F356W
F360M
F410M
Available in APT v25.4: F444W

F250M

F322W2
F410M
F430M
F444W
F460M
Available in APT v25.4: F300M F335M F356W F360M

F182M
F187N
F200W
F210M
F212N

 F250M F277W F300M F335M F356W F360M F410M F430M F444W F460M F480M

Table note:
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.


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

Figure 2. NIRCam filters available for coronagraphy



Field of view

For each choice of coronagraphic mask, the field of view at the detector is a 20″ × 20″ square centered on the image of the coronagraphic mask. 

Figure 3. NIRCam module A coronagraph in the field of view

NIRCam Module A coronagraph in the field of view

The NIRCam coronagraph field of view is projected onto the detectors by optical wedges located on the pupil plane Lyot stops. The imaging region of each coronagraph mask is 20" × 20".


Expected performance

The NIRCam Lyot coronagraphs are expected to detect sufficiently warm Jupiter-type exoplanets, 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 Contrast Considerations for JWST High-Contrast Imaging). 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.

Figure 4 shows the estimated limiting contrast performance of the five NIRCam coronagraphs, under the technical and procedural assumptions of Beichman et al. (2010). Companions with contrasts above the curves would be detectable . (That paper notes the NIRCam coronagraphic performance is limited not by diffraction but rather by telescope scattering or mirror wavefront errors.)

Figure 4. Approximate limiting contrast ratios expected

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 two 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. Adapted from Beichman et al. 2010, Figure 6. More precise estimates may be obtained using the Exposure Time Calculator (ETC).


References

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

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




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