Planning your JWST/NIRSpec observation: pre-imaging and source catalogue

Most observations with NIRSpec Spectroscopy will require high spatial resolution images of the science field previous to performing the spectroscopy. This is due to the fact that the standard NIRSpec target acquisition (TA) needs to acquire reference stars to deliver a position RMS of less than 20mas. NIRSpec TA uses 8 - 20 reference stars with accurate astrometry (< 5 mas), calculates pixel centroids of the stars, transforms their pixel coordinate positions to the ideal sky frame and calculates the slew to accurately place the science targets in the Micro Shutter Array. For some planned observations, very high spatial resolution Hubble Space Telescope images might be already available, and in other cases NIRCam observations will be performed. For a planned NIRSpec observation, we describe in detail the proposed method to generate a high resolution image mosaic to plan NIRSpec spectroscopy. We show some of the data products that have been developed using actual HST observations. We also describe the proposed procedure using simulated NIRCam images for derivation of source catalogs. Rapid availability of these two data products will be crucial for the success of many NIRSpec observations.