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 and an absolute orient accuracy of 60" (0.0167°) or better.

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 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) and anchored to an absolute 60" (0.0167°) orient accuracy or better 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. Note that HST images or mosaics may not have the orientation accuracy of better than 60" (0.0167°) absolute anchoring on the sky, and referencing to Gaia is strongly recommended as a check for the catalog preparation (see note below).  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.

Imaging data used to derive the astrometry for your catalog of reference stars and science targets needs to be carefully vetted against Gaia stars. In particular, the MSA target acquisition procedure has a very limited ability to correct for errors in the initial roll, so it is strongly recommended that the rotation of the reference stars on the sky be aligned with that of the Gaia frame to better than one arcminute (60", 0.0167°) in rotation. Roll offsets larger than 500" (0.14°) are likely to fail. Care should be taken to consider any proper motion of the Gaia stars and reference stars to avoid introducing a spurious roll offset larger than this. This is required to produce the best astrometry possible, and to correct for any lingering small roll offsets that may affect target placements.

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

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