NIRSpec MOS Operations - Slit Losses
JWST NIRSpec MSA-based observations are affected by slit losses due to off-center target placement in the shutters.
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
Slit losses in the MSA shutter and planning constraints
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 loss changes with relative placement of the science sources within the MSA shutters. To mitigate these effects, users can choose from several Source Centering Constraints1 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.
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.
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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 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.
Beck et al. 2016, SPIE, 9910, 12
Planning JWST NIRSpec MSA spectroscopy using NIRCam pre-images