NIRSpec Dispersers and Filters
JWST's NIRSpec has seven filters covering the 0.6–5.3 μm wavelength region and seven dispersive elements that include six gratings and a prism. These filters and dispersive elements are combined to take spectra in the NIR region.
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.
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 three 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 eighth 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
|F140X*||0.8 to 2.0||target acquisition|
|F110W||1.0 to 1.3||narrowband acquisition for brighter targets|
|F070LP||>0.7||0.7 to 1.3 µm spectra|
|F100LP||>1.0||1.0 to 1.9 µm spectra|
|F170LP||>1.7||1.7 to 3.2 µm spectra|
|F290LP||>2.9||2.9 to 5.3 µm spectra|
|CLEAR||0.6 to 5.3||target acquisition or for use with PRISM|
* Bold italics style indicates words that are also parameters or buttons in software tools (like the APT and ETC). Similarly, a bold style represents menu items and panels.
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.
NIRSpec has seven 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 combination||Nominal resolving power||Wavelength 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.
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 three columns: wavelength (micron), dispersion (microns/pixel) and resolution (λ⁄Δλ, unitless). These data are currently in use by the ETC (delivered June, 2016).