A Comparison of MOS spectroscopy with the NIRSpec MSA and other JWST Instruments

JWST multi-object spectroscopy (MOS) with the NIRSpec micro-shutter assembly (MSA) is compared with the NIRISS wide field slitless spectroscopy (WFSS) mode, and the NIRCAM WFSS mode.

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Comparing MOS spectroscopy in NIRSpec, NIRISS, and NIRCam

Main articles: NIRSpec Multi-Object Spectroscopy, NIRISS Wide Field Slitless Spectroscopy, NIRCam Wide Field Slitless Spectroscopy

MOS is an efficient way to obtain the spectra of many sources all within the field of view (FOV) of an instrument at a single pointing. This can also be accomplished using a wide field slitless spectroscopy capability (WFSS) like that offered with the NIRISS and NIRCam instruments. To compare and contrast MOS spectroscopy and wide field slitless spectroscopy, this section will focus on these instrument capabilities as offered by JWST instruments.  

Field of view and operating range

The field of view and wavelength coverage for each instrument mode are compared in Table 1. Here are some differences:

FOV:

  • The field of view (FOV) of the NIRISS/WFSS mode is smaller than that of the NIRspec MSA.
  • For a single module in the NIRCam/WFSS mode, the FOV is also smaller than that of the MSA.

Wavelength Coverage and Spectral Resolution:

  • NIRISS/WFSS operates over a shortened wavelength range from 0.8 to 2.2 μm with low spectral resolution (R ~ 150).
  • The NIRCam/WFSS mode operates over the range from 2.4 to 5 μm with moderate resolution (R ~ 1,600). The same instrument mode includes simultaneous short wavelength (SW; 0.6 to 2.3 µm) imaging over roughly the same field of view via a dichroic. 
  • NIRISS/WFSS and NIRCam/WFSS modes use grisms to disperse the spectra onto their respective detectors.
  • The NIRSpec instrument uses a set of 3 high- or medium-resolution gratings to cover the entire operational wavelength range. Together with matched filters, a minimum of 3 exposures will cover the 1–5 μm range, and additional exposure can extend this coverage to 0.7 μm. NIRSpec MOS mode additionally offers a prism for low resolution spectroscopy.
  • A common practice in the WFSS modes of both NIRISS and NIRCam is to couple the grism with a blocking filter to effectively shorten the spectrum on the detector and limit spectral overlap from other sources in the field, including the background.
  • NIRSpec MOS with the MSA can obtain the entire range from 0.6 to 5.3 μm in the low-resolution PRISM in a single exposure with a much lower probability of contamination.

Spectral Contamination and Slit Losses

One clear advantage of MOS instruments with slit masks is that they can block out sources to prevent contamination by overlapping spectra. Masking out other sources of light allows greater efficiency and sensitivity for faint targets, which can be lost in slitless spectroscopy if they are contaminated by a bright, nearby objectMoreover, because slits block the background as well, MOS spectroscopy with slit masks can reach greater sensitivity than WFSS. Conversely, with WFSS, the dispersed background from areas even quite far from a target occupies the same detector area as the spectrum of the target, leading to added noise.  

With NIRISS/WFSS and NIRCam/WFSS, the spectra of all targets may optionally be observed at 2 perpendicular angles in a single observation (requiring a second exposure) using 2 orthogonal grisms. Spectra taken this way are mapped to different locations on the detectors. Combining extracted spectra observed at both angles is useful for mitigating contamination (since a spectrum contaminated in one angle will hopefully be clean in the orthogonal angle). The strategy also helps with de-blending overlapping science spectra of nearby sources and can improve spectral sampling. 

In NIRISS and NIRCam WFSS modes, there are no slits, hence no slit losses. With the NIRSpec MSA, there are slit losses associated with the MSA bars. Since it is not possible to obtain perfect centering on each source in the fixed grid of the MSA shutters, slit losses will vary from target to target. For point sources, pipeline processing will correct for these losses, although the accuracy of the correction depends on the accuracy of the target acquisition. Alternatively, for extended targets, slit losses from the 0.2" wide MSA shutters can be significant and may be difficult to quantify in some cases. 

Comparative Summary of Techniques

A summary of the advantages and disadvantages of each technique is shown in Table 1. The JWST Wide Field Slitless Spectroscopy article contains more detailed information about WFSS with JWST (both NIRISS and NIRCam instruments). APT Guides are available for each of the instrument modes:

NIRCam Wide-Field Slitless Spectroscopy Template APT Guide
NIRISS Wide-Field Slitless Spectroscopy Template APT Guide
NIRSpec Multi-Object Spectroscopy APT Template

Other articles have been written to describe science use cases in these instrument modes.



