NIRSpec IFU Wavelength Ranges and Gaps
There is a physical gap between the JWST NIRSpec detectors. This affects the IFU observations at high resolutions. The wavelengths that fall in the detector gap are not recoverable.
JWST's NIRSpec supports integral field spectroscopy with an integral field unit (IFU). Light from the source of interest passes through a small (3" × 3") aperture in the MSA mounting frame and travels through the IFU image slicing mirror optics. Figure 1 shows the location of the IFU aperture on the MSA mounting frame between MSA quadrants 3 and 4. The light then is reflected by mirrors to form an image of the source on the image slicer, which consists of 30 stacked mirror surfaces. Figure 2 shows the layout of the slices, numbered from 0 to 29, within the IFU aperture in the plane of the detector. Figure 1 additionally shows the layout of the slices as positioned on the detector, also numbered 0 to 29.
There is a physical gap between the detectors that can be seen in Figure 1 which depicts the detector array boundaries in white. This gap affects NIRSpec high-resolution IFU observations, resulting in a gap in the wavelength coverage. The IFU observing mode article provides the approximate spectral wavelengths lost in the detector gap, but the precise values differ for the individual IFU data slices. This article provides additional information on the exact wavelengths lost in the detector wavelength gap per IFU slice for high-resolution IFU observations.
Short wavelength cutoffs
Some IFU spectra are cut off at the short wavelength end because they project beyond the left edge of detector NRS1. This effect is worst for the F070LP* filter when used with the G140H disperser. Figure 3 illustrates that the spectral cutoffs with this filter/disperser combination occur from ~0.92 to ~0.96 microns, depending on the slice (top to bottom: slice #29 to slice #0, respectively). This blue-end cutoff also occurs using the G140M grating with the F070LP filter, but it occurs at shorter wavelengths in that case (from ~0.85 in slice #0 to ~0.90 microns for slice #29). The full range of the spectra can be examined for a given instrument setup using the JWST ETC.
The presence of a physical gap between detectors affects high-resolution IFU observations because the spectra are long enough to span both NIRSpec detectors. Figure 4 presents an example using disperser G140H and filter F100LP. The limits of the detectors are shown with thick white lines. For clarity, the spectra from the 30 IFU slices are shown in the region around the detector gaps. Note that the wavelength range that falls within the detector gap is different for each slice.
Table 1 Note: Values are in microns.