NIRISS Observing Modes and Templates
A JWST observing program that uses the NIRISS instrument is designed by using one of the NIRISS templates in the Astronomer's Proposal Tool (APT). NIRISS has imaging (0.8 - 5.0 μm) and slitless spectroscopy (0.86- 2.8 μm) capabilities and offers four observing templates: Wide Field Slitless Spectroscopy, Single Object Slitless Spectroscopy, Aperture Masking Interferometry, and Imaging.
Wide Field Slitless Spectroscopy
The NIRISS Wide Field Slitless Spectroscopy (WFSS) mode is designed to obtain spectra in the 0.8 - 2.2 μm wavelength range for every source within the 2.2' x 2.2' FOV of the NIRISS detector. One or more of the NIRISS short wavelength imaging filters must be used as blocking elements in WFSS observations. The user can select either one of the NIRISS WFSS grisms or both of them. Use of both grisms provides orthogonal spectra of the objects within the field of view. The WFSS observations are taken in full frame read-out. This observing mode does not use subarrays. The NIRISS PSFs are under-sampled across the wavelength range used for WFSS and dithering is essential.
Single Object Slitless Spectroscopy
The NIRISS Single Object Slitless Spectroscopy (SOSS) mode is designed for exoplanet transits. SOSS allows high cadence time resolved observations and has two available subarray read-out patterns, SUBSTRIP256 and SUBSTRIP96. SOSS provides two useable orders of cross-dispersed spectra for a single object in the wavelength range 0.6 - 2.8 μm at a resolution of R ≈ 700 in the first order when full frame read-out or the default SUBSTRIP256 subarray read-out are used. In the SUBSTRIP96 subarray read-out, useful data are obtained only for first order and the wavelength range is somewhat reduced. Dithering is not implemented for SOSS mode. Target Acquisition (TA) is required if using a subarray, and strongly encouraged for full-frame read-out. SOSS data will be processed properly in the JWST calibration pipeline only if a TA was performed.
Aperture Masking Interferometry
The NIRISS Aperture Masking Interferometry (AMI) mode offers the highest spatial resolution on JWST and allows high contrast imaging (10-4) at separations of 70-400 mas in four long wavelength filters (≈ 2.8, 3.8, 4.3, and 4.8 μm). The NIRISS AMI mode uses an 80 x 80 pixel subarray for bright targets. Dithering is not recommended. Target Acquisition (TA) is required if using a subarray.
The NIRISS Imaging mode offers imaging capability over the wavelength range 0.8 - 5.0 μm. This mode is currently supported to be used in parallel with NIRCam Imaging as the prime science instrument; NIRISS imaging will be considered as a parallel mode to MIRI Imaging in future cycles. Only full frame read-out is supported.
Step-by-step in APT
The step-by-step instructions for using the NIRISS observing templates in the APT are provided in the following pages: