Abstract
The James Webb Space Telescope near-infrared camera (JWST NIRCam) has two 2'. 2 × 2'.2 fields of view that can be observed with either imaging or spectroscopic modes. Either of two R ∼ 1500 grisms with orthogonal dispersion directions can be used for slitless spectroscopy over λ = 2.4 - 5.0 μm in each module, and shorter wavelength observations of the same fields can be obtained simultaneously. We describe the design drivers and parameters of the grisms and present the latest predicted spectroscopic sensitivities, saturation limits, resolving powers, and wavelength coverage values. Simultaneous short wavelength (0.6 - 2.3 μm) imaging observations of the 2.4 - 5.0 μm spectroscopic field can be performed in one of several different filter bands, either in-focus or defocused via weak lenses internal to NIRCam. The grisms are available for single-object time series spectroscopy and wide-field multi-object slitless spectroscopy modes in the first cycle of JWST observations. We present and discuss operational considerations including subarray sizes and data volume limits. Potential scientific uses of the grisms are illustrated with simulated observations of deep extragalactic fields, dark clouds, and transiting exoplanets. Information needed to plan observations using these spectroscopic modes are also provided.
Highlights
The near-infrared camera (NIRCam) of the James Webb Space Telescope (JWST) has flexible modes that provide wide-field imaging through narrow, medium, or wide-band filters, coronagraphic imaging, and slitless spectroscopy.[1,2,3,4,5]
The large capture range requirement drives the design toward producing spectra at the longest possible wavelengths, and a small minimum detectable piston drives the design toward larger spectral coverage
This can be evaluated by computing the cross-dispersion centroid locations of the fringes at the Long wavelength (LW) and short wavelength (SW) ends of a given filter bandpass
Summary
The near-infrared camera (NIRCam) of the James Webb Space Telescope (JWST) has flexible modes that provide wide-field imaging through narrow-, medium-, or wide-band filters, coronagraphic imaging, and slitless spectroscopy.[1,2,3,4,5] Long wavelength (LW; λ 1⁄4 2.4 to 5.0 μm) Si grisms[6] and λ 1⁄4 1 to 2 μm dispersed Hartmann sensors (DHSs) were developed for the purposes of wavefront sensing and telescope primary segment phasing with NIRCam.[7]. Wavelengths λ < 2.4 μm are imaged by the short wavelength (SW) channel of each module, which has a focal plane of four abutted 2048 × 2048 (2040 × 2040 active) pixel HAWAII-2RG detectors with a plate scale of 32 mas pixel−1. A dichroic beam splitter sends longer wavelengths to the LW channel that has a single 2048 × 2048 (2040 × 2040 active) pixel HAWAII-2RG detector focal plane with a plate scale of 65 mas pixel−1.1,3 This allows simultaneous SW and LW imaging of the same NIRCam field with each module. The following section describes the grism design drivers and adopted design. Their spectroscopic capabilities and expected performance in NIRCam are presented . The paper concludes with a section on observation planning and available tools
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