Table 1. Comparison of wide field slitless spectroscopy with NIRISS and NIRCam, and NIRspec MOS spectroscopy.

NIRSpec/MSANIRISS/WFSSNIRCam/WFSS
FOV

3.4' × 3.6'

2.2' × 2.2'

2.2' × 2.2' for one module, reduced by the filter choice
2.2' × 5.0' for two modules, reduced by the filter choice

Wavelength Range

0.6–5.3 µm (PRISM) in one exposure

0.7–1.89 µm (Band I), 2 filters/exposures required

1.66–3.17 µm (Band II) in one exposure

2.87–5.27 µm (Band III) in one exposure

no imaging possible with NIRSpec

0.62.8 µm (several filters required to cover the range)

0.82.2 µm (several filters required to cover the range)

imaging possible in same observation

2.45.0 µm (several filters required to cover the range)
Optional simultaneous shortwave imaging (0.6–2.3 µm)

Resolving Power

R ~ 100 (PRISM)

R ~ 1,000 (Med-res gratings, Bands IIII)

R ~ 2,700 (High-res gratings, Bands IIII)

R ~ 700 (0.6–2.8 µm)

R ~ 150 (0.8–2.2 µm)

R ~ 1,600

Observable aperture position angles1APA is assigned once observations are scheduledperpendicular APAs are additionally possible in same observationperpendicular APAs are additionally possible in same observation
Slit LossesMSA fixed shutter size, vignetted by MSA barsNoneNone
DitheringRecommended to bridge the detector gap and sample point source spectra at several locations in a shutter and on the detector.  The NIRSpec PSF is critically sampled at most wavelengths and under-sampled at some.Recommended because the NIRISS PSF is under-sampled.Recommended to provide direct imaging of out-of-view sources, and to bridge detector gaps and module gaps.
Contamination and confusion by other sourcesOnly if contaminating sources occupy the same shutterUnavoidable for most sources. Spectra of all sources across the field in dispersion direction may overlapUnavoidable for most sources. Spectra of all sources across the field in dispersion direction will overlap
BackgroundBlocked by the MSA, with only a small amount of leakage, thereby achieving great sensitivityThe entire background that passes through the blocking filter is dispersed, adding noise.The entire background that passes through the blocking filter is dispersed, adding noise. The background of NIRCAM slitless observations is expected to have significant structure caused by the final shape of the pickup mirror illuminating the grism.


Table Note 1: The range of feasible angles for all instrument modes depends on the target pointing ecliptic latitude.



MOS and WFSS data

Figures 1 and 2 show examples of data for the NIRISS and the NIRCam WFSS modes, respectively.  These figures are from a Newsletter article (Vol 30 Issue 2) by Dixon and Willott showing a field of simulated galaxies based on position derived from CLASH (Cluster Lensing and Supernova Survey with Hubble) data, and the WFSS spectral images produce by the NIRISS WFSS observing mode.

Figures 1 and 2. Simulated direct image and WFSS spectroscopy of the same field with NIRISS

A field of simulated galaxies based on a position derived from CLASH (Cluster Lensing and Supernova Survey with Hubble) data. Credit: Van Dixon and Chris Willott STScI Newsletter 2015 (Vol 30, Issue 2).
Figure 3 below is a view of simulated NIRCam Grism WFSS data with 2 cross-dispersed GRISMs.
Figure 3. Simulation of NIRCam WFSS dispersed grism data using different cross filters

Simulated single-module NIRCam observations of a section of the GOODS-S ERS field. Top Panel: full simulations that include estimated dispersed grism backgrounds. Bottom Panel: Same as Top Panel but the backgrounds have been subtracted.
Figure 4 presents simulated NIRSpec MOS data with the R ~ 1,000 G140M grating for comparison with the data products above.

Figure 4. Pre-flight NIRSpec MOS slit data

NIRSpec MOS data acquired with a ground calibration test lamp using the R = 1000 G140M/F100LP short wavelength spectral configuration. The four MSA quadrant spectra are shown. This observation was acquired with a special 5-shutter calibration-only slitlet pattern that had 3 shutters open with 2 closed in-between them. About 100 micro-shutters were opened in all. Bright spectra near the vertical center are from the fixed slits, which are milled into the same mounting plate that contains the MSA. Fainter single-shutter spectra from individual failed open shutters can also be seen.



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