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JWST's NIRSpec has 5 fixed slits (FSs) for high sensitivity single object spectroscopy in the 0.6–5.3 μm wavelength range, using several grating-filter combinations and detector settings.

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The JWST NIRSpec fixed slits (FS) spectroscopy options provide high sensitivity single object spectroscopy over the full 0.6–5.3 μm wavelength region where NIRSpec operates. NIRSpec FSs are designed for high contrast spectroscopy of both the faintest and brightest targets NIRSpec can observe. Example use cases for FSs spectroscopy include, but are not limited to: high sensitivity observations of very faint single targets such as high redshift quasars and isolated low mass brown dwarfs, or bright sources such as solar system objects and young stars that would saturate in the NIRSpec multi-object spectroscopy (MOS) or integral field unit (IFU) spectroscopy modes.

The FS apertures are illustrated in Figure 1, over-plotted on an HST Advanced Camera for Surveys (ACS) Wide Field Channel (WFC) F814 image of a young star.     

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Figure 1. Sky view with JWST NIRSpec FS apertures

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The 4 primary NIRSpec FSs spectroscopy mode apertures are shown over-plotted on an HST ACS/WFC F814 image of the protostar IRAS 20324+4057. The target is within the S200A1 slit, shown at the upper right.

Properties of the FS mode

The apertures for the FSs spectroscopy, MOS and IFU spectroscopy modes are all in the same focal plane in the NIRSpec instrument (called the slit plane). Figure 2 shows the location of all NIRSpec FS apertures in this focal plane, and a zoomed view of their positions and sizes.

The 5 NIRSpec FSs are S200A1, S200A2, S400A1, S1600A1 and S200B1 slits. These are 0.2", 0.2", 0.4", 1.6" and 0.2" wide, respectively. Approximate dimensions of the NIRSpec FSs on the sky are presented in Table 1.  The left slits in Figure 2 are referred to as the "A slits," and the right slit is the "B" slit. The S200B1 aperture is located on the opposite side of NIRSpec’s field of view from the A slits, and is there primarily for redundancy in the unlikely event that the NRS1 (left) NIRSpec detector fails.  The S200B1 slit produces a truncated wavelength range when used with high resolution gratings (see Table 3) and as a result this aperture will not typically be used for science.

The S1600A1 aperture enables stable, high throughput spectroscopy in the FSs spectroscopy mode. It is also optimized for observations of bright stars with transiting exoplanets in the NIRSpec bright object time-series, BOTS, spectroscopy mode.  

Table 1. The NIRSpec FS aperture sizes

FS aperture nameWidth (arcsec)Height (arcsec)

Table note:
The S200B1 slit is for redundancy and will not commonly be used for science.


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Figure 2. NIRSpec FS apertures

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Locations of the JWST NIRSpec FS 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 FSs spectroscopy mode can acquire the highest contrast, highest sensitivity spectra possible with NIRSpec. This is because the FSs are cut into the metal MSA support structure mounting plate, which is extremely opaque. Unlike the MOS or IFU spectroscopy modes, FS data will not be affected by the presence of contaminating objects through failed MSA shutters or by background signal due to the finite contrast of the MSA.  

In NIRSpec FSs spectroscopy mode, subarray readouts can be used to decrease the detector readout time and observe brighter targets than is possible in the FULL detector readout used for the MOS and IFU spectroscopy modes. The FSs spectroscopy mode (and BOTS mode) can be used to observe the brightest targets possible with NIRSpec.  Each of the 5 NIRSpec FSs have matched subarrays that encompass their individual spectra. The ALLSLITS subarray reads an area that includes all the fixed slits spectra. 


Spectral configurations

All disperser and filter combinations available in NIRSpec can be used in the FSs spectroscopy mode. Table 2 outlines the instrument configurations, resolutions and wavelength ranges that can be acquired with the NIRSpec A FSs. 

 Table 2. NIRSpec A FSs instrument configurations, resolutions and wavelength ranges

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PageWithExcerptNIRSpec Dispersers and Filters

The S200B1 slit is on the right side of the NIRSpec instrument and the wavelength range is truncated in the R = 2700 grating settings. Table 3 shows the truncated spectral range for the S200B1 slit in the NIRSpec high spectral resolution mode. As a result of this decreased spectral range, the S200B1 slit is not recommended for regular science use.

Table 3. Truncated wavelength ranges in R = 2700 settings for the S200B1 slit

Spectral configuration

Wavelength range


Detector wavelength gaps

FSs spectra obtained with the high resolution R = 2700 gratings span both NIRSpec detectors. Due to this and the physical separation between the detectors in the focal plane array, there will be small gaps in the spectral coverage for all the A slits. The S200B1 slit is only affected by this in the G140H/F070LP grating/filter combination, but has incomplete spectral coverage to begin with (see Table 3).  When the two S200A1 and S200A2 slits are used in conjunction with each other, they mitigate the wavelength gap for full spectral wavelength coverage. Spectra at wavelengths that fall in the detector gaps are not recoverable in FSs spectroscopy mode when using the S400A1 or S1600A1 slits.  Table 4 outlines the wavelength gap spectral ranges for the A slit R = 2700 instrument configurations in the FS mode. Spectra obtained with the medium resolution gratings or the PRISM do not have a wavelength gap because the complete spectrum will fall onto one detector (NRS1 for the A slits and NRS2 for S200B1; see Figure 5 in the NIRSpec Multi Object Spectroscopy page). 


