The aperture masking interferometry mode in JWST's Near Infrared Imager and Slitless Spectrograph (NIRISS) offers high spatial resolution imaging at 3.8, 4.3 and 4.8 μm for bright objects with ≈10-4 binary contrast at separations of 70–400 mas (with reduced performance at 2.8 μm)
The Aperture Masking Interferometry (e.g. Monnier 2003) (AMI) mode of NIRISS turns the extremely redundant full aperture of JWST into a simpler and more calibratable interferometric array. Light admitted by 7 apertures in an otherwise opaque pupil mask interfere to produce an interferogram on the detector. This interfogram 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 ( \( \delta\theta = 1.22 \, \lambda / D\) , 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). AMI 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. AMI 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 wide-band filter (F277W) in the filter wheel (FW).
See here for an overview of all JWST optics for high-contrast imaging (HCI).
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 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 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.
The AMI mode uses the NRM in conjunction with one of the 3 medium-band filters (F380M, F430M, F480M) or the wide-band F277W filter.
AMI detector array and 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' ≈ 2.4 (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 (after-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
- the effects of hot pixels or bad pixels
- the effects of cosmic ray hits that are not identified in standard processing
- 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 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 sub-pixel 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.
JWST User Documentation Home
Near Infrared Imager and Slitless Spectrograph, NIRISS
NIRISS Pupil Wheel and Filter Wheel
NIRISS aperture masking interferometry: an overview of science opportunities
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
Ford, K. E. S., McKernan, B., Sivaramakrishnan, A. et al. 2014, ApJ, 783, 73
Active Galactic Nucleus and Quasar Science with Aperture Masking Interferometry on the James Webb Space Telescope
Monnier, J. D. 2003, Reports on Progress in Physics, 65, 798
Optical interferometry in astronomy
Sivaramakrishnan, A. & Artigau, E. 2014, STScI Newsletter, Volume 31, issue 01
Aperture-Masking Interferometry with Webb's NIRISS
Thatte, D., Sivaramakrishnan, A., & Stansberry, J, STScI Newsletter, Volume 32, issue 02
Vulcanism on Io with Aperture Masking Interferometry on Webb’s NIRISS