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 2 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 FiltersDithering during an observation can help to recover missing wavelengths.

Figure 1. Sketch of the 4-quadrant MSA projected onto the 2 NIRSpec detectors

Dispersed spectra from sources in the MSA will be projected onto the detectors. There is a physical gap between detectors (the 2 red boxes in the figure).

Obtaining tabular information about wavelength cutoffs for planned MOS spectra

See also: MSA Target Info File

The wavelength cutoffs for each observed source in an exposure are now captured in the MSA Target Info file that can be exported from APT after a MOS Observation has been created using a catalog, and saved to APT.  This file contains a table of columns, including the wavelengths at both the blue and red edges of the two NIRSpec detectors if the nominal wavelength range for a given source has been cut off.

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 3 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 3 different shutters in the MSA (overplotted)

ETC-produced spectra of a target in G235H/F170LP at 3 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.

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