NIRSpec BOTS Wavelength Ranges and Gaps

A physical gap between the JWST NIRSpec detectors affects the S1600A1  slit used for BOTS observations at high resolutions. Wavelength coverage is also affected by subarrays. Wavelength ranges and gaps are given here for all grating-filter and subarray combinations. 

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JWST NIRSpec supports bright object time-series (BOTS) spectroscopy with the S1600A1 fixed slit, located in the MSA mounting frame. Wavelength coverage for a given disperser-filter combination may be interrupted due to the gap between the 2 detectors, as indicated in Table 1 for the high-resolution gratings (G140H, G235H, and G395H). Similarly, the nominal wavelength ranges can be shortened for subarrays other than SUB2048The wavelength coverage for all of the BOTS grating-filter and subarray combinations are presented in Table 1. 

Table 1. NIRSpec BOTS wavelength coverage for each disperser-filter and subarray combination (in units of  μm)

Disperser-filterSUB2048SUB1024ASUB1024BSUB512 or SUB512S
G140H/F100LP0.971.31, 1.351.830.971.07, 1.591.831.071.31, 1.351.591.071.19, 1.471.59
G235H/F170LP1.662.20,, 2.673.071.792.20, 2.272.671.792.00, 2.472.67
G395H/F290LP2.873.72, 3.825.182.873.03, 4.515.183.033.72, 3.824.513.033.38, 4.174.51

 PRISM spectra do not map to an area of the detector covered by the SUB1024A subarray for the S1600A1 aperture used for BOTS observing mode.

The full spectral range sampled by the disperser-filter combination (except for gaps mentioned above) is measured by the SUB2048 subarray options in BOTS mode. The SUB1024A and SUB1024B subarrays read out only half of the detector pixels in the dispersion direction. SUB1024A reads out both the red and blue ends of the waveband, and SUB1024B reads out the inner region of the waveband with a small separation between due to the detector gap. The smaller SUB512 and SUB512S options were designed to cover the full wavelength range for the PRISM. They span only a sub-range of the high-resolution grating data and could also be used for high-resolution spectra on very bright sources.

Figure 1 depicts the nominal spectral coverage of the S1600A1 aperture for the 9 available disperser-filter combinations when SUB2048 is used. The limits are expressed in units of μm. The gaps in the spectra are shown in yellow. The exact position of the wavelength gap 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. Spectra at wavelengths that fall in the detector gaps are not recoverable when using the S1600A1 slit. Spectra obtained with the medium-resolution gratings or the PRISM do not have a wavelength gap because the complete spectrum will fall onto one detector, NRS1.

Figure 1. Wavelength coverage using aperture S1600A1 with SUB2048 (full coverage).

The range of available wavelengths for each grating-filter combination using the fixed slit aperture S1600A1 and subarray SUB2048. (Note that BOTS mode must be used with a subarray, however.) Wavelength gaps are shown in yellow and wavelength cutoffs due to detector edges are shown as well. All numbers are given in units of μm.

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