JWST High-Contrast Imaging Optics
High-contrast imaging (HCI) optics for 3 JWST instruments have been designed to suppress diffracted light from a source to make its fainter companions more observable.
Three JWST instruments offer various high-contrast imaging (HCI) optics to suppress diffracted light from the host and thereby make companion sources—both point and extended—more observable.
- NIRCam coronagraphic imaging:
- Five Lyot-type coronagraphs (3 round, 2 bars) are available.
- All allowed NIRCam coronagraphic configurations are listed at the NIRCam Filters for Coronagraphy article.
- NIRCam coronagraphs with round coronagraphic masks (occulters) work best in narrow and medium bands centered at 1.92, 3.23, and 4.35 µm.
- Coronagraphs with bar-shaped occulters work best in narrow and medium bands in the ranges 1.7–2.2 µm and 2.4–5 µm.
- MIRI coronagraphic imaging:
- One Lyot-type coronagraph and three 4-quadrant phase mask coronagraphs (4QPMCs) are available.
- MIRI 4QPMs work only in narrow bands centered at 10.65, 14.40, and 15.50 µm.
MIRI's Lyot-type coronagraph works only in a broad band centered at 23 µm.
- NIRISS aperture masking Interferometry: one aperture masking interferometer (AMI).
- NIRISS/AMI works best with medium bands centered at 3.8, 4.3, and 4.8 µm.
- NIRISS’s 65 mas pixels satisfy the Nyquist criterion at 4 µm and performance at shorter wavelengths is reduced. Nevertheless, because the wide filter centered at 2.77 µm spans a deep absorption feature of water, its use may be particularly relevant for exoplanetary research.
In addition, HCI can be carried out using basic imaging modes of the observatory (Rajan et al., 2015; Durcan, Janson, and Carson, 2016), as well as using IFU strategies similar to Konopacky et al. (2013). These modes are not yet covered in the documentation.
Main article: NIRCam Coronagraphic Occulting Masks and Lyot Stops
See also: NIRCam Filters for Coronagraphy
The HCI optics of Lyot-type coronagraphs are pairs of binary masks of different types: one occulter and one Lyot stop. The first mask—the occulter or "coronagraphic mask"—lies on the first focal plane of the imaging instrument, where it blocks light from the center of the host image, but allows any other light to pass. The effective radius of the occulter is the inner working angle (IWA), which is about as close to the host as one can work. The second mask—the Lyot stop—lies in the plane of the re-imaged pupil, where it blocks light from the host that has been diffracted at the edges of the primary mirror segments, the secondary mirror support structure, and the occulting mask itself. In other words, the Lyot stop suppresses the diffraction spikes and rings that commonly appear in direct images of bright stars. (The Webb PSF software may be used to get familiar with the detailed morphology of occulted coronagraph images.)
If the apparent separation between the feature of interest and the host is greater than the IWA, companion light passes the occulter, and—after losing a bit of light on the Lyot stop—the light from the feature of interest reaches the detector. On JWST, NIRCam has 5 sets of Lyot-type coronagraphic optics (3 with round and 2 with bar-shaped occulters), and MIRI has one Lyot-type coronagraph, with a round occulter. (See Figure 2.)
Four-quadrant phase-mask coronagraph (4QPMC)
Main article: MIRI Coronagraph Masks
The HCI optics of 4QPMCs are pairs of a phase mask on the focal plane and a Lyot stop (apodizer) on the pupil plane. The phase mask, which is transparent at the appropriate wavelengths, imparts a 180° phase shift to light passing through 2 quadrants on the diagonal. Light from a source centered on the common point of the 4 quadrants interferes destructively. As in a Lyot-type coronagraph, the Lyot stop attenuates light diffracted from the edges of the telescope aperture and support structures, in the pupil plane, and from phase and amplitude aberrations on the wavefront. The optical advantage of a 4QPMC is a very small IWA in terms of λ/D (IWA = 1 λ/D at λ = 10–16 μm), which somewhat compensates for the lower, diffraction-limited, spatial resolution at MIRI's long wavelengths. The price paid is a strong sensitivity to optical aberrations and source misalignments.
Aperture masking interferometry (AMI)
Main article: NIRISS Non-Redundant Mask
The HCI optic of an AMI is a non-redundant mask (NRM) in the pupil plane. The mask is opaque except for holes, in the case of NIRISS/AMI, 7 holes. The number of unique (i.e., non-redundant) baselines between pairs of holes in the NIRISS/AMI NRM is N × (N − 1)/2, or 21, where N is the number of holes in the NRM. Each baseline creates a single fringe pattern at focus, and the 21 fringes interfere to create an interferogram, which is actually just a PSF, The PSF's fine structure is more than twice as sharp as the corresponding full aperture PSF, but with much wider wings.
An NRM interferogram possesses certain observables—closure phases and fringe amplitudes—that can be calibrated using a PSF reference observation to remove many instrumental effects. For higher contrast observations, the reference star is expected to be placed within a few to 10 mas of the target star when both are commanded to the center of the pixel. Such repeated placement is aimed at mitigating residual pixel-to-pixel variations that remain after flat fielding and other routine image calibrations are performed. These observables allow the fitting of basic models, such as binary or triple point sources, and simple extended structures. Binary point source flux ratios of up to about 10 stellar magnitudes should be achievable. The search space of an NRM extends inwards to a separation of λ/2B, where B is the hole-to-hole length of the longest baseline. For fitting binary models as well as for true imaging—that is, not using closure relations or model-fitting—the NRM on NIRISS has an inner working angle (IWA) of about 70 mas. Beyond half an arcsecond, NIRCam's coronagraphs provide better contrast.
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