The aperture masking interferometry (AMI) mode in JWST's Near Infrared Imager and Slitless Spectrograph (NIRISS) offers high spatial resolution, moderate contrast imaging at 2.8, 3.8, 4.3, and 4.8 μm (e.g., binary point source contrasts as high as ≈10^-4), at separations between ~70–400 mas. At 2.8 μm it delivers reduced performance.

See also: NIRISS Observing Modes, NIRISS Aperture Masking Interferometry APT Template, NIRISS AMI Recommended Strategies, NIRISS AMI Science Use Case

NIRISS's aperture masking interferometry (AMI) mode (Monnier 2003) turns the full aperture of JWST into an interferometric array. Light admitted by 7 holes or sub-apertures in an otherwise opaque pupil mask interferes to produce an interferogram on the detector. The mask is designed such that each baseline (i.e., the vector linking the centers of 2 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." (A full aperture, on the other hand, can be considered as made up of infinite sub-apertures and multiple sets of sub-apertures to generate the same baseline making the aperture extremely redundant.) The interferogram created by the aperture mask has a sharper core than that provided by normal "direct" imaging.

The advantage is significant: while the ability to separate closely spaced objects with normal imaging is given by the familiar Rayleigh criterion (separation $//<![CDATA[ \begin{array}{l}\delta\theta = 1.22 \, \lambda / D\end{array} //]]>$ , where $//<![CDATA[ \begin{array}{l}\lambda\end{array} //]]>$ is the wavelength of light and D is the diameter of the telescope), interferometry can resolve objects as close as $//<![CDATA[ \begin{array}{l}\delta\theta = 0.5 \, \lambda / D\end{array} //]]>$ (the Michelson criterion). The AMI mode allows planetary or stellar companions that are up to ~9 magnitudes fainter than their host star and separated by ~70–400 mas to be detected and characterized. It can also be used to reconstruct high resolution maps of extended sources, such as active galactic nuclei.

The AMI mode is enabled by a non-redundant mask (NRM) in the pupil wheel (PW), which is used in conjunction with one of 3 medium-band filters (F380M, F430M, F480M) or a wideband filter (F277W) in the filter wheel (FW).

See the JWST High-Constrast Imaging (HCI) article for a discussion on the various JWST modes that enable high-contrast imaging (HCI). Additional NIRISS-related HCI information is provided in HCI NIRISS Limiting Contrast.

For a walk-through on developing a JWST observing program using NIRISS AMI, please refer to the science use case article NIRISS AMI Observations of Extrasolar Planets Around a Host Star.

The PSF produced by the NRM has a narrow central diffraction core surrounded by an extended skirt resulting from overlapping fringes produced by each pair of apertures. These features give AMI 2 distinct advantages when compared to coronagraphy or full aperture imaging:

1. The sharp core of the PSF produces better signal-to-noise ratios close to a bright host star than full aperture imaging.
2. The interferometric fringes in the outskirts of the NRM PSF are easily measured due to their relative brightness and wider angular extent, making instrumental effects easier to calibrate out of science data

By comparing the distribution of flux in an interferogram obtained for a target of scientific interest (e.g., the binary shown in the left panel of Figure 3) with the distribution obtained for a reference star that is known to be (or strongly suspected of being) single (e.g., the right panel of Figure 3), AMI observations allow secure detections of faint companions at separations that are not accessible to the coronagraphic modes of JWST. AMI observations acquired through multiple filters allow the spectral properties of the secondary to be measured.

Advantages and disadvantages of space-based aperture masking interferometry

Space-based near-infrared interferometry has 3 advantages over similar ground-based interferometry. The first is that fringe visibilities (or relative photometry) from space are many orders of magnitude more stable than ground-based visibilities. Atmospheric scintillation corrupts ground-based visibility amplitudes, which renders symmetric structures hard to recover reliably. Good fringe visibilities provide more reliable disk brightness and structure.

Secondly, JWST's low thermal background enables fainter targets to be observed, since ground-based 3–5 µm interferometry can be thermal background-limited.

The third advantage is that fringe phases, closure or kernel phases derived from space-based data, are an order of magnitude more accurate than ground data can provide. Interferometric phase data are only sensitive to radially anti-symmetric structure in the target, so fringe phases and amplitudes from space-based data provide leverage for high-contrast binary point sources where the symmetric component is essentially the bright central star and the anti-symmetric component is dominated by the faint companion.

