NIRSpec Multi-Object Spectroscopy
One of the showcase observing modes of JWST is the NIRSpec multi-object spectroscopy (MOS) mode using the micro-shutter assembly (MSA). The MSA can obtain simultaneous spectra of many science sources within a 3.6' × 3.4' field of view.
The JWST NIRSpec multi-object spectroscopy (MOS) mode provides multiplexing 0.6–5.3 μm spectroscopy capabilities over a 3.6' × 3.4' field of view. This mode uses tiny configurable shutters in the micro-shutter assembly (MSA) to acquire dozens to hundreds of spectra of astronomical sources within a single exposure. This is a very powerful feature for spectroscopic surveys. For example, potential use cases for the NIRSpec MOS mode include, but are not limited to: spectral characterization of the faintest objects in our universe, surveys to investigate galaxy formation and evolution, stellar population studies, star cluster formation, and the evolution and properties of extended solar system bodies.
The NIRSpec MSA consists of four quadrants of 365 × 171 shutters that can be individually opened and closed to create the spectral slit configurations for this multi-object spectroscopy mode. In Figure 1, the four NIRSpec MSA quadrants are plotted on a Hubble Space Telescope WFC3 F555W image of the Tarantula Nebula in the Large Magellanic Cloud.
The NIRSpec MSA can be opened in contiguous columns of several shutters to create what are called “slitlets” to acquire MOS spectra of science sources of interest. Figure 2 shows a zoomed-in view of several three-shutter slitlets, configured to open on (faint) sources, over-plotted on the Hubble Ultra Deep Field WFC3 UVIS image.
Properties of the MSA
The MSA consists of four quadrants, each with 171 rows of 365 shutters, totaling ~250,000 shutters. The four quadrants are labeled Q1, Q2, Q3, and Q4, as shown in Figure 3. Each MSA shutter is 0.20" × 0.46" (width in the dispersion direction × height).
MSA shutters are mounted on a fixed support grid. Support bars between all shutters are 0.069" wide, so that the shutter pitch (center-to-center distance) is 0.27" in the dispersion direction and 0.53" in the spatial (cross-dispersion) direction.
The area in each MSA quadrant is about 95" in the dispersion (X) direction by 87" in cross-dispersion (Y), and the total extent of the MSA sky field of view is 3.6' × 3.4'. The MSA quadrants are separated from one another by ~23" along the dispersion (X) axis (that is, between the inner edges of the tan-shaded shutter arrays shown in Figure 3 from the right edge of Q3 to the left edge of Q1, and similarly between Q4 and Q2). Note that this gap not the same as the gap between detectors in the dispersion (X) direction, which is ~ 18". It is the detector gap that is responsible for missing wavelength information in spectral observations with the MSA. The MSA mounting plate also separates Q1 from Q2, and Q3 from Q4, by about 37" in the spatial (Y) direction. The fixed slits, located in this 37" gap between quadrants, are always open.
To create an MSA configuration, the ~250,000 shutters can be individually selected to open or close. The NIRSpec MSA is configured to open sets of science shutters using a two-step process: (1) the MSA magnet arm sweeps across the MSA to open all shutters in the quadrants, then (2) the magnet moves back across the quadrants to close unused shutters and leave open those configured to observe specific science sources. This configuration process takes about 90 seconds.
Some of the shutters in the NIRSpec MSAs are defective and either permanently "failed open" or "failed closed." There are presently a small number (less than twenty) permanently failed open shutters; these are detrimental to science because spectra from spurious sources can overlap and contaminate science spectra. Additionally, approximately 15% of the quadrant shutters are permanently failed closed or impacted by electrical shorts and therefore inoperable (see Figure 4).
Of those shutters that are failed closed, there are three types:
- Some failed closed shutters are physically stuck closed;
- Some shutters used to be failed open but were deliberately blocked closed during the manufacturing process to limit detrimental contaminating spectra;
- Some entire rows and columns of shutters in the MSA are masked closed due to electrical shorts in the MSA that prevent them from being addressed properly by the configuration magnet.
The NIRSpec observation planning software will search for optimal MSA shutter science configurations and automatically plan around all failed shutters.
A few considerations for MOS observing:
- In addition to failed open shutters, even operable shutters that are commanded shut during an exposure have finite contrasts. This results in small light leaks that can contaminate observed spectra from planned sources. This is the same problem that affects IFU spectra. Further details about the problem and mitigation strategies, which are similar to those for IFU observations, are described in the NIRSpec Micro-Shutter Assembly, NIRSpec MSA Leakage Correction for IFU Observations, and MSA Leakage Subtraction Strategies articles.
- The small size of individual NIRSpec MSA shutters means that detailed and careful planning must be carried out for most observations of compact sources. High quality astrometry (to 5–10 mas) is not required, but strongly recommended, particularly for sources smaller than the size of a shutter. Accuracy of target acquisition, and hence the flux calibration, is directly related to the input astrometric accuracy. The best astrometry is likely to come from HST images of the field or a JWST NIRCam mosaic image.
- Larger or extended sources may be observed by configuring the MSA into a long slit or other suitable geometry. In this use case, detailed astrometry is not required, but pointing precision will be poorer.
