NIRISS Imaging Recommended Strategies
Advice on how to optimize JWST NIRISS imaging 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:
- 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).
- 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.
- 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 aperture strategy choices for Exposure Time Calculator
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. It is recommended to keep the source aperture extraction radius under 2". 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.
See also: NIRISS Bright Limits
Bright objects within the NIRISS field of view may affect the images in some other part of the field of view after dithering. As a general guideline, stars brighter than magnitude 15 in the short wavelength filters and brighter than 12.5 in the long wavelength filters will saturate in the first few groups. Hence if many stars of this brightness are present in the area, or if stars that are many magnitudes brighter than this may appear in the field of view, this is likely to cause persistence issues. The WISE W1 and W2 filter images are useful as guides to assess the brightness of objects in the NIRISS long wavelength filters, while the 2MASS images are useful as guides to assess the brightness of objects in the NIRISS short wavelength filters.