NIRSpec MOS Operations - Catalogs and Images

JWST NIRSpec multi-object spectroscopy (MOS) mode observations require images and the associated catalogs to plan the observations. Optimal data calibration results from planning catalogs with internal relative accuracy of 5–10 mas.

One of the featured observing modes of NIRSpec is multi-object spectroscopy (MOS) with the micro-shutter assembly (MSA), which consists of roughly a quarter million configurable shutters that are 0. 20" × 0.46" in size. The NIRSpec MSA shutters can be opened in adjacent rows to create flexible and positionable spectroscopy slits (slitlets) on prime science targets of interest. Because of the very small shutter size, the NIRSpec MSA spectral data quality will benefit significantly from accurate astrometric knowledge of the positions of planned science sources, as well as the MSA target acquisition (MSATA) reference stars.

NIRSpec MOS spectroscopy and the MSATA process was designed to execute using catalogs with relative astrometric accuracy from the 5 to 50 mas level. Optimal flux calibration of NIRSpec MOS science will require internal relative accuracy of 5 to 10 mas (1-sigma) in the catalog used for observation planning. NIRSpec MOS observations planned with catalogs to 50 mas accuracies (e.g., such as from Sloan Digital Sky Survey imaging) will execute with uncertain source placement in science apertures, resulting in lower accuracy flux and wavelength calibration (Figure 2 in the MSATA article shows the source placement uncertainty as a function of input catalog accuracy).

A 5–10 mas level of relative astrometric accuracy can presently be achieved using images from the cameras on the Hubble Space Telescope (HST). Tiled mosaics of multiple HST image fields may have slightly decreased accuracy compared to the 5 mas level, but still represent an improvement over the typical in-field relative accuracy in lower resolution space-based images (i.e., Spitzer Space Telescope) or ground-based cameras. Dithered images from the HST ACS or WFC3 UVIS cameras cover a majority of the NIRSpec MSA footprint, and can provide the necessary in-field accuracy for NIRSpec spectroscopy planning. HST images used for planning NIRSpec MSA spectroscopy should preferentially have been acquired within the past decade for optimal spectroscopic planning due to appreciable proper motions (~10 mas or more) in Galactic field stars. Images from NIRCam on the JWST will also acquire imaging for high precision source catalogs; therefore, the capability for same-cycle imaging with NIRCam to plan NIRSpec MSA-based science programs will be possible.

For MOS observations of bright science targets, the GAIA catalog may meet the astrometric accuracy requirements for NIRSpec MSA planning. However, the relatively shallow depth of GAIA implies that many science targets will not have GAIA coordinates, and will therefore still require HST or JWST images. When constructing a catalog for MOS planning, it is critical to ensure that the positions of the science targets and the MSATA reference stars have been measured in the same image or mosaic.

Additional Information on source images and catalogs for NIRSpec MOS spectroscopy planning is included in these articles:

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