With JWST NIRSpec’s multi-object spectroscopy (MOS) mode, the micro-shutter assembly (MSA) can obtain simultaneous spectra of many science targets 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 targets 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.
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 dispersion by 87" in cross-dispersion, and the total extent of the MSA sky field of view is 3.6' × 3.4'. There are ~23" gaps between the four MSA quadrants in the dispersion axis (that is, between Q1 and Q3, Q2 and Q4 as shown in Figure 3). The MSA mounting plate separates Q1 from Q2, and Q3 from Q4, by about 37". Fixed slits, located in this 37" gap between quadrants, are always open.
Some of the shutters in the NIRSpec MSA s are defective and either permanently "failed open" or "failed closed." There are presently a small number (less than two dozen) permanently failed open shutters; these are detrimental to science because spectra from spurious sources can overlap and contaminate science spectra. Additionally, approximately 14% of the quadrant shutters are permanently failed closed and therefore inoperable (see Figure 4).
There are three types of failed closed shutters:
- 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.
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 † |
MSA spectra are projected onto the two NRS1 and NRS2 NIRSpec detectors. In the NIRSpec MOS mode, some shutters will not capture the full spectral range at the high resolution, R ~ 2,700, configuration settings. This is because the right-most MSA shutters in MSA quadrants 1 and 2 (see Figure 3) project the long wavelength spectra off the right side of detector NRS2 in the R = 2,700 modes. (See Figure 5 for a view of the MSA shutter data in R ~ 1,000 mode).
Detector wavelength gaps
See also: NIRSpec MOS Wavelength Ranges and Gaps
In the MOS mode, gaps in spectral coverage caused by the physical distance between the two detectors, referred to as detector wavelength gaps, depend on which MSA shutters are opened on sources. Unlike fixed slits (FS) and integral field unit (IFU) observations, which only have wavelength gap losses in the R ~ 2,700 high spectral resolution mode, all grating and filter combinations in the MOS mode can have shutters that, if open, will lose spectra in the wavelength 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 wavelength gap and acquire complete spectra of science targets.
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 1
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 has four 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, 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).
Options for dithering
Most observations with JWST will require dithering. The NIRSpec MOS mode provides three options for creating offsets or dithers:
- Nodding: The telescope can be repositioned slightly between exposures to place the targets in different shutters within the slitlets of an MSA configuration. This is called nodding.
- Fixed dither option: This option can be used to move sources to a different location on the MSA, also resulting in a new spectral position on the detector.
- Flexible dither option: For this option, the MPT finds an optimal pointing for multiplexing in an MSA configuration, then searches for subsequent optimal MSA configurations that maximize observed targets at new offset positions.
The NIRSpec MOS dithering page provides an in-depth review of the available options that are briefly described here.
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 are not contaminated.
Some key pages relevant for observing with the JWST NIRSpec Multi-Object Spectroscopy observing mode:
- JWST Multi-Object Spectroscopy Description
- NIRSpec MOS Observing Mode Summary
- NIRSpec MOS Wavelength Ranges and Gaps
- NIRSpec MOS Hardware Description
- NIRSpec MOS Operational Considerations
- NIRSpec MSA Planning Tool
- Strategies and Recommendations:
- NIRSpec MOS Use Case Example: NGC 346
- NIRSpec MOS Use Case Example: MOS observations of distant galaxies
- NIRSpec Spectroscopy Processing Algorithm Documentation
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
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