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.
See also: 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 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 from 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 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
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‡ (30,000 e–) |
for Ngroups = 2*
† Inner working angle (IWA) for deepest contrast. Beyond 400–500 mas NIRCam coronagraphs provide higher contrasts.
‡ When 2 neighboring pixels accumulate charge at very different rates, the brighter pixel “spills” photoelectrons on to its neighbor, but the reverse does not occur. This effect becomes pronounced above about 30,000 e– in the bright pixel. We mitigate this effect in AMI data by setting a signal limit lower than the true non-linearity-based saturation limit for the NIRISS detector.
* For Ngroups = 1 the bright limit will be approximately 0.60 mag brighter.
** The NIRISS magnitudes in filters F277W and F380M roughly correspond to WISE W1 magnitudes. The NIRISS magnitudes in filters F430M and F480M roughly correspond to WISE W2 magnitudes. There is a ±0.05 magnitude uncertainty due to the conversion from NIRISS magnitude to WISE magnitudes, which is a function of the spectral shape of the source. The magnitudes of the WISE and NIRISS filters should match for an average A0V star and WISE magnitudes are predicted to be slightly smaller than the NIRISS magnitudes for later spectral types. There is an additional uncertainty of order ±0.1 in the simulated NIRISS magnitudes for a given spectrum due to uncertainties in the NIRISS throughputs and quantum efficiency.
Sivaramakrishnan, A., et al. 2014 STScI Newsletter 31 1
Aperture-Masking Interferometry with Webb’s NIRISS
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