On the other hand, JWST's largest baseline is 6.5 m which is significantly smaller than that of single-dish ground imaging (8–10 m) or multiple telescope interferometers (up to hundreds of meters). Combining AMI data with ground-based near-simultaneous interferometric data can improving the resulting science.

AMI exposure sequence

See also: NIRISS AMI Template APT Guide, NIRISS Non-Redundant Mask, NIRISS Target Acquisition, NIRISS Detector Subarrays, NIRISS Filters

An AMI observation sequence usually involves a target acquisition (TA) followed by images using the NRM in combination with the desired filters. A TA is required to ensure accurate and reproducible placement of targets within the small subarray that is typically used for AMI. The TA procedure is optional for applications of AMI that require full frame exposures.

Target acquisitions are accomplished by taking short integrations in a predefined subarray through the F480M filter in the FW and either the NRM (for brighter targets) or CLEARP element (for fainter targets) in the PW. The TA procedure autonomously determines the centroid of the brightest object in the TA subarray. Accurate knowledge of the position of the source at this location is used to command a small slew that places it accurately in the subarray used for AMI science.

The TA is followed by the science exposures, which use the NRM in the PW and one or more of the 4 filters in the FW available to AMI:  F277W, F380M, F430M, and F480M. The science exposure may be taken with a subarray or a full frame aperture, depending on the science requirements or the brightness of the source. Optionally,  one or more direct images using the CLEARP aperture and the same suite of FW filters as those used for the NRM images may be obtained for PSF characterization or related analyses.

This entire sequence is typically repeated for a nearby "reference star," which is single and ideally of similar magnitude and color. When contrast limits are not very demanding, a reference star from an unrelated observation, or possibly a synthetic reference PSF can be used. For more demanding cases, the science target(s) and reference star(s) observations should not be separated by any adjustment of JWST's primary and secondary mirrors.

Filters used with NRM to enable AMI mode 

See also: NIRISS Filters, NIRISS Non-Redundant Mask, NIRISS Sensitivity, NIRISS Bright Limits, HCI NIRISS Limiting Contrast 

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 4 shows the transmission curve of the AMI filters.

Table 1. NIRISS AMI filter properties

AMI detector array and subarrays

See also: NIRISS Detector Subarrays

Science targets for the AMI mode are typically bright point sources. The AMI mode usually uses an 80 × 80 subarray (which includes 4 rows of reference pixels that are not sensitive to light). The subarray can be read out quickly enough to ensure that sources as bright as M ≈ 3 (Vega magnitude system) will not saturate in the F480M filter. For faint targets, the full frame mode can be used. Target acquisition for the AMI mode uses a 64 × 64 subarray. More details are available on the NIRISS subarrays article.

AMI Flat field errors

Analysis of on-sky flat field measurements indicate that current flat fielding errors do not affect AMI detection limits substantially. Point source simulations of a target and calibrator star separated by 2 mas in each of the detector X and Y axes were modeled along with two sources of noise: flat field errors (from on-sky flat measurements) and photon noise. These simulations were analyzed to derive detection limits, which are shown in Figure 5. The simulations considered a shallow exposure to detect 10⁸ photons and a deeper exposure to detect 10¹⁰ photons. Detection limits for these two exposures differ by a factor of 10, or 2.5 magnitudes, which is consistent with photon noise-limited results.

AMI dither patterns

See also: NIRISS AMI Dithers, NIRSS Dithers

Dithering is available but not required for AMI mode observations. Dithered AMI data may help to mitigate:

• residual errors in the flat field,
• effects of hot pixels or bad pixels,
• effects of cosmic ray hits that are not identified in standard processing, and
• inter-pixel capacitance variations.

Since these effects occur on different spatial scales on the detector, dither patterns involving spatial offsets of many pixels as well as small fractions of a pixel may be required to treat them.

The dither patterns for the AMI mode are implemented as "primary" dithers that perform ~30 pixel offsets (with up to 4 positions within the 80 × 80 pixels science subarray) in conjunction with "secondary" dithers that are subpixel offsets (0.20–0.33 pixels) designed to obtain adequate pixel phase coverage. More details on the available AMI dither patterns may be found on the NIRISS dithers page. 

References

Artigau E., Sivaramakrishnan A., Greenbaum A. Z. et al., 2014, SPIE, 9143E, 40A