NIRSpec MOS Operations - Slit Losses

JWST NIRSpec MSA-based observations are affected by slit losses due to off-center target placement in the shutters.


This article has not yet been updated to include in-flight performance.

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One of the main observing modes of NIRSpec is multi-object spectroscopy (MOS) with the micro-shutter assembly (MSA). The MSA consists of a fixed grid of roughly a quarter million configurable shutters that are 0.20" × 0.46" in size. Its shutters can be opened in adjacent rows to create customizable and positionable spectroscopy slits (called "slitlets") on prime science targets of interest. At any given pointing, MSA targets will map to different positions within each target shutter. Properly accounting for the slit loss errors requires precise knowledge of those positions within the shutters. Because of the very small shutter size, NIRSpec MSA spectral data quality will benefit significantly from accurate Catalog astrometry of planned science source positions.

NIRSpec MOS calibration slit loss uncertainties vs. planning Catalog accuracy

See also: NIRSpec MOS Operations - Pre-Imaging Using NIRCam

Slit losses in the MSA shutter and planning constraints

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The NIRSpec MSA is comprised of a fixed grid of micro-shutters. Consequently, science sources of interest cannot all be perfectly centered within their configured spectral slits and, as a result of the very small MSA shutter aperture size, moderate flux can be lost outside of the slit. Because the FWHM of the PSF increases with wavelength, slit throughput loss is also a function of wavelength. Likewise, slit losses are affected by the relative placement of the science sources within the MSA shutters. To mitigate these effects, users can choose from several Source Centering Constraints in NIRSpec MSA Planning Tool (MPT). The definition of these constraints are given in Table 1 in the NIRSpec MPT - Planner article. Figure 1 presents the worst-case point source flux throughput in an MSA shutter as a function of wavelength for the different available Source Centering Constraints.
Figure 1. NIRSpec MSA shutter point source throughput as a function of wavelength for different source centering constraints  

These are the worst case throughputs resulting from geometrical slit loss for a point source observed through an MSA shutter plotted as a function of wavelength. The curves relate to the different options of Source Centering Constraint (Table 1) in the NIRSpec MSA Planning Tool. The larger PSF at 5 μm relative to the fixed shutter size results in more slit loss through the shutters. The Tightly Constrained, Constrained, and Midpoint curves correspond to 85%, 75%, and 62% of the flux, respectively, compared to a perfectly centered source at 2.95 μm. The Unconstrained curve (magenta) refers to positions anywhere inside the mid-bar area of the shutter. Note that below 1 μm, the fraction of the power in the wings of the PSF is increased relative to that in the core, even though the PSF core is narrower at shorter wavelengths. Hence, except for the Unconstrained case, slit throughput declines (slit losses increase) below 1 μm. The opposite is true for the Unconstrained case, because in this scenario, more of the PSF wings are captured by the slitlet. 

The NIRSpec MSA science sources could be located anywhere within their planned shutter centering constraint. Consequently, the actual slit throughput for a source may be larger than that of the selected constraint level shown, up to the value shown for a perfectly centered source (black curve). The worst case shutter throughput occurs when sources are located near the corners of the constrained area in the shutter, since the PSF is truncated on two sides. In the Unconstrained case (magenta), a science source of interest might be centered behind both the horizontal and vertical MSA bars in the shutter corner. In this situation only a small fraction of the flux of a point source will make it into the open shutter to be measured by the spectrograph. The Unconstrained option is recommended only for spatially extended targets where additional flux would make it into the slit, compared to the point source calculation presented in Figure 1.

Slit loss uncertainty as a result of Catalog astrometric accuracy

The NIRSpec calibration pipeline will apply a slit loss throughput correction for point sources based on the planned position within an MSA shutter. A perfectly centered point source with optimal TA astrometric accuracy (5 mas) will have no excess flux error; the slit loss can be calibrated at the optimal level achievable by the pipeline (estimated to be approximately a ~6% term).  However, if the TA astrometric accuracy is relaxed, the slit loss correction will also carry a greater uncertainty.

Figure 2 shows that reduced astrometric accuracy in the planning Catalog will result in an additional flux calibration uncertainty primarily due to an associated slit loss uncertainty. These calculations are done assuming a point source observed in the corner of its MSA shutter Source Centering Constraint, resulting in worst case slit loss throughput uncertainty. These calibration errors are very difficult to correct because of imprecise knowledge of the final source centering in a NIRSpec MSA slit. The curves are anchored to the wavelength defining the source centering constraint: 2.95 μm. The figure describes the geometrical slit throughput only, and does not represent the total end-to-end instrument throughput or sensitivity.

Figure 2 shows that the accuracy of the flux calibration of the science sources will depend on the Catalog relative astrometric accuracy. The ability to effectively limit slit losses using the different Source Centering Constraints in MPT will also depend on the Catalog relative astrometric accuracy. These factors should be taken into consideration when deciding whether or not pre-imaging with NIRCam would benefit the spectroscopic observations. As seen from Figure 2, in field-relative astrometry of 5–10 mas or better is needed to limit the excess flux calibration error for point source observations. Figure 2 in the NIRSpec MSA Target Acquisition article shows that this level of accuracy results an overall target acquisition uncertainty better than 20 mas.

Figure 2. NIRSpec MSA slit loss calibration error vs. input Catalog relative astrometric accuracy 

The estimated excess slit loss calibration error is plotted against input astrometric Catalog planning accuracy. These plots present the worst case calibration slit loss error that can result based on the accuracy of the target acquisition. The colored curves represent shutter centering constraints used in the planning process. A point source at the center of an MSA shutter planned using 30 mas of relative astrometric Catalog accuracy can have up to 10% of excess slit loss calibration error because of imperfect TA that results from the relaxed Catalog constraints. 


Beck et al. 2016, SPIE, 9910, 12
Planning JWST NIRSpec MSA spectroscopy using NIRCam pre-images

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    Relative throughput of different source centering constraints are clarified.
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