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The JWST NIRISS non-redundant mask (NRM) enables the Aperture Masking Interferometry mode which is a high-spatial resolution, moderate-contrast imaging technique.

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Introduction

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.

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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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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

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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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Transmission curves of AMI filters, based on measurements at cryogenic temperatures by the manufacturer.

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Table 1. NIRISS AMI filter properties

Filter

λ
(μm)

Δλ/λ

IWA1
(mas)

Fraction of flux in
brightest pixel (max)

Saturation2 (72,000 e) for NGROUPS = 2
(Vega)

     NIRISS FiltersWISE
F277W2.7826.3%890.052

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W1 = 7.2
F380M3.835.4%1200.028

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W1 = 4.2

F430M

4.295.0%1400.022

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W2 = 3.5
F480M4.826.4%1500.018

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W2 = 3.1

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IWA

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1  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 mags brighter.


 


 

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Related links

NIRISS Observing Modes

NIRISS Aperture Masking Interferometry

NIRISS Detector

JWST Detector MULTIACCUM Integration
NIRISS Detector Readout Patterns
NIRISS Detector Subarrays

NIRISS Operations

NIRISS Target Acquisition
NIRISS AMI Dithers

NIRISS Performance

NIRISS AMI-Specific Treatment of Limiting Contrast
NIRISS Bright Limits

NIRISS Sensitivity 

Recommended Strategies

NIRISS AMI Recommended Strategies

NIRISS AMI Science Use Case

NIRISS AMI Observations of Extrasolar Planets Around a Host Star

Observing Methods

JWST High-Contrast Imaging
JWST Moving Target Observations
Instrument-Specific Considerations for Moving Targets  

Exposure Time Calculator

JWST Exposure Time Calculator Overview
JWST ETC NIRISS Target Acquisition

Astronomer's Proposal Tool

JWST Astronomers Proposal Tool Overview
NIRISS Aperture Masking Interferometry Template APT Guide
JWST APT Special Requirements 

 

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References

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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 data
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Last updated

Published July 11, 2017


 

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Published March 02, 2017


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UpdatedJuly 11, 2017
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TeamNIRISS 
AuthorRavindranath, La Massa 
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