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.
Comparing MOS spectroscopy in NIRSpec, NIRISS, and NIRCam
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 FoV and wavelength coverage for each instrument mode are compared in Table 1. The 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. 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 and NIRCam WFSS modes use grisms to disperse the spectra onto their respective detectors, but the NIRSpec instrument uses a set of three 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 object. Moreover, 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 two perpendicular angles in a single observation (requiring a second exposure) using two orthogonal grisms. Spectra taken this way are mapped two 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 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:
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.
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
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
0.6–2.8 µm (several filters required to cover the range)
0.8–2.2 µm (several filters required to cover the range)
imaging possible in same observation
2.4–5.0 µm (several filters required to cover the range)
optional simultaneous shortwave imaging (0.6–2.3 µm)
R ~ 100 (PRISM)
R ~ 1,000 (Med-res gratings, Bands I–III)
R ~ 2,700 (High-res gratings, Bands I–III)
R ~ 700 (0.6–2.8 µm)
R ~ 150 (0.8–2.2 µm)
R ~ 1,600
|Observable aperture position angles1||APA is assigned once observations are scheduled||perpendicular APAs are additionally possible in same observation||perpendicular APAs are additionally possible in same observation|
|Slit Losses||MSA fixed shutter size, vignetted by MSA bars||None||None|
|Dithering||Recommended 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 sources||Only if contaminating sources occupy the same shutter||Unavoidable for most sources. Spectra of all sources across the field in dispersion direction may overlap||Unavoidable for most sources. Spectra of all sources across the field in dispersion direction will overlap|
|Background||Blocked by the MSA, with only a small amount of leakage, thereby achieving great sensitivity.||The 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 32 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.