JWST Slit Spectroscopy

JWST provides slit spectroscopy in two instruments: Near Infrared Spectrograph (NIRSpec) and Mid-Infrared Instrument (MIRI).

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Main articles: MIRI Low-Resolution Spectroscopy, NIRSpec Fixed Slits Spectroscopy

Spectroscopic slits are narrow apertures in the optical path designed to allow light only from the intended scientific target onto the detector array. These slits are usually narrow in the dispersion direction, which limits the size of the target in the wavelength direction and improves the spectral resolution. JWST offers spectroscopy in fixed slits in two instruments, NIRSpec and MIRI, which generate spectra covering wavelength ranges from 0.6 to 5.3 μm and 5 to approximately 12 μm, respectively. 



Available spectroscopic slits 

Main articles: NIRSpec Fixed Slits, MIRI Spectroscopic Elements

NIRSpec offers slit spectroscopy in 5 apertures, which are described in detail on the NIRSPEC Fixed-Slit Spectroscopy article. MIRI offers slit spectroscopy in a single aperture with the low-resolution spectroscopy (see the MIRI LRS APT Template article)


Table 1. NIRSpec and MIRI spectroscopic slits

InstrumentSlitSize (arcsec)Size (pixels)
NIRSpecS200A1, S200A2, (S200B1)0.2 × 3.22 × 32
NIRSpecS400A10.4 × 3.654 × 36
NIRSpecS1600A11.6 × 1.616 × 16
MIRILRS0.51 × 4.75 × 43


The NIRSpec S1600A1 slit is optimized for time-series observations.

Summary of available options

The NIRSpec slits can produce spectra with resolutions of roughly 100, 1,000, and 2,700. At the lowest resolution, a full spectrum from 0.7 to 2.3 μm can be generated in one grism setting. At the higher resolutions, 4 settings are required for full wavelength coverage.

The LRS on MIRI produces a spectrum from 5 past 12 μm, with a spectral resolving power increasing from ~40 at 5 μm to ~200 at 12 μm. The long wavelength cut-off for the LRS is a soft number because the prism transmission decreases rapidly from 10 μm to longer wavelengths while the resolution is growing higher. For very red sources, it is possible to obtain meaningful data all the way out to 14 μm, but the quality of the flat fields and photometric calibration will begin dropping past 10 μm and will not be reliable past 12 μm.



Comparison to ground-based slit spectroscopy

The primary advantage to spectroscopy from JWST is that telluric absorption does not limit the wavelength coverage, making it possible to observe in spectral regions inaccessible from the ground and to obtain continuous spectra from 0.6 μm all the way to the LRS sensitivity cut-off past 12 μm. In addition, JWST is unaffected by atmospheric turbulence, so that the spectroscopic slit truncates less of the light from a target and the angular resolution in the cross-dispersion direction is closer to the diffraction limit.  Finally, the cold operating temperatures of JWST and the resulting low backgrounds from the telescope lead to much better mid-infrared sensitivity than can be achieved from the ground.



Dithers and nods

 Main articles: JWST Dithering Overview, NIRSpec Dithers and NodsNIRSpec FS Dither and Nod Patterns, MIRI LRS Dithering

Dithering in slit spectroscopy may help mitigate the effects of bad pixels, detector effects, and improve spatial and spectral sampling. In addition, the different pointings can provide background observations around point (or compact) sources. Where the step size is relatively large (larger than the PSF size), this is typically referred to as "nodding". 

NIRSpec fixed slit spectroscopy offers both a "nodding" with large-scale offsets and sub-pixel dithering. A variety of options are available for both types of dithers. The user can select either one strategy, or combine both. The MIRI LRS template offers an ALONG SLIT NOD 1 option, where the target is placed alternately at 30% and 70% of the slit length (approx. +/- 0.9 arcsec or 8.25 px from the slit centre). A MAPPING mode is also offered, where the user can define a number of offsets in the spatial and/or spectral direction. These dither types can be combined with mosaic settings to perform a dither pattern at each mosaic pointing position.

For more information, please see these articles:

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Near-infrared spectroscopic options

Observers interested in spectra in the 0.6–5.3 μm range have options on 3 of the 4 JWST instruments.  

NIRISS and NIRCam both offer wide-field slitless spectroscopy, making it possible to obtain spectra of multiple targets in the same field simultaneously at 0.8–2.3 μm and 2.4–5.0 μm, respectively. NIRISS also offers single-object slitless spectroscopy covering the range 0.6–2.8 μm.  In crowded fields or in fields with complex backgrounds, these slitless modes can make it difficult to disentangle the spectrum of individual targets.

For more on spectroscopy with NIRISS and NIRCam, please see these articles:

NIRSpec offers 2 further options. The micro-shutter assembly (MSA) offers Multi-Object Spectroscopy, and in areas in the sky where backgrounds and crowdedness are issues, it gives definite advantages over wide field slitless modes.  The integral field unit (IFU) will produce full spectra in 900 spatial elements in a 3" × 3" field.

For more on these NIRSpec modes, please see these articles:

The NIRSpec fixed slits are ideal for a spectrum of an individual target.  Unlike the slits in the MSA, the danger of leakage of spectral information from any other aperture into the aperture of interest is minimized.  And for point sources, it generates spectra with a simpler optical path and better sensitivity than the IFU.



