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 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

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: 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 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.

Exposure depth estimation for binary point source