Advice on how to optimize JWST NIRISS imaging observations is covered in this article.

See also: NIRISS Imaging, NIRISS Imaging APT Template, JWST Parallel Observations, APT Coordinated Parallel Observations

NIRISS imaging is offered as a prime observing mode and as a coordinated parallel observation when NIRCam imaging or NIRCam wide field slitless spectroscopy (WFSS) is a prime observing mode. NIRISS imaging is also a required component of NIRISS WFSS observations, with a direct image taken before and after each set of grism exposures in a filter.

Guidelines for considering NIRISS imaging observations over NIRCam imaging, choosing exposure parameters, selecting aperture extraction sizes in the Exposure Time Calculator, and designing an efficient science program are given below.

NIRISS imaging or NIRCam imaging?

NIRCam is the primary near-infrared imager for JWST. However, there are science cases in which a user may prefer to use NIRISS imaging:

1. NIRISS is more sensitive to low surface brightness features between 0.8–2.5 μm than NIRCam, which is applicable to science goals like studying galactic tidal tails at "cosmic noon" (z ~ 2), due to the larger pixel scale of 0.066"/pixel compared with the pixel scale of NIRCam short wavelength channel (0.031"/pixel).
   
   
2. NIRISS offers a "simple" field of view of 2.2' × 2.2' while NIRCam offers 2 modules each covering 2.2' × 2.2' with a 44" gap between modules, and 4"–5" gaps between detectors within each module in the short-wavelength channel. For cases where the position of a target is not known to great accuracy (e.g., tens of arc seconds), such as target of opportunity requests to identify electromagnetic counterparts to gravitational wave sources, NIRISS may be preferable to optimize observation planning. Without precise astrometry, observations with NIRCam run the risk of the target of interest falling in a detector or module gap.
   
   
3. For science programs using the NIRISS WFSS mode, a direct image is taken before and after each set of grism exposures in a filter. Some observers may choose to take additional imaging exposures in filters in which they do not take grism exposures. NIRISS imaging in this case would optimize both the science return, by using the same instrument set-up as the WFSS observations, and observing efficiency, by saving on observatory overheads associated with switching to another instrument.

Recommended strategies for the Exposure Time Calculator

Aperture strategy choices

See also: JWST ETC Imaging Aperture Photometry Strategy, NIRISS Filters

The Exposure Time Calculator (ETC) is used to calculate signal-to-noise ratios (SNRs) for an observation based on input exposure parameters. When determining exposure parameters in the ETC, users can select the aperture radius from which the flux is extracted and the background subtraction method. We recommended the filter-dependent source extraction radii listed in Table 1, which are based on the measured 80% encircled energy radii for a point source.  The recommended sky annulus for extracting the background region has an inner radius of 2 times the extraction radius and an outer radius of 4 times the extraction radius.

Table 1. Recommended aperture extraction radii for point sources for use in the ETC

Note: These extraction radii are for point sources. If the source is extended, it is up to the user to define the region of interest for calculating the SNR. It is also up to the user to ensure that other sources from the ETC scene are not included in the background extraction area, unless this effect is intended.

Effect of charge migration on near-infrared detectors

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," see Goudfrooij et al. 2024). 

In the Exposure Time Calculator, this value, 34,400  e^-,  is considered to be the effective saturation limit, which is the signal limit at which charge migration occurs in the F090W filter. This value 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 PSF. The JWST calibration pipeline uses this limit in the charge_migration step in calwebb_detector1.

The pipeline accounts for charge migration by discarding the affected pixels when fitting an integration ramp to determine the flux of a source (see Goudfrooij et al. 2024). Users may thus wish to exceed this signal limit threshold for bright objects as long as there are at least ~3–5 groups in an integration ramp prior to the onset of charge migration. If there are only a few groups unaffected by charge migration, the flux measurement will be less reliable.

References

Goudfrooij, P., Grumm, D., Volk, K., Bushouse, H., 2024, PASP, 136, 4503
An Algorithm to Mitigate Charge Migration Effects in Data from the Near Infrared Imager and Slitless Spectrograph on the James Webb Space Telescope.