See also: NIRSpec Dispersers and Filters
All of the available disperser and filter combinations can be used in NIRSpec MOS mode. Table 1 below outlines usable instrument configurations, spectral resolutions and wavelength ranges.
Table 1. Spectral configurations available in NIRSpec MOS mode
|Disperser-filter combination||Nominal resolving power||Wavelength range † |
† Wavelength range values presented here are approximate. Note that the nominal spectral ranges for medium and high-resolution dispersers may be shortened due to red-end detector cutoffs. The cutoff wavelengths depend on the target aperture location (slit or shutter). Detailed limits are found on the wavelength ranges and gaps pages for the IFU, FS, and BOTS, and in the ETC. Information on wavelength ranges for MOS, which depend on the position of the shutter in the MSA, can be determined using the MSAViz Tool.
MSA spectra are projected onto the two NIRSpec detectors (NRS1 and NRS2). In the NIRSpec MOS mode, some shutters will not capture the full spectral range at the high resolution, R ~ 2,700, configurations. This is because the right-most MSA shutters in MSA quadrants 1 and 2 (Figure 3) project the longest wavelengths beyond the right edge of detector NRS2 in the R = 2,700 configurations. Alternatively, R~1000 spectra do not extend past the right hand side of NRS2 (see Figure 5).
Detector wavelength gaps
See also: NIRSpec MOS Wavelength Ranges and Gaps
In the MOS mode, there are gaps in spectral coverage caused by the physical distance between the two detectors, referred to as detector wavelength gaps. The range of wavelengths lost in the gap are different for different shutters in the MSA since the spectra from different shutters maps to different locations on the detectors. Unlike fixed slits (FS) and integral field unit (IFU) observations, which suffer wavelength gap losses only in the R ~ 2,700 high spectral resolution mode, all grating and filter combinations in the MOS mode have shutters that lose wavelengths to the gap.
The NIRSpec MOS mode has a specialized MSA Planning Tool (MPT) within the JWST Astronomers Proposal Tool (APT) software. Using this planning tool, it is possible to create dither options to move targets by ~18" (or more) in the dispersion direction. This 18" is the approximate minimum dither distance necessary to span the detector wavelength gap and acquire complete spectra of science sources. Also, it is possible to inspect MSA configurations designed for MOS observations using the MPT to ensure that wavelengths of interest fall into operable regions on the detectors (in areas unaffected by the detector gap or the long wavelength cutoffs, for instance). The tool is called the NIRSpec MSA Spectral Visualization Tool (MSAViz).
The detector wavelength gap discussed here is different from the gap between quadrants of the MSA shown in Figure 3.
NIRSpec MOS mode exposures are only acquired in FULL frame 2048 × 2048 detector pixel readout; no subarrays are used.
NIRSpec MOS exposure times are tied to the timing of the detector readout patterns. There are four readout patterns available for NIRSpec MOS observations:
- NRSRAPID *
The readout patterns are split over two readout modes: (1) traditional and (2) 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 averages four frames (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, but unlike the traditional readout equivalent, NRSIRS2 has five frames averaged into a single group. These IRS2 readout patterns improve performance and sensitivity in long exposure MOS observations of faint objects.
Additional information on NIRSpec MOS exposure specification and how this translates to exposure time and sensitivity can be found using the JWST Exposure Time Calculator (ETC). Users interested in determining which readout pattern is best for their science should refer to the NIRSpec Detector Recommended Strategies article.
* Bold italics style indicates words that are also parameters or buttons in software tools (like the APT and ETC). Similarly, a bold style represents menu items and panels.
Options for dithering
Most observations with JWST will require dithering. This is especially true for NIRSpec since the PSF is under-sampled at most wavelengths. The NIRSpec MOS mode provides two options for creating offsets or dithers:
- Nodding: The telescope can be repositioned slightly between exposures to place the sources in different shutters within the slitlets of an MSA configuration.
Fixed Dithers: Larger telescope dithers which position the sources onto different shutters of the MSA, and their spectra onto different areas of the detectors.The NIRSpec MOS dithering page and the NIRSpec MOS Recommended Strategies page provide an in-depth review of the available options mentioned here. Users interested in learning which dithering strategies are recommended for their science should read the NIRSpec Dithering Recommended Strategies article.
What do NIRSpec MOS data look like?
Figure 5 shows NIRSpec MOS mode data acquired with a ground calibration test lamp using the R = 1,000 G140M/F100LP short wavelength spectral configuration. The four MSA quadrant spectra are shown. This observation was acquired with a special five-shutter calibration-only slitlet pattern that had three shutters open with two closed in-between them. Two of the slitlet configurations are highlighted. Failed open shutters cause contaminating single shutter spectra. The MSA planning software is designed to automatically optimize an MSA configuration around failed open shutters so that science spectra of observed sources are not contaminated by dispersed light from these shutters.
Dorner, B., Giardino, G., Ferruit, P. et al. 2016, A&A, 592, A113
A model-based approach to the spatial and spectra calibration of NIRSpec onboard JWST
Kutyrev, A.S., Collins, N., Chambers, J. et al. 2008 SPIE, 7010, 70103d
Microshutter arrays: High contrast programmable field masks for JWST NIRSpec