NIRISS AMI Recommended Strategies
The JWST NIRISS Aperture Masking Interferometry mode (AMI) 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. Advice for choosing proposal parameters is given.
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 seven 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.
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 AMI-specific treatment of limiting contrast observing techniques article. Advice for choosing proposal parameters, optimizing observation set-up, and designing efficient AMI proposals is given below.
Choosing an optimal calibrator for a PSF reference star
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 athttp://ad.usno.navy.mil/wds/single/singleframe.html that should be checked to see if the proposed calibrator has been observed interferometrically and found to be single. The European equivalent at http://www.jmmc.fr/searchcal_page.htm 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
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.
Estimating number of groups for an exposure
It is recommended to observe the maximum number of groups in an integration prior to the onset of saturation. 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:
cp_e_per_frame = cpf × count rate × ph_corr × tframe (1),
where cpf is the central pixel fraction; ph_corr (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 seconds.
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 NGroups. The total number of electrons in the central pixels in the entire exposure is:
cptot = cpf × tot_e (2),
where tot_e is the total number of requested photons in the NRM PSF.
Table 1. Parameters for estimating number of Groups for an AMI observation
Photometric correction b
|Filter zero point c|
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:
- cptot < 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 saturation. In this case, NGroups is:
ngroups = cptot / cp_e_per_frame (3)
This value will be NGroups 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.
cptot > 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:
Tsat = sat_e/(count rate × ph_corr × cpf) (4)
ngroups = Tsat/tframe (5)
In addition to these two scenarios, there are two limiting cases:
- Very bright sources: Saturation is reached in under 1 group (e.g., F480M, magnitude 2.41 source). This means the brightness limit for the filter is exceeded and some pixels may saturate.
- Very faint sources: Number of integrations exceeds the maximum number of allowed integrations in an exposure (>10000). Another observation can then be created to garner additional photons.
See the NIRISS AMI Science Use Case for a worked example.
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
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.
Perrin, M. D., et al. 2014 Proc. SPIE 9143, 91433X
Updated point spread function simulations for JWST with WebbPSF