NIRSpec Dispersers and Filters

JWST's NIRSpec has 7 filters covering the 0.6–5.3 μm wavelength region and 7 dispersive elements that include 6 gratings and a prism. These filters and dispersive elements are combined to take spectra in the NIR region. 

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NIRSpec is sensitive to nearly a full decade in wavelength: 0.6–5.3 µm. The NIRSpec optical path contains 2 wheel mechanisms, the filter wheel assembly (FWA) and the grating wheel assembly (GWA), which provide disperser-filter combinations to cover the NIRSpec wavelength range. The NIRSpec filters and dispersers form matched sets to cover the wavelength ranges shown in Figure 1, with one configuration in low spectral resolution mode (R ~ 30–300), and 4 configurations in medium (R ~ 1,000) and high (R ~ 2,700) spectral resolution mode.  

The FWA contains seven transmission filters, plus an OPAQUE position. The GWA contains 6 diffraction gratings, a double-pass prism, and a flat mirror. 

Figure 1. NIRSpec wavelength coverage
A pictorial representation of the NIRSpec wavelength coverage and the number of configurations necessary to cover the full ~0.6–5.3 µm spectral range in the low, medium and high spectral resolution settings.

NIRSpec filters


Words in bold italics are
parameters or buttons in 
like APT & ETC. Similarly, a bold 
style represents menus & panels.

NIRSpec's 7 transmission filters are: F140X, F110W, F070LP, F100LP, F170LP, F290LP, and CLEAR. Four of the filters are long-pass filters, i.e., they have a well defined cut-on wavelength and transmit all longer wavelengths. The other 3 filters are bandpass filters, with both a cut-on and cut-off wavelength, and are mainly used for target acquisition, or with the prism. Table 1 summarizes the properties of the transmission filters, and Figure 2 shows their optical throughput. The LP and CLEAR filters are matched with gratings to define the wavelength regions used for NIRSpec science.

There is also an 8th NIRSpec FWA position, OPAQUE, that is used as an instrument shutter. It blocks light from entering NIRSpec during calibration exposures and whenever NIRSpec is not being used. 

Table 1. Transmission filters

NameBandpass (µm)Purpose
F140X0.8 to 2.0Target acquisition
F110W1.0 to 1.3Narrowband acquisition for brighter targets
F070LP>0.70.7 to 1.3 µm spectra
F100LP>1.01.0 to 1.9 µm spectra
F170LP>1.71.7 to 3.2 µm spectra
F290LP>2.92.9 to 5.3 µm spectra
CLEAR0.6 to 5.3Target acquisition or for use with PRISM

Note that the nominal spectral ranges for each filter may be shortened due to detector cutoffs. For the F070LP filter in particular there are blue end wavelength detector cutoffs that occur. The cutoff wavelengths depend on the target aperture location (slit or shutter), but are worse for the IFU. These cutoffs are described separately for each mode in the following articles: NIRSpec IFU Wavelength Ranges and Gaps, NIRSpec FS Wavelength Ranges and Gaps, NIRSpec BOTS Wavelength Ranges and Gaps, and NIRSpec MOS Wavelength Ranges and Gaps. The ETC can also be used to see where the cutoffs occur for all modes except MOS. Information on ranges for MOS, which depend on the position of the shutter in the MSA, can be derived using the MSAViz Tool.

Figure 2. Transmission filter throughput

Transmission filter throughput

The optical throughput of each of the NIRSpec transmission filters. The top plot shows the filters used for science spectral configurations. The bottom plot shows F110W and F140X, which are the filters for NIRSpec target acquisition. Users are strongly encouraged to use the ETC to make and compare source S/N estimates rather than attempt to model these independently. However, the system throughput curves (a component of which is the filter transmission) are available for download from the JWST ETC.

