The JWST NIRISS non-redundant mask (NRM) enables the aperture masking interferometry mode which is a high spatial resolution, moderate-contrast imaging technique.
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
Figure 3 shows the transmission curves for these filters.
Sivaramakrishnan, A., et al. 2014 STScI Newsletter 31 1
Artigau, E., Sivaramakrishnan, A., et al. 2014, arXiv:1406.6882
Sivaramakrishnan, A. et al., 2009, SPIE, 7440
An image-plane algorithm for JWST's non-redundant aperture mask data