Parent page: NIRCam Instrumentation
NIRCam has grisms available for R = λ/Δλ ~ 1,600 slitless spectroscopy at long wavelengths (2.4–5.0 µm). These may be used during commissioning to help align JWST's mirrors (Shi et al. 2008) and afterward for science observations. Table 1 lists the properties of the grisms.
Each NIRCam module has two grisms installed on the long wavelength (LW) channel, which disperse spectra in orthogonal directions, along detector rows (grism R) and columns (grism C). For each observation, a grism in the LW pupil wheel is used in combination with a filter in the LW filter wheel. Broader filters yield wider spectral range and longer spectra (in detector pixels) at the expense of higher background and source confusion.
Table 1. NIRCam grism properties
Two observing modes listed in Table 2 make use of the grisms: wide field slitless spectroscopy and grism time series. In most cases, only first order spectra are visible. Fainter second order spectra of some sources may be visible when F250M, F300M, F277W, or F322W2 filters are used in combination with the grism, though only the wide filters can be used for the grism time-series mode.
Table 2. NIRCam grism observing modes
The grism spectral resolving power, R = λ/Δλ, is ~1,600 at 3.95 µm (the undeviated, zero-order wavelength), and decreases to R ~ 1,150 at 2.5 µm. It can be approximated by the function (shown in Figure 1) given in Section 3.2 of Greene et al. (2017):
R ~ 3.35(λ/µm)4 − 41.9(λ/µm)3 + 95.5(λ/µm)2 + 536(λ/µm) + 240,
which is the result of a spline curve fit to the optical model prediction of the grism FWHM spectral resolving power R versus wavelength for point sources.
In each NIRCam module, the two grisms, grism R and grism C, disperse spectra along detector rows and columns, respectively. This dispersion does not change measurably with wavelength, and changes are small with field position (2% or less). Dispersion directions (increasing wavelength) are given for NIRCam ideal coordinates (Xideal, Yideal) in Table 3 and Figure 2. Table 4 shows the dispersion lengths measured in pixels for the available filters, with the dispersions spanning roughly 1,000 pixels per micron of wavelength (Figure 3). Dispersion lengths in pixels are determined between the 5% power limits at wavelengths λ1 and λ2 and converted to pixels (1 nm/pixel).
Table 3. NIRCam grism dispersion directions
Table 4. NIRCam grism dispersion lengths in filters for the two modes
Figures 4 and 5 show NIRCam module A first and second order sensitivities that include contributions from the JWST and NIRCam optics, filters, detector quantum efficiency (QE), and grism. The module B grism is less sensitive by ~30%, and is not shown.
The module A grisms have higher throughput efficiency due to anti-reflection coating on both sides. Module B grisms are only coated on the smooth (non-grooved) side, resulting in 30% lower transmission as well as ghosts (faint reflections) of some bright spectra.
Fabrication of the NIRCam silicon grisms is described by Jaffe et al. (2008) and Gully-Santiago et al. (2010). The grism diameter is 48 mm, with 42 mm optically usable. The spacing between grooves is 15.36 µm. The blaze angle is offset slightly from the prism angle, as it was chosen to maximize efficiency near the mean wavelength of the LW modules (3.7 µm), which is offset from the undeviated wavelength (3.95 µm). Table 5 lists the basic manufacturing specifications of the grisms.
Table 5. NIRCam grism hardware properties
Greene, T., Beichman, C., Gully-Santiago, M. et al. 2010, SPIE 7731
Greene, T., Chu, L., Hodapp, K. W. et al. 2016, SPIE 99040E
Greene, T. et al. 2017, JATIS, 035001
Gully-Santiago, M., Wang, W., Deen, C. et al. 2010, SPIE 77393S
Jaffe, D. T., Wang, W., Marsh, J. P. et al. 2008, SPIE 70103L
Shi, F., King, B. M., Sigrist, N. et al. 2008, SPIE 70102E