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, the table below gives IWAs for the ends and the middle of the bar.
For MIRI's phase masks (4QPMs) and the NRISS aperture masks (AMI), the nominal IWA = λ/D for the table's fiducial wavelengths.
Closer-in science (s < IWA) may be possible, but we warn 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. At the bottom of this article, you can find plots of the estimated throughput of the companion versus s for relevant combinations of coronagraphic mask and filter, for both MIRI and NIRCam. 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). A variety of NIRCam filters are permitted for coronagraphy. For MIRI and NRISS, filters are available that match the fiducial wavelength.
|mask name||fiducial λ (μm)|
non-redundant mask on pupil plane
MIRI: throughput vs. apparent separation for combinations of coronagraphic mask and filter
NIRCam: throughput vs. apparent separation for combinations of coronagraphic mask and filter
High contrast imaging articles
Perrin et al. 2014, “Updated point spread function simulations for JWST with WebbPSF”, Proc. SPIE. 9143,
Perrin et al. 2012, “Simulating point spread functions for the James Webb Space Telescope with WebbPSF”, Proc SPIE 8842, and
Perrin 2011, Improved PSF Simulations for JWST: Methods, Algorithms, and Validation, JWST Technical report JWST-STScI-002469.