NIRSpec MOS Wavelength Ranges and Gaps

There is a physical gap between the JWST NIRSpec detectors, where MOS spectra of any resolution may lose wavelength coverage. High-resolution spectra may also have short- and long-wavelengths cut off at the outer edges of the detector. These effects depend on both the position of the shutter and the selected disperser. 

Observations with the NIRSpec Micro-Shutter Assembly (MSA) can be affected by wavelength cutoffs due to the physical edges of the two detectors and the separation between them. How these cutoffs and gaps map to wavelengths depends on the shutter position and the disperser that was chosen for the observation. The full operating range for this instrument mode is from 0.6 to 5.3 µm. The nominal wavelength ranges for each disperser are given in NIRSpec Dispersers and Filters. Dithering during observation can help to recover missing wavelengths.

Figure 1. Sketch of the 4-quadrant MSA projected onto the two NIRSpec detectors

Dispersed spectra from sources in the MSA will be projected onto the detectors. There is a physical gap between detectors (the two red boxes in the figure).

Visualizing wavelength cutoffs for observation plans 

The JWST Exposure Time Calculator (ETC) can be used to show sample spectra (including cutoffs and wavelength gaps) from representative shutters across the MSA for each disperser-filter combination. For example, Figure 2 shows the ETC-produced spectra of one target in G235H/F170LP at three different MSA positions on quadrant 3. The spectra are overplotted on one another. Note the shift in the gap depending on shutter position. 

Figure 2. Signal-to-noise plot from the JWST ETC showing the S/N for three different shutters in the MSA (overplotted)

ETC-produced spectra of a target in G235H/F170LP at three different MSA positions on quadrant 3. The spectra are overplotted on one another to illustrate that the position of the gap shifts depending on shutter position. The gap in the center is from a shutter position at the center of the quadrant, while the gaps to the left and right are from shutter positions at the upper right, and lower left of Q3, respectively.
Additionally, the JWST NIRSpec MSA Spectral Visualization Tool (MSAViz) can be used to visualize the spectra of sources in a planned exposure on the detectors. It indicates the wavelength gaps and cutoffs, which the user can also output from the tool. Figures 3 and 4 were generated from the MSAViz tool. Figure 4 shows the locations of spectra on the detectors from several representative shutters, for each of the possible disperser-filter combinations. The tool is available on GitHub.
Figure 3. The MSA configuration used to generate spectral plots seen in figures below

Long slits (~40 shutters in length) were opened in Q2 and Q3 of the NIRSpec micro-shutter assembly. The three slits in each of the quadrants were selected to clearly show the wavelength differences on the detector in spectra formed by them. The grey columns and rows depict areas on the MSA impacted by electrical shorts, and scattered grey shutters represent inoperable (failed) shutters.
Figure 4. Spectral visualizations generated with MOSviz tool

The nominal first order spectra for the F170LP filter and G235M grating combination using the MSA configuration shown in Figure 3. The dark rows are from inoperable shutters that are in the configured slitlet. The wavelengths lost in the spectral gap depend on the position of the shutters of each slitlet in the MSA and the disperser used (G235M in this case).


The nominal first order spectra for the F290LP filter and G395H grating combination using the MSA configuration shown in Figure 3. The wavelengths lost in the spectral gap depend on the position of the shutters of each slitlet in the MSA and the disperser used (G395H in this case). The high-resolution spectra of some slit positions suffer from long wavelength cutoffs in addition to gaps in wavelength. This effect happens even for slits in Q3 and Q4 if they are located in the right half of those quadrants, close to the gap.


The nominal first order spectra for the CLEAR filter and PRISM combination using the MSA configuration shown in Figure 3. The wavelengths lost in the spectral gap depend on the position of the shutters of each slitlet in the MSA and the disperser used (PRISM in this case). The spectra of shutters in Q1 and Q2 can suffer from gaps in wavelength, as seen in the lower left spectra on NRS2.


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