The JWST NIRCam grisms obtain R ~ 1,600 slitless spectroscopy at 2.4–5.0 µm in two science observing modes: wide field and single-object time series. The grisms may also help align JWST's mirrors.


Introduction

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

ParameterValue
Dispersion1 nm/pixel
Resolving power, R =  λ/Δλ~1,600 at 4 µm

Undeviated wavelength (measured)

3.95 µm (3.92–3.98)

 


Observing modes

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

ModeAvailable filtersUsage
Wide field slitless spectroscopy

Medium: F250M, F300M, F335M, F360M, F410M, F430M, F460M, F480M

Wide: F277W, F322W2, F356W, F444W

For a wide field with a multitude of (possibly overlapping) spectra
Grism time seriesWide: F277W, F322W2, F356W, F444WTo monitor bright, isolated, time-varying objects



Resolving power

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.

Figure 1. NIRCam grism spectral resolution versus wavelength

The FWHM spectral resolving power R = λ/Δλ ~ 1,600 at 3.95 μm, limited somewhat by the circular beam factor and diffraction. At shorter wavelengths, the resolving power is limited further by pixel sampling of the PSF, but this may be improved by dithering multiple observations. Reproduced from Figure 2b of Greene et al. (2017).


Dispersion

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 

Element

Dispersion
within: 

Dispersion
direction in
module A 

Dispersion
direction in
module B 

Measured
dispersion
(nm/pixel) 

Grism R

Rows

+Xideal

-Xideal

1.004

Grism C

Columns

+Yideal

+Yideal

1.006

Figure 2. NIRCam grism dispersion directions

Dispersion directions (increasing wavelength) for the row (R) and column (C) grisms along the ideal coordinate axes (Xideal, Yideal) in each module. Reproduced from Figure 4 of Greene et al. (2017).


Table 4. NIRCam grism dispersion lengths in filters for the two modes

Filter

λ1

λ2

Dispersed
pixels

Observing mode

F277W

2.395

3.179

784

Wide Field + Grism Time Series

F322W2

2.413

4.083

1670

Wide Field + Grism Time Series

F356W

3.100

4.041

941

Wide Field + Grism Time Series

F444W

3.835

5.084

1249

Wide Field + Grism Time Series

F250M

2.401

2.609

208

Wide Field

F300M

2.800

3.205

404

Wide Field

F335M

 3.142

 3.606

 464

Wide Field

F360M

3.386

3.869

483 

Wide Field

F410M

3.810 

 4.366

 556

Wide Field

F430M

 4.140

4.426 

 286

Wide Field

F460M

4.486

4.788

302

Wide Field

F480M

4.614

5.048

434

Wide Field

Example dispersion lengths of grism spectra in various filters, measured in pixels between the 5% power limits at wavelengths λ1 and λ2. The filters available for the grism time series and wide field slitless spectroscopy modes are shown in Table 2. 

Figure 3. NIRCam grism dispersion lengths for module A

The four wide filters available for both observing modes are shown, along with all the medium filters for WFSS. Each grism element is used in combination with a filter. Broader filters produce longer spectra and only first order spectra are visible in most cases. Fainter second order spectra of some sources may be visible when using F250M, F300M, F277W, or F322W2 (in the F250M, F277W and F322W2 examples shown, the first order spectra span much of the detector, and the second order spectra fall outside). The red boxes show the full detector width (2,040 pixels) along the x-axis. Placement along the y-axis is arbitrary for clarity.


Sensitivity

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.

Figure 4. NIRCam grism + filter sensitivities (1st order)

NIRCam module A first order sensitivities including JWST and NIRCam optics, filters, detector quantum efficiency (QE), and grism. The module B grism is less sensitive by ~30% (not shown).

Figure 5. NIRCam grism + filter sensitivities (2nd order)

NIRCam module A second order sensitivities including JWST and NIRCam optics, filters, detector quantum efficiency (QE), and grism. The module B grism is less sensitive by ~30% (not shown).


Throughput

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.

Figure 6. NIRCam grism throughputs (1st order)

First order throughputs of the module A and B grism and available filters (module A filters shown, module B is similar), including all JWST and NIRCam optics and detector quantum efficiencies. The grism throughput must be multiplied with that of a selected filter. The module B grisms are AR coated on only one side and therefore have throughputs ∼30% lower than the module A grisms.  

Figure 7. NIRCam grism throughputs (2nd order)

Second order throughputs of the module A and B grism and available filters (module A filters shown, module B is similar), including all JWST and NIRCam optics and detector quantum efficiencies. The grism throughput must be multiplied with that of a selected filter.  


Hardware

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

ParameterValue
Prism angle6.16°
Blaze angle5.75°
Blaze wavelength3.7 µm
Groove frequency65 grooves/mm
Maximum thickness8.0 mm
Diameter48.0 mm, circular with side flats
Clear aperture42.0 mm, circular
AR coating2.4 to 5.0 µm
Peak transmission

70.5% at 3.7 µm (module A)
52% at 3.7 µm (module B)


Figure 8. NIRCam grism photograph

NIRCam grism photograph

Reproduced from Figure 6 of Greene et al. (2010).




References

Greene, T., Beichman, C., Gully-Santiago, M. et al. 2010, SPIE 7731
NIRCam: development and testing of the JWST near-infrared camera

Greene, T., Chu, L., Hodapp, K. W. et al. 2016, SPIE 99040E
Slitless Spectroscopy with the James Webb Space Telescope Near-Infrared Camera (JWST NIRCam)

Greene, T. et al. 2017, JATIS, 035001
λ = 2.4 to 5 μm spectroscopy with the James Webb Space Telescope Near-Infrared Camera

Gully-Santiago, M., Wang, W., Deen, C. et al. 2010, SPIE 77393S
High-performance silicon grisms for 1.2-8.0 μm: detailed results from the JWST-NIRCam devices

Jaffe, D. T., Wang, W., Marsh, J. P. et al. 2008, SPIE 70103L
Fabrication and test of silicon grisms for JWST-NIRCam 

Shi, F., King, B. M., Sigrist, N. et al. 2008, SPIE 70102E
NIRCam Long Wavelength Channel grisms as the Dispersed Fringe Sensor for JWST segment mirror coarse phasing








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