NIRISS AMI Recommended Strategies

Recommendations for crafting a NIRISS aperture masking interferometry mode (AMI) observing program are presented. This mode 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. 

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See also: NIRISS Aperture Masking Interferometry, NIRISS AMI Template APT Guide, NIRISS AMI Science Use Case, HCI NIRISS Limiting Contrast, NIRISS Non-Redundant Mask

The aperture masking interferometry (AMI) mode of NIRISS enables high-contrast imaging by turning 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. 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 3 medium-band filters (F380M, F430M, F480M) or a wideband filter (F277W) in the filter wheel (FW).

NIRISS AMI observations can be read out in full frame mode or with the SUB80 subarray.

target acquisition (TA) is required when using a subarray and strongly recommended for full frame readout to ensure that the target is always placed on the same detector pixel. Note the recommended TA mode for AMI observations as a function of target brightness near the bottom of the target acquisition article. 

Details about the limiting contrast accessible by AMI are available on the HCI NIRISS Limiting Contrast observing techniques article. Advice for choosing proposal parameters, optimizing observation set-up, and designing efficient AMI proposals is given below.

Effects of charge migration in near-infrared detectors in the AMI mode

When 2 neighboring pixels accumulate charge at very different rates, the brighter pixel "spills" photoelectrons to its neighboring pixels, but the reverse effect does not occur. This charge migration causes the full width half maximum of the point spread function (PSF) to be larger for bright point sources compared to faint point sources (the so-called "brighter-fatter effect"). 

Analysis from commissioning data shows that charge migration has a ~1% effect on the brightest pixel at an accumulated charge of 30,000 e-. This effect is mitigated in AMI observations by considering 20,000 e- to be the effective saturation limit, which is lower than the true non-linearity based saturation limit for the NIRISS detector. Thus, the JWST Exposure Time Calculator will give a saturation warning message when this effective saturation limit is exceeded in the brightest pixel of the AMI PSF. This limit may be revised pending the results of on-going analysis to understand the onset, impact, and mitigation of charge migration in AMI observations.

Choosing an optimal calibrator for a PSF reference star

AMI or kernel phase imaging (KPI) use the non-redundant mask (NRM) or CLEARP pupil in the AMI mode. These require observations of the target and a point spread function (PSF) reference star. The PSF reference star is used to calibrate out instrumental contributions to the interferometric observables of closure (or kernel) phases and visibility amplitudes. Closure phases are the sum of the fringe phases from 3 holes. Kernel phases are certain linear combinations of fringe phases (Martinache 2010).  A closure phase must theoretically be zero for a point source (see Lawson 2000Monnier 2003, and Martinache 2010 to learn more about these observables). The PSF reference star needs to be effectively single, and of similar brightness (within ~0.5 magnitude) and spectral type as the science target. To calibrate out detector non-linearities, a science target and its calibrator should have similar peak pixel counts in an integration, and a similar number of groups in an integration. It is also best if the calibration star is not too far from the science target, since changing the spacecraft attitude may change the temperature of the primary mirror segments slightly, which may affect the measured closure or kernel phases (Sivaramakrishnan et al. 2023, Kammerer et al. 2023).

It is important to check that the reference star is effectively single. There are 2 web resources that allow one to check whether a star is single. First there is the list of single stars maintained by the U.S. Naval Observatory that should be checked to see if the proposed calibrator has been observed interferometrically and found to be single. The European equivalent is also useful for this check. If a star has not been checked by ground-based interferometry the next possibility is to see if there are HST observations, which provide the best possible angular resolution to resolve a companion. If the target is not present in these databases, then the remaining way to check for a possible companion is to look at the near-infrared spectral energy distribution to see if there is evidence of an excess that may be due to a fainter and cooler companion or dust. (A hot companion such as a white dwarf star is unlikely to be bright enough compared to the primary star to be of concern at 3 to 5 μm.)

Observing calibrator(s) close in time to the target

Words in bold are GUI menus/
panels or data software packages; 
bold italics are buttons in GUI
tools or package parameters.

Ideally, for higher contrast needs, science target and PSF reference star observations should be scheduled close in time, so that the telescope is in a similar state, thermal or otherwise, for all the observations. Also, they should be observed using the same telescope optical configuration, so no wavefront correction should occur between any of the observations. 

In the Astronomers Proposal Tool (APT), this link between science target and PSF reference star observations is enforced by clicking on the Special Requirements tab, adding a Timing requirement of Group/Sequence Observations Link, selecting target(s) and calibrator(s) from the Observation list box and choosing the Non-interruptible option.

Target Acquisition

Target acquisition (TA) is required when using a subarray in the AMI mode and strongly recommended when using full frame readout. Two TA modes are offered: AMIBRIGHTwhich uses the F480M filter crossed with the NRM in the pupil wheel for decreased throughput to observe bright acquisition stars, and AMIFAINT, which uses the F480M filter crossed with the CLEARP filter in the pupil wheel for increased throughput to observe faint acquisition stars. Users are advised to the the Exposure Time Calculator to determine the exposure parameters for their TA observations.

Although the target will be placed with high precision in the center of a pixel when doing a target acquisition, there will be shift in position in the science exposure due to filter-to-filter offsets. Figure 1 shows the measured PSF offsets by filter from commissioning observations after using the AMIBRIGHT mode. Offsets using the AMIFAINT mode have not been measured. As of APT v. 2023.5, these filter-dependent offsets are automatically applied when moving from the target acquisition source in AMIBRIGHT mode to the science target to ensure the most accurate placement of the science target in the center of the target.

