NIRISS has a non-redundant mask (NRM) which 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 7 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 due to the mirror edges, struts supporting the secondary mirror, and secondary mirror. The 15% throughput 7-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 (inner working angle) can be resolved.
Figure 1. NIRISS non-redundant mask (NRM)
Left: NIRISS's titanium NRM prior to blackening. Right: A full-scale prototype NRM showing the JWST primary mirror segments and secondary mirror supports engraved on it. In this re-imaged pupil plane, the diameter of the circumscribing circle of the full pupil is nominally 40 mm. The holes are undersized to allow pupil placement error of up to 3.8% of the pupil diameter. (Sivaramakrishnan et al, 2014).
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.
Figure 2. NIRISS AMI point spread function (from cryovac data)
An image (on a linear stretch) of a thermal point source taken during cryovacuum testing, using the NIRISS F380M filter. The sharp core with a dark area around it is one feature of this NRM image. Another feature is the extended "fringing" around this core. The contrast present in these fringes help explain why NRM images push theoretical resolution limits, and why relative astrometry with NRM is so sensitive. The science data coordinate system directions are shown in this cropped (35 × 35 pixels) image.
Filters used with NRM to enable AMI Mode
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.
Figure 3. Filters for use with AMI mode
Transmission curves of AMI filters, based on measurements at cryogenic temperatures by the manufacturer.
Table 1. NIRISS AMI filter properties
Fraction of flux in
brightest pixel (max)
Saturation2 (72,000 e–) for NGROUPS = 2
| || || || || ||NIRISS Filters||WISE|
|PageWithExcerpt||NIRISS Bright Limits||W1 = 7.2|
|PageWithExcerpt||NIRISS Bright Limits||W1 = 4.2|
|PageWithExcerpt||NIRISS Bright Limits||W2 = 3.5|
|PageWithExcerpt||NIRISS Bright Limits||W2 = 3.1|
1 Inner working angle (IWA) for deepest contrast. Beyond 400–500 mas NIRCam coronagraphs provide higher contrasts.
2 For NGROUPS=1 the bright limit will be approximately 0.75 mags brighter.