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.

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Inner working angle (IWA)

Main articles: NIRCam Coronagraphic ImagingMIRI Coronagraphic ImagingNIRISS Aperture Masking Interferometry

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 (sIWA) 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)

InstrumentMask typeMask namefiducial λ (μm)N
(λ/D)
IWA
(arcsec)




NIRCam


round

MASK210R2.1


6

0.40"
MASK335R3.00.57"
MASK430R4.60.87"


short-λ
bar 


MASKSWB


2.1

2

0.14"
40.27"
60.41"


long-λ
bar 


MASKLWB


4.6

20.30"
40.59"
60.89"



MIRI


4-quadrant
phase mask

MASK106510.65

1
0.34"
MASK1440
14.400.46"
MASK1550
15.500.49"
LyotMASKLYOT2332.16"



NIRISS



non-redundant mask 


MASK_NRM

2.77



1

0.089"
3.80.12"
4.30.14"
4.80.15"

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

Figure 1. MIRI coronagraphic masks: throughput vs. apparent separation 

MIRI: throughput vs. apparent separation for combinations of coronagraphic mask and filter. The dashed lines show the apparent separations for 50% transmission.


NIRCam: throughput vs. apparent separation for combinations of coronagraphic mask and filter

Main articles: NIRCam Coronagraphic Occulting Masks and Lyot StopsNIRCam Filters for Coronagraphy

Figure 2. NIRCam coronagraphic masks: throughput vs. apparent separation

NIRCam: throughput vs. apparent separation for combinations of coronagraphic mask and filter. The dashed lines show the apparent separations for 50% transmission.


References

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




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