All NIRspec needs is HST/WFC3 pre-imaging? The use of Milky Way Stars in
  WFC3 Imaging to Register NIRspec MSA Observations
  The James Webb Space Telescope (JWST) will be an exquisite new near-infrared
observatory with imaging and multi-object spectroscopy through ESA's NIRspec
instrument with its unique Micro-Shutter Array (MSA), allowing for slits to be
positioned on astronomical targets by opening specific 0.002"-wide micro
shutter doors.
  To ensure proper target acquisition, the on-sky position of the MSA needs to
be verified before spectroscopic observations start. An onboard centroiding
program registers the position of pre-identified guide stars in a Target
Acquisition (TA) image, a short pre-spectroscopy exposure without dispersion
(image mode) through the MSA with all shutters open.
  The outstanding issue is the availability of Galactic stars in the right
luminosity range for TA relative to typical high redshift targets. We explore
this here using the stars and $z\sim8$ candidate galaxies identified in the
source extractor catalogs of Brightest of Reionizing Galaxies survey
(BoRG[z8]), a pure-parallel program with Hubble Space Telescope Wide-Field
Camera 3.
  We find that (a) a single WFC3 field contains enough Galactic stars to
satisfy the NIRspec astrometry requirement (20 milli-arcseconds), provided its
and the NIRspec TA's are $m_{lim}>24.5$ AB in WFC3 F125W, (b) a single WFC3
image can therefore serve as the pre-image if need be, (c) a WFC3 mosaic and
accompanying TA image satisfy the astrometry requirement at $\sim23$ AB mag in
WFC3 F125W, (d) no specific Galactic latitude requires deeper TA imaging due to
a lack of Galactic stars, and (e) a depth of $\sim24$ AB mag in WFC3 F125W is
needed if a guide star in the same MSA quadrant as a target is required.
  We take the example of a BoRG identified $z\sim8$ candidate galaxy and
require a Galactic star within 20" of it. In this case, a depth of 25.5 AB in
F125W is required (with $\sim$97% confidence).
