NIRSpec Fixed Slits

The NIRSpec fixed slits (FS) spectroscopy and bright object time-series (BOTS) spectroscopy modes use the 5 FS apertures machined into the mounting plate of the micro-shutter assembly (MSA).

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NIRSpec has 5 fixed slits (FS) cut into the metal that separates the 4 micro-shutter assembly (MSA) quadrants. All apertures can be used in NIRSpec FS spectroscopy mode, but the 1.6" × 1.6" wide aperture alone is used for the bright object time-series (BOTS) observing mode. These FS apertures are always open, even if the multi-object spectroscopy (MOS) mode or integral field unit (IFU) spectroscopy mode is being used. By construction, the fixed slits spectra project onto a different part of the detector than the MSA or IFU spectra, and therefore there is no overlap or contamination from failed open MSA shutters. As a result, the FS can provide the most sensitive, highest-contrast spectroscopy available with NIRSpec.

Fixed slits positions on the MSA focal plane

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NIRSpec's fixed slits are named based on their location on the metal structure and their aperture width. Images of the slits with 'A' in their name, S200A1S200A2S400A1S1600A1, project onto detector NRS1, and the slit with 'B' in its name, S200B1, projects onto detector NRS2 as shown in Figure 1.
Figure 1. Fixed slit apertures

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Locations of the JWST NIRSpec fixed slit entrance apertures in the micro-shutter assembly (MSA) focal plane are shown. The red rectangle on the left is a zoomed section in the main figure that shows the A-side slits. A zoomed view of the S200B1 slit is shown on the right. The gray regions in the background show the footprints of the detectors NRS1 (left) and NRS2 (right) that would be seen looking through the instrument toward the detector plane.

Fixed slits dimensions on the sky

Table 1 presents the width and length of each fixed slit as projected onto the sky, as well as the naming convention. The name of each fixed slit also includes its width in mas. The 0.2" slits provide full spectral resolution, the 0.4" slit and 1.6" × 1.6 " apertures have spectral resolutions that are degraded by the spatial PSF or physical source size. The S200B1 slit is provided for redundancy. The S200B1 spectra can be truncated at long wavelengths, so this slit is not expected to be commonly used for science. 

Table 1. Dimensions of fixed slits as projected onto the sky

Fixed slit nameWidth (arcsec)Length (arcsec)Note

When used with S200A1, it provides full wavelength coverage across the inter-detector gap

S1600A11.61.6High throughput slit used in BOTS mode
S200B10.23.3Redundant with S200A1

Fixed slits throughput

Like all slit-based spectrographs, the fixed slit apertures are not perfect rectangles. Due to their small physical sizes and the construction process, the fixed slit apertures have irregularities along their edges. These irregularities impact throughput and can affect spectral resolution and wavelength calibration. The locations of the rough slit edges are shown in Figure 2 (red circles), and 1-D curves of the average transmission along the slit length are shown in Figure 3.  Note that the data reduction pipeline will take the actual transmissions of the slits into account. However, when combined with pointing uncertainty, these irregular slit edges can be sources of flux uncertainty for FS science. The FS nod offset positions have been updated to be slit-specific to avoid these irregularities and get the best possible throughput for point sources observed with the FS. The S1600A1 aperture is wide enough that it is not significantly affected by these throughput issues.

Figure 2. Images of the NIRSpec fixed slits

Images of the NIRSpec fixed slits

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A composite photograph showing 4 of the 5 fixed slits. The red circles highlight particularly large features that affect throughputThe BOTS aperture is large enough that it doesn't have features that affect transmission and is therefore not shown. The S400A1 slit is a composite of 2 images.

Figure 3. Transmission curves of the fixed slits

Transmission curves of the fixed slits

The normalized throughput curves for NIRSpec's fixed slits, running from −0.5 (the bottom of the slit) to +0.5 (the top of the slit).

Notable updates
    Added a sentence to clarify that slit irregularities are common
Originally published