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Introduction

Parent page: NIRISS Instrumentation
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.

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Figure 1. NIRISS non-redundant mask (NRM)

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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).

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.

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Figure 2. NIRISS AMI point spread function (from cryovac data)

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This is 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 the core. The contrast present in these fringes helps 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

Main article: NIRISS Filters
See also: NIRISS Sensitivity, NIRISS Bright Limits, NIRISS AMI-Specific Treatment of Limiting Contrast 

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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.

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Figure 3. Filters for use with AMI mode

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A figure showing transmission curves of AMI filters, based on measurements at cryogenic temperatures by the manufacturer.

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Table 1. NIRISS AMI filter properties

Filterλ (μm)Δλ/λIWA (mas)Fraction of flux
in brightest
pixels (max) 
Saturation (72,000 e)
for Ngroups = 2
(Vega)





     NIRISS filtersWISE

F277W

2.78

26.3%

89

0.052

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W1 = 7.2

F380M

3.83

5.4%

120

0.028

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W1 = 4.2

F430M

4.29

5.0%

140

0.022

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W2 = 3.5

F480M

4.82

6.4%

150

0.018

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W2 = 3.1

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IWA

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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.

 

 

 





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References

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

Greenbaum, A.Z., Pueyo, L., Sivaramakrishnan, A., Lacour, S., 2015, ApJ, 798, 68

An image-plane algorithm for JWST's non-redundant aperture mask dataJWST technical documents





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