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Last Updated  Mar 24, 2017


The aperture masking interferometry mode in JWST's Near Infrared Imager and Slitless Spectrograph (NIRISS) offers high spatial resolution imaging at 2.8, 3.8, 4.3 and 4.8 μm for bright objects with ≈ 10-4 predicted contrast at separations of 70–400 mas.

Introduction

The aperture masking interferometry (AMI) mode of NIRISS uses the wave nature of light to produce images with high spatial resolution at near-infrared wavelengths.  It is a simple and time-honored technique. Light admitted by 7 apertures punched in an otherwise opaque mask interfere with each other to produce an interferogram on the detector, which has a sharper core than 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 10 magnitudes fainter than their host star and separated by 70 – 400 mas to be detected and characterized.  AMI  can also be used to build up 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).

Figure 1. Prototype of the NIRISS non-redundant mask

NRM in the NIRISS pupil

A  prototype of the NIRISS non-redundant mask, which shows the layout of the 7 hexagonal apertures (holes) in the mask with respect to the JWST primary mirror segments and secondary-mirror supports.  Pairs of these apertures define 21 unique ("non-redundant") vector separations ("baselines") that produce an interferogram on the detector.  The holes transmit ~15% of the light incident on the mask. They are smaller than the re-imaged mirror segments to allow for small misalignments in the optical system. Photo credit: Anand Sivaramakrishnan.
Figure 2. Elements in the NIRISS pupil and filter wheel used by the AMI mode

Optical elements used by the AMI mode

Optical elements available to the NIRISS AMI mode are shown by the green dashed circles. AMI is enabled by using the non-redundant mask in the pupil wheel in combination with one of four filters in the filter wheel.
Figure 3. NIRISS AMI simulations

NIRISS AMI simulation of an 8 magnitude star with a fainter companion

Simulated images through the F430M filter of binary (left column) and single (right column) point sources are compared as direct images (top row) and interferograms produced by the NRM (bottom row).  In these simulations, the binary system consists of an 8th magnitude star with a 9th companion at a separation of 207 mas.  Arrows indicate how the interferograms produced by the NRM change in the presence of a binary source. Simulations by Thatte and Sivaramakrishnan (JWST-STScI-004484).
The PSF produced by the NRM, which is sometimes called the "interferogram", has a narrow central diffraction core surrounded by an extended skirt in which the interferometric fringes produced by the apertures in the mask appear as distinct "blobs."  These features give AMI two distinct advantages compared with coronagraphy:

  1. the outer regions of the PSF contain a greater fraction of the total flux relative to the intensity of the PSF peak
  2. the bright interferometric fringes in the outskirts of the NRM PSF are easily detectable due to their relative brightness and distant location from the core.

By carefully 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 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.

 


AMI Filters

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.

Figure 4: Filters for use with AMI mode

 


AMI subarrays

Science targets for the AMI mode are typically bright point sources.   The AMI mode usually uses an 80 × 80 subarray, which can be readout quickly enough to ensure that sources as bright as M' ≈ 4 (Vega magnitude system) will not saturate.  For faint targets, the full-frame mode can be used. Target acquisition for the AMI mode uses a 64 × 64 subarray. 


AMI dither patterns

Dithering is available but not required for AMI mode observations.  In general, acquiring dithered AMI data will help to mitigate

  • residual errors in the flat field
  • the effects of hot pixels
  • the effects of cosmic ray hits that are not identified in standard processing
  • intra-pixel sensitivity 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 are required to treat them.

The dither patterns for the AMI mode are implemented as "primary" dithers that perform 40-pixel offsets in a rectangular pattern (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.  


 

Related links

Near Infrared Imager and Slitless Spectrograph, NIRISS

NIRISS Overview

References

Artigau, E., Sivaramakrishnan, A.,  Greenbaum, A. Z. et al., 2014, SPIE, 9143E, 40A, 
NIRISS aperture masking interferometry: an overview of science opportunities

Greenbaum, A.Z., Pueyo, L., Sivaramakrishnan, A., Lacour, S., 2015, The Astrophysical Journal, 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, The Astrophysical Journal, 783, 73, 
Active Galactic Nucleus and Quasar Science with Aperture Masking Interferometry on the James Webb Space Telescope

Sivaramakrishnan, A. & Artigau, E. 2014, STScI Newsletter, Volume 31, issue 01, 
Aperture-Masking Interferometry with Webb's NIRISS

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