NIRSpec FS Operations
Operational features of the JWST NIRSpec fixed slits (FS) spectroscopy science mode include target acquisition methods and dithering options.
The JWST NIRSpec fixed slits (FS) spectroscopy options provide high sensitivity single object spectroscopy over the full 0.6–5.3 μm wavelength region where NIRSpec operates. NIRSpec Fixed Slits are designed for high contrast spectroscopy of both the faintest and brightest targets NIRSpec can observe. Example use cases for FS spectroscopy include, but are not limited to: high sensitivity observations of very faint single targets such as high redshift quasars and isolated low mass brown dwarfs, or bright sources such as solar system objects and young stars that would saturate in the full detector readout used in the NIRSpec multi-object spectroscopy (MOS) or integral field unit (IFU) spectroscopy modes.
Target acquisition for FS mode
The NIRSpec wide aperture target acquisition (TA) method, WATA 1, is the default target acquisition method for the FS observing mode. This method corrects any errors in absolute telescope pointing (and absolute knowledge of the source coordinates) by executing a TA centroid of the science target and offsetting it to the center of the wide aperture. After centroiding the target in the wide aperture, an offset is applied to move to the aperture that will be used for science. Details about NIRSpec target acquisition in the FS observing mode can be found in the article NIRSpec Target Acquisition.
The NIRSpec standard target acquisition (TA) method MSATA may also be used for NIRSpec science observations performed with the NIRSpec fixed slits (FS) observing mode. During the MSATA procedure, a set of TA reference objects are observed through the open micro-shutters, and centroids are calculated and used to accurately correct the initial spacecraft pointing and position angle. The achievable TA performance depends significantly on the relative astrometric knowledge of both the reference stars and the science source positions used to plan the TA and the science. The MSATA procedure is designed to work best when the relative astrometric accuracy of the target field is in the range of 5 to 50 mas. The S200A1 and S200A2 fixed slits have the same width as NIRSpec MSA shutters. For these slits, the optimal TA accuracy of about 20 mas requires a relative astrometric accuracy of 5 mas in the planning catalog. MSATA and WATA are not equally good choices for the FS target acquisition. For MSATA there is a two-phase MOS process involving an orient assignment. MSATA reference stars must be provided by the observer and may require pre-imaging. Further information about selecting MSATA reference stars can be found in the article on MSATA Reference Star Selection Recommended Strategies. Additionally, MSATA has significantly more overhead charged to the observer.
The article NIRSpec Target Acquisition Recommended Strategies presents some considerations to help the observer decide which is best for their program.
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FS dither and nod options
See also: NIRSpec Dithering Recommended Strategies
The NIRSpec fixed slits (FS) spectroscopy mode has several dither and nod options available. In FS mode, the dithers are small scale subpixel (<0.1") offsets of the target position, which will be used to mitigate detector effects, help remove cosmic rays, and particularly improve spectral and/or spatial sampling. The nods in FS mode are larger offsets (>0.5") used to subtract nearby background flux. As a result, the nod options are best used for targets that are not significantly spatially extended on the scale of the offset. The FS mode nods provide the primary offset positions along the spectral slits, and the dither options provide optional subpixel offsets to improve sampling. The nod positions and subpixel dither offsets can be used separately or together.
In the high resolution gratings (G140H, G235H, and G395H), spectra fall across the gap between detectors. The 0.20" A slits (designated S200A1 and S200A2) can both be used to observe one source in the same visit at two different positions on the detector, to cover the wavelength gap caused by the separation between detectors.
See also: NIRSpec FS and IFU Mosaic APT Guide
The mosaic tools available in the JWST Astronomers Proposal Tool (APT) can be used to define FS mosaics or slit scans across a spatially resolved astronomical scene of interest. This option can be used to acquire spatially resolved spectroscopy in a field that is too bright and would saturate in the full frame detector readout used by the IFU mode of NIRSpec. The available FS subarrays shorten the exposure group time to enable science on brighter sources.
The NIRSpec IFU has finer spatial sampling (0.1") and will more efficiently acquire resolved spectroscopy in astrophysical scenes, so the FS mosaic option should only be used on the brightest sources observable with NIRSpec.
FS background subtraction
The NIRSpec FS nod options are offset positions along the cross dispersion length of the slit that can be used to subtract nearby background flux. The nod patterns have 2-point, 3-point, and 5-point options available.
In addition to nod offsets, the 0.20" A slits (designated S200A1 and S200A2) can both be used to observe one source in the same visit. Observations using both of these Fixed Slits can be used for background subtraction, particularly on targets that are extended on spatial scales that are larger than the FS nod offset distances. Observations using both the S200A1 and S200A2 Fixed Slits must either use FULL frame readout, or a subarray that is large enough to capture spectra from both slits (ALLSLITS ). Data can be acquired in both the S200A1 and S200A2 slits by selecting this combined option in the pull-down menu for Slit in the APT interface.