Last Updated Mar 24, 2017
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.
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.
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|
1.03 – 3.1µm
|2.3 – 6.9µm|
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.
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.)
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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