NIRISS Aperture Masking Interferometry
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.
NIRISS's aperture masking interferometry (AMI) mode (Monnier 2003 turns the full aperture of JWST into an interferometric array. Light admitted by seven 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 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. A full aperture on the other hand can be considered as made up of infinite sub-apertures and multiple sets of subapertures generate the same baseline making the aperture extremely redundant. The interferogram created by the aperture mask has a sharper core than is 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 \delta\theta = 1.22 \, \lambda / D , where $\lambda$ is the wavelength of light and D is the diameter of the telescope), interferometry can resolve objects as close as \delta\theta = 0.5 \, \lambda / D (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 three medium-band filters (F380M, F430M, F480M) or a wideband filter (F277W) in the filter wheel (FW).
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 in which the fringes produced by the apertures in the mask interfere with each other to produce complicated patterns. These features give AMI two distinct advantages compared with coronagraphy or full aperture imaging:
- The sharp core of the PSF produces better signal-to-noise ratios close to a bright host star than full aperture imaging.
- 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.
AMI exposure sequence
An AMI observation sequence usually involves a 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 four 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
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
|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.
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 four 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 page.
AMI dither patterns
Because of persistence (latent images) dithering is discouraged. 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 four 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.
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