What can you do with JWST if you are looking for faint sources or an extended structure with really small separations? The inner working angles of the NIRCam coronagraphic modes will limit you to separations greater than 290 mas at 2.4-5.0 µm, and only with the bar occulting masks.

Aperture Masking Interferometry (AMI) mode of JWST’s Near Infrared Imager and Slitless Spectrograph (NIRISS) may provide the answer, with its unique capability of high-resolution, moderate contrast (10^-4) imaging at a separation as small as 70 mas.

How does it work? AMI turns the full aperture of JWST into an interferometric array. This is accomplished by adding a 7-hole “non-redundant” aperture mask, which provides 21 unique (“non-redundant”) pairs of vector baselines to sample the incoming wavefront observed through one of the medium-band filters (F380M, F430M, F480M) or the broadband F277W filter. The analysis of these interferometric data reveal source structure, and faint companions.  Detections are often photon noise limited.

The AMI mode of NIRISS represents the first implementation of aperture masking on a space-based platform. The stability of space-based imaging is expected to provide highly calibratable fringe amplitudes, because of JWST’s photometric stability. When combined with mature analysis techniques developed for ground-based interferometry, AMI can be a powerful platform for diverse science cases, such as:

• High-Resolution, Moderate-Contrast Imaging of Point Sources
  Enables detection of planets and structures within ~70-500 mas of their parent star at 3 - 5 µm.  At separations above ~400 mas coronagraphic imaging is preferred.
• High-Resolution Imaging of Extended Sources
  Provides data for synthesis imaging and other image reconstruction methods (e.g., galactic nuclei, young extra-solar giant planets, solar system objects).
• Exoplanet Characterization
  The three medium-band filters together with F277W isolate spectral features that characterize the temperature and gravity of giant planets and brown dwarfs.

• Bright Object Imaging
  Identify structure in extremely bright objects, e.g. Jupiter's moon Io, Wolf-Rayet stars.

For more information about NIRISS AMI and the technique of aperture masking interferometry, please refer to:

• NIRISS Aperture Masking Interferometry
• NIRISS AMI Observations of Extrasolar Planets Around a Host Star
• NIRISS AMI Recommended Strategies