Table 4. Wavelength gap ranges for the A slits 

FS apertureSpectral configurationDetector wavelength gap range
S200A1  G140H/F070LP   None
 G140H/F100LP 1.302–1.339
 G235H/F170LP 2.1822.244
 G395H/F290LP 3.6853.789
S200A2 G140H/F070LP   None
 G140H/F100LP 1.3471.385
  G235H/F170LP 2.2592.321
 G395H/F290LP 3.8153.919
S400A1 G140H/F070LP   None






 G235H/F170LP 2.2032.265

The G140+F070LP configuration in all slits of NIRSpec FSs mode does not have a gap in wavelength because the science data lie all on one detector (the left detector, NRS1).



NIRSpec FS data can be acquired in FULL frame 2048 × 2048 detector pixel readout, the ALLSLITS subarray 2048 × 256, or subarrays that are matched to each slit (2048 × 64). The subarrays have names connected to their FS apertures, as shown in Table 5. NIRSpec FS subarray exposures allow for observations of brighter targets than possible with the FULL frame detector readout because of shorter frame read times. 

Table 5. The matched subarray names for the FS apertures

FS aperture

Matched subarray name









S200A1 and S200A2


Table 6. Detector frame readout time options for NIRSpec FS subarrays



Frame time (s)


2048 × 2048

10.7368 or 14.5888 (IRS2)


2048 × 256






2048 × 64


FULL frame detector readout can use standard readout and the IRS2 option (as explained in the exposure specification section). Expanded options for subarrays exist for the S1600A1 aperture so that very bright targets can be observed. These are described in more detail in the NIRSpec bright object time-series, BOTS, spectroscopy mode page.

Exposure specification

NIRSpec FS exposure times are tied to the timing of the detector readout patterns and subarrays. There are 4 readout patterns available for NIRSpec FS observations:

  • NRS 

The readout patterns are split over 2 readout modes, traditional and improved reference sampling and subtraction (IRS2).  The traditional mode, which is used for the NRSRAPID and NRS readout patterns, is similar to the detector readout for NIRCam and NIRISS. In FULL detector readout, NRSRAPID has a single frame (10.7 s), and NRS has 4 frames averaged into a single group (42.8 s).

The IRS2 mode, which is used for the NRSIRS2RAPID and NRSIRS2 readout patterns, intersperses reference pixels within the science pixel reads to improve noise characteristics achievable during data processing, resulting in longer frame times and higher data volumes.  Like the traditional readout, the NRSIRS2RAPID is a single frame, and NRSIRS2 has 4 frames averaged into a single group. These IRS2 readout patterns improve performance and sensitivity in long exposure FS observations of faint objects. However, IRS2 readout patterns can only be used if FULL frame detector readout patterns are used, so FS subarray readouts cannot take advantage of the improved performance offered by IRS2.  

Additional information on NIRSpec FS exposure specification and how this translates to exposure time and sensitivity can be found using the JWST Exposure Time Calculator (ETC) page.

Options for dithering

Most observations with JWST will require ditheringThe NIRSpec FS spectroscopy mode has several dither and nod options available. Dithers are offsets of the target position to even out or mitigate detector effects, cosmic rays, or improve spatial sampling. Nod offsets are also used in data processing to subtract nearby background flux.  

The options for the NIRSpec FS dithers include:

  • Primary nodding: One (single position with no slit dither), 2, 3 or 5 nods are available.
  • Subpixel offsets: These are used to improve spectral sampling of the line spread function or spatial sampling of the point spread function. Subpixel offset options are SPATIAL, SPECTRAL or BOTH. Selecting these options will increase the number of acquired exposures.

The NIRSpec FS Dithers and Nods page provides an in depth view of the available options briefly described above.

What do NIRSpec FS data look like?

Figure 3 shows NIRSpec FS mode data acquired with a ground calibration test lamp using the R = 2700 G140H+F100LP short wavelength spectral configuration. Spectra from the 5 FSs are visible in these ALLSLITS subarray data. The FS apertures are always open, so if a source or background emission falls through them, their signals will be acquired, even when the other NIRSpec observing modes are used.  

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Figure 3. NIRSpec FS sample data

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An example of a calibration line lamp image of the NIRSpec FS mode data. Each NIRSpec FS has its own matched subarray, shown here is the ALLSLITS configuration which captures all five FSs in one exposure.



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nameRelated links

Related links

JWST User Documentation Home,
Near Infrared Spectrograph, NIRSpec
NIRSpec Overview
NIRSpec Observing Modes
NIRSpec Fixed Slits
NIRSpec Bright Object Time-Series Spectroscopy
JWST Astronomers Proposal Tool, APT
JWST APT website
JWST Exposure Time Calculator, ETC
JWST ETC website

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Birkmann, S. 2016 ESAC JWST "On Your Mark" Workshop (ppt) (pdf)
Single Object Spectroscopy and Time Series Observations with NIRSpec

JWST technical documents

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Last updated

Updated July 10, 2017

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Published January 3, 2017



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UpdatedJanuary 3, 2017
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