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. 

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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. Unlike other NIRSpec observing modes, wavelength losses due to the gap cannot be fully recovered by dithering within the IFU aperture. 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.

Figure 1. The IFU aperture in the NIRSpec aperture plane

The IFU aperture location is shown on the mounting frame with respect to the NIRSpec fixed slits and MSA quadrants. It is marked by a small blue square halfway between the upper left quadrant (Q3) and the bottom left quadrant (Q4). The 30 pseudo-slits mapped to the detector are also shown as a column of slits in blue to the left of quadrants #3 and #4.

Figure 2. IFU slices in the plane of the detectors

The NIRSpec IFU uses a slicer mirror to image the field of view into 30 slices. Each slice is 0.1" wide and 3" long on the sky. Here the slices are shown in the plane of the detectors.

Short wavelength cutoffs

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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 μm, 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 μm for slice #29).  The full range of the spectra can be examined for a given instrument setup using the JWST ETC.
Figure 3. IFU short-wavelength cutoffs with the F070LP filter

The missing wavelengths for each of the IFU slices due to the NRS1 detector edge using the G140H disperser.
The range of nominal wavelengths for the G140H/F070LP configuration is shown. Slit #29 (top) and slit #0 (bottom) show the extrema of these differences. In the worst case, wavelengths below 0.96 μm are cut off (slice #0, bottom). At the top (slice #29) wavelengths below 0.92 μm are cut off. Spectra of the fixed slits appear in the middle of the figure. The detector cutoffs are less severe for spectra of the fixed slits over the same wavelength range, as seen in the figure.

Wavelength gaps 

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.

Figure 4. IFU missing wavelengths

The missing wavelengths for each of the IFU slices due to the detector gap.
Top: The complete range of nominal wavelengths for the G140H/F100LP configuration is shown. Spectra of the fixed slits also appear in the middle of the figure.
Bottom: A clipped portion of the nominal wavelength range near the gap is shown to more clearly show that the missing wavelengths differ for each slice, and to better differentiate the cutoff values using the color key. Slit #29 (top) and slit #0 (bottom) show the extrema of these differences. Spectra of the fixed slits over the same clipped wavelength range are shown, but in this case, the wavelengths land on the NRS2 detector and are recorded. Only a portion of the range from slit S200A2 falls off the right edge of NRS2.

Table 1 lists the detector gap wavelength cutoff values determined from on-orbit data for the numbered IFU virtual "slits" with the high-resolution gratings for grating-filter combinations G140H/F100LP, G235H/F170LP, and G395H/F290LP. Note that when using the grating-filter combination G140H/F070LP the resulting spectra do not have any gaps because the spectra do not extend beyond NRS1.

Table 1. Wavelength cutoffs of the detector gap for the IFU virtual slits

Table note: Values are in microns.

Latest updates
    Corrected value for SLIT_ID 25, G395H_MIN

    Updated based on commissioning data
Originally published