NIRSpec dispersers 

NIRSpec has 7 dispersers in the GWA:

  • three high resolution, R ~ 2,700, gratings (G140H, G235H, and G395H)
  • three medium resolution, R ~ 1,000, gratings (G140M, G235M, and G395M)
  • a low resolution, R ~ 100, double-pass prism (PRISM)

The full wavelength range of NIRSpec can be sampled in one exposure using the prism. However, each diffraction grating can only provide clean spectra over a factor of two in wavelength due to order contamination. The second order λ/2 spectra end up on exactly the same detector pixels as photons with wavelength λ (albeit with reduced efficiency). As a result, when using the diffraction gratings the wavelength limit on the short side (blueward) is defined by the throughput of the long pass filters, and the limit on the long side (redward) is defined by the wavelength where second-order light contaminates the spectrum. To obtain data over the entire 0.6–5.3 µm wavelength range using the gratings, spectra over smaller wavelength ranges are obtained using matched dispersers and filters, and then combined.

To avoid order contamination, each disperser is only used with its paired transmission filter(s), as shown in Table 2. Also shown are each combined grating-filter wavelength range and the nominal resolving power, which is defined as the resolving power at the center of the nominal wavelength range. Grating transmissions for the NIRSpec medium and high resolution dispersers are presented in Figure 3, and the wavelength dependence of the resolving powers is shown in Figure 4.

The last position in the GWA is a plane mirror, which provides undispersed imaging of the sky and is only used for target acquisition or field position verification during science operations.

Grating Wheel Position Sensor

The grating wheel position can vary slightly between exposures with different grating selections. There is a grating wheel position sensor that removes any zero point wavelength shift (in post processing) due to slight variations in the position of the grating wheel. NIRSpec is required to deliver a wavelength accuracy to better than 1/8 of a spectral resolution element, or approximately 15 km/s for spectra taken with the high-resolution gratings. The instrument model wavelength calibration is expected to meet the wavelength calibration accuracy requirement, making Autocals unnecessary. Autocals can add significant overhead to an observation.

Table 2. Available disperser-filter combinations

Disperser-filter combinationNominal resolving powerWavelength range



Wavelength range values presented here are approximate. Note that the nominal spectral ranges for medium and high-resolution dispersers may be shortened due to red-end detector cutoffs. The cutoff wavelengths depend on the target aperture location (slit or shutter). Detailed limits are found on the wavelength ranges and gaps pages for the IFU, FS, and BOTS, and in the ETC. Information on wavelength ranges for MOS, which depend on the position of the shutter in the MSA, can be determined using the MSAViz Tool.

Figure 3. Transmission curves of the NIRSpec dispersers

The transmission curves of the NIRSpec dispersers as a function of wavelength. The top plot shows the transmission for the prism mode (R ~ 100), the middle plot shows the medium (R ~ 1,000) resolution dispersers and the bottom plot shows the transmission for the high (R ~ 2,700) resolution dispersers.

Figure 4. Resolving power of the NIRSpec dispersers

The resolving power of NIRSpec's gratings and prism as a function of wavelength. The resolving power is computed assuming a spectral resolution element size of 2.2 pixels (typically a fully illuminated aperture for the IFU, the MOS and the 200 mas slits). Note: The resolution for wider S1600A1 and  S400A1 apertures may be degraded, especially for extended sources, but also for point sources at wavelengths long enough that the PSF width is > 2.2 pixels.

Dispersion curves for the NIRSpec dispersers

The dispersion and resolution curves as a function of wavelength for the different dispersers for the NIRSpec instrument are shown in Figures 5-11. Files containing the tabulated data used to produce these curves can be downloaded from the following links. These are binary fits tables that contain 3 columns: wavelength (μm), dispersion (μm/pixel) and resolution (λ⁄Δλ, unitless). These data are currently in use by the ETC (delivered June, 2016). 








Figure 5. Dispersion and Resolution for the PRISM

Figure 6. Dispersion and Resolution for the G140M grating

Figure 7. Dispersion and Resolution for the G140H grating

Figure 8. Dispersion and Resolution for the G235M grating

Figure 9. Dispersion and Resolution for the G235H grating

Figure 10. Dispersion and Resolution for the G395M grating

Figure 11. Dispersion and Resolution for the G395H grating

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