JWST High Contrast Imaging Inner Working Angle
The JWST inner working angle (IWA) governs how close to the host the companion can be and still be observable.
Inner working angle (IWA)
The inner working angle (IWA) is approximately the smallest apparent separation (s) between host and companion sources at which the companion is detectable—if it is bright enough, of course.
The design specification is IWA = Nλ/D, where the nominal aperture diameter is D = 6.5 m, λ is a fiducial wavelength, and N = 6 for the NIRCam round occulters, N = 4 for the center of NIRCam bar occulters, N = 3 for MIRI Lyot-type coronagraph, and N = 1 for MIRI's 4QPMs and NIRISS's AMI.
For the coronagraphs, IWA is fixed and approximately equal to the 50% transmission radius of the coronagraphic mask (occulter). For NIRCam's bar occulters, Table 1 gives IWAs for the ends and the middle of the bar.
For MIRI's phase masks (4QPMs) and the NIRISS aperture masks (AMI), the nominal IWA = λ/D for the table's fiducial wavelengths.
Closer-in science (s < IWA) may be possible, but we caution observers that even if the throughput is not zero, it is increasingly reduced by the light lost on the coronagraphic mask, and the PSF of the companion is increasingly distorted by the same effect. Furthermore, the raw contrast Craw(s) will be increased (worsened) by the combination of reduced companion PSF and increased host PSF, at the same apparent separation s. These effects are likely to be very difficult to calibrate. Plots of the companion's estimated throughput versus s for relevant combinations of coronagraphic mask and filter, for both MIRI and NIRCam, can be found in Figures 1 and 2. PSFs computed by WebbPSF will show the expected distortion of the companion image based on current values of the aberrations in the JWST telescope optics.
Table 1. JWST nominal inner working angles (IWAs)
|Instrument||Mask type||Mask name||fiducial λ (μm)||N|
1 Inner working angle for deepest contrast.
A variety of NIRCam filters are permitted for coronagraphy. For MIRI and NRISS, filters are available that match the fiducial wavelength.
MIRI: throughput vs. apparent separation for combinations of coronagraphic mask and filter
Main article: MIRI Coronagraph Masks
NIRCam: throughput vs. apparent separation for combinations of coronagraphic mask and filter
Perrin, M., et al. 2014, Proc. SPIE. 9143, 91433X
Updated point spread function simulations for JWST with WebbPSF
Krist, J., et al., 2010, SPIE, 77313J
The JWST/NIRCam coronagraph flight occulters
Boccaletti, A., et al. 2015, PASP, 127, 633
The Mid-Infrared Instrument for the James Webb Space Telescope, V: Predicted Performance of the MIRI Coronagraphs