NIRSpec FS Wavelength Ranges and Gaps
A physical gap between the JWST NIRSpec detectors affects fixed slit observations of the "A"-type slits at high resolutions. The affected wavelength ranges and gaps are available for all fixed slits and grating-filter combinations.
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Table 1. NIRSpec "A"-type fixed slits wavelength ranges in FULL frame
|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 on the wavelength ranges and gaps are found in the ETC.
There is a physical gap between the detectors which affects NIRSpec fixed slits observations, resulting in a gap in the wavelength coverage. The wavelength ranges and wavelength gaps are presented here for all the NIRSpec fixed slits using all of the grating-filter combinations. Note that, except for some of the BOTS subarrays, the available fixed slit subarrays do not affect wavelength coverage.
Apertures S200A1, S200A2, S400A1, and S1600A1
Figure 1 shows the spectral coverage of the S200A1 aperture, using the 9 available grating-filter combinations. The limits are expressed in units of μm. Gaps in the spectra are shown in yellow. Spectra obtained with the medium-resolution gratings or the PRISM/CLEAR do not have a wavelength gap because the complete spectrum will fall onto one detector, NRS1. Note that the exact position of the wavelength gap for the high-resolution gratings depends on the grating wheel assembly (GWA) tilt angle and will vary slightly from observation to observation, typically on a scale less than 0.025 μm for the prism and 0.01 μm for the gratings. The JWST Exposure Time Calculator (ETC) computes these gaps and includes them in plots of count rate or signal-to-noise ratio (SNR) as shown in Figure 2. In this figure, the extracted flux is shown as a function of wavelength for 3 grating-filter combinations using the S200A1 aperture.
The exact wavelength limits for each slit depends on whether and where cutoffs occur due to detector edges. As seen in Figure 1, detector cutoffs occur at the blue end for the F070LP filter and the G140M and H gratings, and at the red end for the G395M and H grating.
Figures 3, 4, and 5 show the spectral coverage of the S200A2, S400A1, and S1600A1 apertures, respectively.
The S200A1 and S200A2 slits are separated by an amount large enough so that combining the spectra obtained from both slits will provide a means to recover wavelengths lost in the gaps of each of their spectra. Within the FS template in APT, it is possible to specify the slit option S200A1 and S200A2. This will create separate exposure specifications, one for S200A1, and one for S200A2. These exposures will occur in the same visit. Spectra at wavelengths that fall in the detector gaps are not recoverable in fixed slits spectroscopy mode when using the S400A1 or S1600A1 slits.
The S200B1 aperture is located on the opposite side of NIRSpec’s field of view from the "A"-type slits and is there primarily for redundancy. The S200B1 slit produces a truncated wavelength range when used with high-resolution gratings and as a result, this aperture will not typically be used for science. The S200B1 slit is also affected by the detector gaps in the G140H/F070LP and G140M/F070LP grating-filter combinations as shown in Figure 6.