Figure 1. Measured filter-dependent target offsets from pixel center when using the AMIBRIGHT target acquisition mode

The figure shows measured offsets between the peak of the target PSF and pixel center after target acquisition in the AMIBRIGHT mode. APT automatically applies these filter-dependent offsets between the TA observation and the science observation to more accurately place the science target in the center of the pixel. Adapted from Figure 5 in Sivaramakrishnan et al. 2022.

Exposure depth estimation for binary point source

The JWST Exposure Time Calculator (ETC) performs signal-to-noise ratio (SNR) calculations for the JWST observing modes. Sources of interest are defined by the user and assigned to scenes which are used by the ETC to run calculations for the requested observing mode.

For the purpose of this calculation, assume that the flux ratio for the target HD 218396 and the planetary companion we wish to detect is ~10-4. According to Ireland (2013), the number of photons necessary to detect this contrast is:

1.5 x Nhole/ (contrast ratio)2, where Nhole refers to the number of apertures (holes) in a mask.

Since there are 7 apertures in the AMI NRM, this translates to:

73.5 / (contrast ratio)2

Considering the fact that NRM has not been used in space before, use a slightly more conservative value of:

100 / (contrast ratio)2= 100 / (0.0001)2 = 1010

Therefore, the goal of our calculation is to detect 1010 photons from the target in the ETC simulations.

Recommended parameters for strategy choices for Exposure Time Calculator

See also: JWST ETC Imaging Aperture Photometry Strategy

When determining exposure parameters in the ETC, users can select the aperture radius from which the flux is extracted and the background subtraction method. The NIRISS AMI image has extended wings which can be used in data analysis. This aperture contains most of the AMI PSF flux. For the AMI mode, choosing noiseless skybackground is recommended because the extended PSF makes background subtraction difficult in AMI subarray data. The main noise contributors to AMI are systematics and photon noise. In any case, AMI analysis solves for any pedestal or background in the data.

For the aperture extraction radius, the following filter-dependent values for point sources are recommended:

Table 1. ETC aperture extraction radius for point sources





The ETC aperture radius defaults to the largest of these sizes, 2.5".

AMI Dithers

See also: NIRISS AMI Dithers

Five-point SUBPIXEL dithering is recommended for AMI observations. This dither pattern is designed to enable subsampling using the drizzling technique developed for Hubble data taken with pixel shifts that are wider than the Nyquist sampling limit. This subpixel sampling dither pattern should benefit shorter wavelength AMI filters.

When to take a direct image

The photometry from the AMI observations is expected to be of similar accuracy to that achieved in regular imaging; the largest additional uncertainty in the AMI mode compared to normal imaging is the aperture corrections, as the PSF has more extended structure from the NRM than is present in normal imaging. A direct image may be useful as a check on the photometry and to see if there are other stars in the vicinity of the science target to aid in the data analysis. Note, however, that the regular imaging has a more restrictive bright limit than the NRM imaging, and this will be a limitation on the photometry for brighter objects. As a general guideline, any star bright enough to need to use the bright mode in acquisition is likely to also be too bright to be unsaturated in direct images.

ETC calculations for AMI do not include a direct imaging component. To run calculations for direct imaging, use the NIRISS imaging mode in ETC.



Hirata, C. M., Choi, A. 2019, PASP 132, 1007 (ADS)
Brighter-fatter effect in near-infrared detectors – I. Theory of flat auto-correlations

Ireland, M. J. 2013 MNRAS 433, 2
Phase errors in diffraction-limited imaging: contrast limits for sparse aperture masking

Kammerer, J., Cooper, R. A., Vandal, T., et al., 2023, PASP, 135, 1043
The Near Infrared Imager and Slitless Spectrograph for JWST. V. Kernel Phase Imaging and Data Analysis

Lawson P. R. 2000
Principles of Long Baseline Stellar Interferometry
Course notes from the 1999 Michelson Summer School, held August 15-19, 1999. Edited by Peter R. Lawson. Published by NASA, Jet Propulsion Laboratory, California

Martinache, F. 2010, ApJ, 724, 464
Kernel Phase in Fizeau Interferometry

Monnier J. D. 2003, Reports on Progress in Physics, 66, 789 
Optical interferometry in astronomy

Perrin, M. D., et al. 2014 Proc. SPIE 9143, 91433X
Updated point spread function simulations for JWST with WebbPSF

Sivaramakrishnan, A., Tuthill, P., Lloyd, J. P., et al., 2023, PASP, 135, 1043
The Near Infrared Imager and Slitless Spectrograph for the James Webb Space Telescope – IV. Aperture Masking Interferometry.

Websites to check whether a star has been observed interferometrically and found to be a single star:

U.S. Naval Observatory

Jean-Marie Mariotti Center

Latest updates
    • Updated to note that APT 2023.5 (release date Aug. 24 '23) has filter-dependent offsets applied between TA in AMIBRIGHT mode and the science target, so removed the suggestions that users could apply their own OFFSET Special Requirement when defining observations.
    • Updated discussion of charge migration.

    Updated to add Figure 1 to show the target offset relative to TA location. Other Cycle 2 updates made.

  •  Included discussion of charge migration and recommended background and source aperture extraction sizes for ETC. 

    Added note that users need to use ETC imaging calculations to derive exposure parameters for AMI direct imaging
Originally published