# NIRISS AMI Recommended Strategies

Advice for choosing proposal parameters for the JWST NIRISS aperture masking interferometry mode (AMI), which 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.

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 wide-band filter (F277W) in the filter wheel (FW).

NIRISS AMI observations can be readout 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").

Lab testing and data from cryovacuum testing shows that this effect becomes pronounced in the NIRISS detector above 30,000 e in the bright pixel. We mitigate this effect in AMI observations by considering 30,000 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.

# Choosing an optimal calibrator for a PSF reference star

Data analysis with the non-redundant mask (NRM) requires 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 phases (CP) and visibility amplitudes. Closure phases are the sum of the fringe phases from three holes. A closure phase must theoretically be zero for a point source (see Lawson 2000 and Monnier 2003 to learn more about these observables). The PSF reference star needs to be single, and of roughly the same magnitude (in the NIRISS filters used for the AMI observation) and spectral type as the science target. It is also best if the calibration star is relatively close to the science target on the sky, since changing the spacecraft attitude may change the temperature of the primary mirror segments slightly and thus affect the Fourier phases observed by the NRM.

Checking that the star is single is the most important factor. There are two 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 the 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. (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

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 Requirement 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.

# Exposure depth estimation for binary point source

The JWST Exposure Time Calculator (ETC) performs signal-to-noise (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, we 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, we 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.

# Estimating number of groups for an exposure

Main article: Step-by-Step ETC Guide for NIRISS AMI Observations of Extrasolar Planets Around a Host Star

It is recommended to observe the maximum number of groups in an integration prior to the onset of "effective saturation", which is defined as the limit where charge begins accumulating on pixels that neighbor the central pixel when observing a point source (30,000 e/s). Observers should use the JWST Exposure Time Calculator (ETC) to determine this number, which is reported in the ETC Reports pane. The analytical calculations below are useful for estimating this threshold.

For each filter, we used analytical noiseless NRM PSFs in an aperture of side ~1" (31 × 31 pixels) to estimate NGroups. This PSF can be simulated using the JWST PSF simulation tool WebbPSF (Perrin et al. 2014).

The number of photons per frame in the brightest pixel of the NRM PSF is:

 (6)

where  cpf is the central pixel fraction; phcorr (photometric correction) is the NRM throughput relative to the clear pupil, combined with the aperture correction for the 1" aperture; and tframe is the frame time, which for the AMI observing mode using the SUB80 subarray is 0.07544 s.

Table 1 gives the values of the central pixel fraction (cpf) for each filter that can be used by the AMI mode, the NRM throughput relative to the clear pupil, and the zero points of the filters. These values can be used to calculate the maximum number of groups prior to effective saturation (NGroups sat). The total number of electrons in the central pixels in the entire exposure is:

 (7)

where tot_e is the total number of requested photons in the NRM PSF.

Table 1. Parameters for estimating maximum number of groups for an AMI observation

Filtercpf a

Photometric correction b

(ph_corr)

Filter zero point c
F227W0.04480.13226.14
F380M0.02820.12723.75
F430M0.02340.12623.32
F480M0.01940.12423.19

a Central pixel fraction.

b NRM throughput relative to the clear pupil/aperture correction for 31 × 31 aperture.

c Using count rates for Vega spectrum scaled to V = 9.0.

Depending on the brightness of the source and the number of required photons needed to achieve science goals, there are two scenarios that determines NGroups sat:

1. $cp_{tot} < sat\_e$

This is usually the case for faint objects in which the required number of total photons is reached before the brightest pixel reaches effective saturation. In this case, NGroups sat is:

 (8)

This value will be the number of groups for the observation as long as it is less than the maximum number of groups allowed by APT (i.e., < 800). In this case, the observations has one integration.
If the number of groups exceeds 800, then more than one integration will be necessary to achieve the exposure time required to detect the necessary number of photons.

2. $cp_{tot} > sat\_e$

In this case, saturation in an integration is reached before the requested number of photons are detected and we therefore need an exposure with multiple integrations.

The time to reach saturation is:

 (9)

and

 (10)

In addition to these two scenarios, there are two limiting cases:

1. Very bright sources:Saturation is reached in under one group (e.g., F480M, magnitude 2.41 source). This means the brightness limit for the filter is exceeded and some pixels may saturate.

2. Very faint sources: Number of integrations exceeds the maximum number of allowed integrations in an exposure (>10,000). Another observation can then be created to garner additional photons.

## Analytically estimating number of groups: worked example

As a worked example for estimating the maximum number of counts analytically, we use the target of the AMI Example Science Program to observe HD 218396 (magnitude M = 5.26, Vega) using NRM and the F480M filter. The goal of this program is to detect 1010 photons to achieve the desired contrast ratio. Charge migration, where charge accumulates in pixels neighboring the central pixel when observing a point source, occurs at 30,000 e, so we use this value as the saturation limit (sat_e).

Using equation (2) and Table 1:

Since cptot > sat_e, we use equations (4) and (5) and Table 1 to find NGroups sat:

This value of NGroups sat is calculated for a noiseless PSF and should be used only as an initial estimate. The actual value may be slightly lower than this value.

# Recommended parameters for strategy choices for Exposure Time Calculator

Main article: 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, we recommend choosing noiseless skybackground 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, we recommend the following filter-dependent values for point sources:

Table 2. ETC aperture extraction radius for point sources

Filter

(")

F277W1.6
F380M2.0
F430M2.3
F480M2.5

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

# 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 simulations for direct imaging, use the NIRISS imaging mode in ETC.

# Designing an efficient science program with multiple filters

Main article: NIRISS Pupil and Filter Wheels

Target acquisition is performed with the F480M filter prior to the start of science observations. Thus, when using the F480M filter for science, it is most efficient to start an exposure sequence with the F480M filter.

Since overheads are associated with each mechanism move, it is recommended to select subsequent filters in sequential order in the filter wheel, as illustrated in Figure 1. If all the filters are to be used, then the sequence F480M, F380M, F430M, and F277W produces the least motion of the filter wheel.

# References

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

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

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

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

Published 28 Nov 2017 16 Sep 2019 Included discussion of charge migration and recommended background and source aperture extraction sizes for ETC. 30 Apr 2018 Added note that users need to use ETC imaging calculations to derive exposure parameters for AMI direct imaging