High Resolution Imaging with the NIRISS Non-Redundant Aperture Mask
Through the use of a non-redundant aperture mask (NRM), NIRISS will provide JWST's highest resolution imaging. This will be particularly useful for high-contrast imaging of sources around bright stars, as well as for measuring the structural properties of the nuclei of galaxies, star-clusters, and other sources bright enough to take advantage of this observing mode.
For exoplanets, an important challenge is to detect faint objects close to a bright point source. The NRM is designed to detect point sources that are separated by 0.1 - 0.5 arcsec with a brightness (contrast) ratio as small as 10-4-10-5. The NRM on NIRISS is optimized to operate between 3.8 and 5.0 μm. Model atmospheres of gas giant planets predict that the spectral energy distribution peaks beyond the M-band (4-5 microns), which is difficult to observe from the ground due to the high background. The spectral region beyond ~2.5 μm is thus a unique discovery space for JWST for exoplanet detection and characterization. NIRISS is capable of resolving objects at 0.2" with a contrast of 9.5 magnitudes and aims to achieve the same resolution for point sources with a contrast ratio of 12.0 – 12.4 magnitudes.
Design and Features of the Non-Redundant Aperture Mask (NRM)
- The 7 hole mask is placed in the Pupil Wheel of the NIRISS. When projected back to the Primary Mirror (PM), the subapertures of the mask are located near the centers of 7 of the 18 PM segments. The hexagonal holes are undersized relative to the primary mirror segments in order to allow for the possibility of shifts ("pupil shear") between JWST's pupil image at the location of the mask.
- The mask turns the full aperture of a telescope into an interferometric array and is designed such that each baseline (i.e., the vector linking the centers of two holes) is unique
and forms fringes with a unique spatial frequency in the image plane. Since each spatial frequency is sampled only once the mask is called non-redundant. The resulting PSF is
created by several coherent fringe patterns overlaid across one another.
- When projected onto the primary mirror, the longest baseline (BL) and shortest baseline (BS) correspond to 5.28 m and 1.32 m, respectively.
- The longest baseline provides a resolving power of 0.5λ/D, compared to the traditionally accepted full aperture Rayleigh criterion limit of 1.22λ/D.
- Resolution (0.5λ/BL) ~75 mas at 4.6 micron
- Nominal FOV (0.5λ/BS) ~0.4" at 4.6 micron
- Throughput 15%, Contrast sensitivity ~10 mag
- Unlike conventional PSF co-addition, the information extracted from the NRM data can be averaged to reduce noise, even in the presence of slowly varying artifacts ("speckles").
Dual-Wheel Configuration for Aperture-Masking Interferometry
The NRM (highlighted in green) in the Pupil Wheel may be used only with the medium band filters (highlighted in green) in the Filter Wheel.