NIRISS Non-Redundant Mask
The JWST NIRISS non-redundant mask (NRM) enables the aperture masking interferometry mode which is a high spatial resolution, moderate-contrast imaging technique.
Main article: NIRISS Aperture Masking Interferometry
NIRISS has a non-redundant mask (NRM) that enables aperture masking interferometry (AMI), which is a high resolution, moderate-contrast imaging technique. Examples of science enabled by this mode include direct imaging of binaries, exoplanets, transitions disks, and the environments around bright active galactic nuclei.
The NRM is an opaque element with seven hexagonal apertures located in the pupil wheel. The apertures are undersized with respect to the projected dimensions of the mirror segments to avoid image degradation from the mirror edges, struts supporting the secondary mirror, and secondary mirror. The 15% throughput seven-holes mask offers simultaneous, multi-baseline interferometry and is used with the NIRISS long wavelength medium-band (F380M, F430M, and F480M) and F277W broadband filters.
When viewing a point source, the NRM offers 21 unique baselines defined by the pairs of holes in the mask to create a sharply peaked interferogram. None of the 21 separations between the different aperture pairs that define the baselines are the same, and hence, this element's reference as a "non-redundant" mask. Objects as close as δθ = 0.5 λ/D (inner working angle) can be resolved.
The interferogram generated by the NRM has a PSF with a profile that is more than twice as sharp as the full aperture PSF, but with much broader wings. The fringe patterns created by the multiple holes in the mask are imprinted in the broad wings, allowing the retrieval of information at smaller spatial scales than possible with the full aperture PSF.
Filters used with NRM to enable AMI Mode
Main article: NIRISS Filters
See also: NIRISS Sensitivity, NIRISS Bright Limits, NIRISS AMI-Specific Treatment of Limiting Contrast
NRM will be used in conjunction with F277W, F380M, F430M, or F480M; these filters were chosen to capture spectral regions of scientific interest. The bandpasses are relatively narrow to preserve the non-redundancy of the u − v (i.e., spatial frequency) coverage. The properties, including estimated saturation limits in the NIRISS filter bandpasses and in the WISE W1 (3.4 μm) and W2 (4.2 μm) bands, are listed in Table 1.Figure 3 shows the transmission curves for these filters.
Table 1. NIRISS AMI filter properties
|Filter||λ (μm)||Δλ/λ||IWA† (mas)||Fraction of flux|
|Saturation‡ (72,000 e–) |
for Ngroups = 2
W1 = 7.2
W1 = 4.2
W2 = 3.5
W2 = 3.1
† Inner working angle (IWA) for deepest contrast. Beyond 400–500 mas NIRCam coronagraphs provide higher contrasts.
‡ For Ngroups = 1 the bright limit will be approximately 0.75 mag brighter.
Sivaramakrishnan, A., et al. 2014 STScI Newsletter 31 1
NIRISS aperture masking interferometry: an overview of science opportunities
Artigau, E., Sivaramakrishnan, A., et al. 2014, arXiv:1406.6882
NIRISS Aperture Masking Interferometry: An overview of science opportunities
Sivaramakrishnan, A. et al., 2009, SPIE, 7440
Planetary system and star formation science with non-redundant masking on JWST