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. Currently, NIRCam coronagraphy is limited to Module A only.
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.
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 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
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
|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)|
|4.6 µm (center)|
0.40″ (2.1 µm)
0.57″ (3.35 µm)
0.87″ (4.30 µm)
0.23″ (1.82 µm)
0.25″ (2.00 µm)
0.27″ (2.12 µm)
| F250M |
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.
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.
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 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.
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.)
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