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.
A 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 in-flight data shows that charge migration has a ~1% effect on the brightest pixel at an accumulated charge of 26,000 e-. This effect is mitigated in AMI observations by considering 26,000 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. The JWST calibration pipeline uses this limit in the charge_migration step in calwebb_detector1.
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 2000, Monnier 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.
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: AMIBRIGHT, which 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 Exposure Time Calculator to determine the exposure parameters for their TA observations.
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
| Filter | Radius (arcsec) |
|---|---|
| F277W | 1.6 |
| F380M | 2.0 |
| F430M | 2.3 |
| F480M | 2.5 |
The ETC aperture radius defaults to the largest of these sizes, 2.5".
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.
References
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.
Related links
Websites to check whether a star has been observed interferometrically and found to be a single star:
U.S. Naval Observatory http://www.astro.gsu.edu/wds/single/singleframe.html
Jean-Marie Mariotti Center http://www.jmmc.fr/searchcal_page.htm