Mid-infrared spectroscopic options

MIRI can generate spectra with the LRS slit, the LRS slitless mode, the medium-resolution spectrograph (MRS).  The LRS slitless mode will generate spectra without any truncation of signal due to slit throughput, but the cost is reduced sensivity and spectral resolution.  The MRS produces spectra from 4.9 to 28.3 μm with resolutions between 1,600 and 3,400 (out to 18 μm) and 1,200 and 1,800 (past 18 μm) with pixel scales ranging from 200 to 270 mas.

MIRI offers no wide field spectroscopic modes.  However, if anyone is determined to obtain spectral information of many sources at once, they can generate a spectral energy distribution (SED) by stepping through all 9 of the imaging filters.  The result will be an SED from 5.6 to 25.5 μm, effectively a spectrum with a resolution of ~5, of every source within the 74" × 113" imaging field.  This mode can be thought of as a very low-resolution spectrograph (VLRS) for multiple objects.

For more on these modes, please see these articles:

If a resolution higher than 10 is desired, the LRS is the most efficient option. The MRS offers higher resolution, two spatial dimensions, and coverage to longer wavelengths, but it is less sensitive.



Planning observations with the spectroscopic slits

Main articles: JWST ETC Aperture Spectral Extraction Strategy, MIRI LRS Template APT Guide, NIRSpec Fixed Slit Spectroscopy APT Template
See also: JWST ETC MIRI Target Acquisition, JWST ETC NIRSpec Target Acquisition

Anyone interested in obtaining spectra with the fixed slits on JWST will need to prepare an observing proposal.  Two important tools for that process are the Exposure Time Calculator (ETC), which helps estimate the integration times needed to obtain the desired signal-to-noise ratio, and the Astronomer's Proposal Tool (APT), which actually generates the observations and is required to submit a proposal. Note that the ETC can also be used to perform target acquisition exposure calculations; the user is highly recommended to make use of this capability for slit spectroscopy observations.

The goals of the observer should inform their choice of observing mode and slit size.  Higher spectral resolution and narrower slits will facilitate the study of emission and absorption lines.  For example, the MRS would be preferable to the LRS on MIRI, and for NIRSpec, the S200 slits would be preferable to S1600A1.  Studies of the continuum or broad dust features would benefit from lower resolutions.



Transmission losses

It is inevitable that the spectroscopic slit will truncate some part of the point spread function (PSF), leading to transmission losses in the resulting spectrum.  These losses are a function of wavelength, because the size of the PSF is a function of wavelength.  They also depend on the location of the source within the slit, and this will vary primarily due to pointing errors as the telescope shifts a source from the target acquisition field to the slit. Given the importance of source placement in the slit, target acquisition is recommended for slit spectroscopy observations.

For JWST, these pointing errors can be expected to be on the order of 7 mas (the precise value of the mean error depends on the distance the telescope moves).  Pointing errors on this scale are almost negligible compared to the size of even the smallest fixed slit, 220 mas on NIRSpec.  Consequently, it's expected that the transmission function (transmission vs. wavelength) will be almost the same from one spectrum to the next and thus straightforward to correct in the pipeline.

A couple of caveats are important.  First, the transmission function for an extended target, even if it is compact, will differ from that for a point source, which could lead to more significant issues, especially if the target is mispointed.  Second, mispointings are more likely when an offset source is used for target acquisition.  In that case, any errors in relative positions between the source chosen for target acquisition and the scientific source could seriously impact the quality of the spectrum.  As an example, even relative positional errors of only 50 mas are halfway from the centerline of the smallest NIRSpec slit to its edge, if the user is unlucky enough for the error to be in that direction.  For this reason, TA on the science target is usually the safest option, provided that it is a point source.



Scattered light and saturation

The NIRSpec slits are relatively safe from scattered light because they are in the gap between the two halves of the MSA and most of the shutters will be closed.

For the MIRI LRS, on the other hand, contamination from the imager field of view should be considered. During LRS observations, any sources in the imager field will also be dispersed. Because of the broad passband of the LRS prism, these sources will easily saturate in the field. Very bright or saturated sources can cause detector artifacts along rows and columns around the bright source. These can in principle cause systematics in the LRS spectrum during slit spectroscopy observations. Very bright sources located in the imager field may also cause some light to scatter into the LRS slit spectral region. If possible, observers should avoid having very bright sources in the imager portion of the detector field of view whilst exposing with LRS, in particular in the region immediately adjacent to the slit (this can be checked using the Aladin visualization option in APT).  If they find a potentially troublesome source, it may be necessary to constrain the roll angle of the telescope to keep this source off of the imaging array during an integration.The full detector read out will be available to observers, allowing any such sources, if present, to be identified, which can help with data analysis.



Spectroscopic data

Main article: Data Processing and Calibration Files

The JWST data reduction pipeline will generate spectra of the targeted science objects in the spectroscopic slits. These data will be calibrated and presented in flux density units (Fν) vs. wavelength (in μm).  Observers do not have to obtain calibration data themselves.

Flux calibration involves two components. Observatory requirements are that absolute flux calibration (i.e., photometry) be good to 10% for NIRSpec and 15% for MIRI; it's anticipated that these requirements will be exceeded in cycle 1. Point-to-point flux calibration (i.e., relative or spectroscopic flux calibration) will be substantially better than the absolute flux calibration.  Wavelength calibration should meet the requirement of 10% of the resolution element (as measured by its full width at half maximum).




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    Updated the along slit nod locations for MIRI LRS dithering