Last Updated Mar 24, 2017
The sizes of the three round occulting masks are optimized for coronagraphic observations at λ ~ 2.1, 3.35, and 4.3 µm. At these wavelengths, the inner working angle IWA ~ 6 λ/D, where D = 6.5 m is the effective diameter of the JWST entrance aperture. IWA is the minimum-detectable apparent separation between the (occulted) bright source and the nearby faint source of scientific interest, assuming photometric criteria for detection are satisfied.
The 2 bar-shaped coronagraphic masks have tapered widths to accommodate ranges of short or long wavelengths with IWA ~ 4 λ/D.
When using each mask, a user may choose from multiple filters. For the round masks, the chosen filter may deviate from the optimal wavelength with some restrictions.
The NIRCam coronagraphs were not designed for optimal performance below 2 µm. Chromatic aberrations are induced in NIRCam's short wavelength channel by the optical wedges included in the Lyot stop elements used to project the occulting masks onto the detectors. These aberrations increase the PSF size along one axis, especially for wide filters, complicating PSF subtraction of the reference star. Furthermore, the coronagraph substrate antireflective coating has poor transmission below 1.9 µm (Figure 1).
Ground-based coronagraphs typically offer better performance (lower chromatic aberrations, wavefront errors, and IWAs) below 2 µm, except near 1.8 µm where the atmosphere is opaque. Therefore, NIRCam coronagraphy is not available below 1.7 µm.
Table 1. Permitted filters for use with each coronagraphic occulting mask
|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.
Related linksKrist, J,. et al. 2010, SPIE, 7313J
The JWST/NIRCam coronagraph flight occulters
On this page
JWST Observatory and Instrumentation