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Last Updated  Mar 24, 2017


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

Introduction

NIRCam coronagraphy enables high-contrast imaging, in which the diffracted light of a bright object is suppressed to reveal much fainter objects nearby.

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.

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 the bright object.

NIRCam's three round coronagraphic masks have inner working angles IWA = 0.40″, 0.63″, and 0.81″ (radius), corresponding to 6λ/D at 2.1, 3.35 and 4.1 μm, where λ is the observed wavelength and D = 6.5 m is the nominal diameter of the JWST aperture. 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 two 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
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 4 lines of information at the top are the (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 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
Descriptionsmall roundmedium roundlarge roundnarrow barwide bar
Nominal wavelength(s)
(IWA)
2.1µm
(6 λ/D) 

3.35µm
(6 λ/D) 

4.3µm
(6 λ/D)

1.03 – 3.1µm
(4 λ/D)

2.3 – 6.9µm
(4 λ/D)
Permitted filters

F182M
F187N
F200W
F210M
F212N

F250M
F300M
F322W2
F335M
F356W
F360M
F410M

F250M
F300M
F322W2
F335M
F356W
F360M
F410M
F430M
F444W
F460M 

F182M
F187N
F200W
F210M
F212N

F250M
F277W
F300M
F335M
F360M
F410M
F430M
F444W
F460M
F480M

Orange: Effective wavelength is altered by the transmission of the coronagraph optical mount (COM), which has a long-pass anti-reflection coating that cuts on longward of 1.9 µm.

Throughputs of all filters are available at NIRCam Filters.

 


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 2. 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. 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 3 shows the estimated limiting contrast performance of the 5 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 3. Approximate limiting contrast ratios expected

Approximate limiting contrast ratio, required for 5-sigma 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. Adapted from Beichman et al. 2010, Figure 6. More precise estimates may be obtained using the Exposure Time Calculator (ETC).
 


 

Related links

Near Infrared Camera, NIRCAM

NIRCam Overview

NIRCam Coronagraphic Occulting Masks and Lyot Stops

NIRCam Filters for Coronagraphy

References

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

Krist, J. et al. 2010
"The JWST/NIRCam coronagraph flight occulters"
Proc. SPIE 7731, Optical, Infrared, and Millimeter Wave, 77313J

Krist, J. et al. 2009
"The JWST/NIRCam coronagraph: mask design and fabrication"
Proc. SPIE 7440, Techniques and Instrumentation for Detection of Exoplanets IV, 74400W

Krist, J. et al. 2007
"Hunting Planets and Observing Disks with the JWST NIRCam Coronagraph"
Proc. SPIE 6693, Techniques and Instrumentation for Detection of Exoplanets III, 66930H

Green, J. et al. 2005
"High contrast imaging with the JWST NIRCAM coronagraph"
Proc. SPIE 5905, Techniques and Instrumentation for Detection of Exoplanets II, 59050L
 

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