A New Method for Wide-Field Near-IR Imaging with the Hubble Space
  Telescope
  We present a new technique for wide and shallow observations using the
near-infrared channel of Wide Field Camera 3 (WFC3) on the Hubble Space
Telescope (HST). Wide-field near-IR surveys with HST are generally inefficient,
as guide star acquisitions make it impractical to observe more than one
pointing per orbit. This limitation can be circumvented by guiding with gyros
alone, which is possible as long as the telescope has three functional gyros.
The method presented here allows us to observe mosaics of eight independent
WFC3-IR pointings in a single orbit by utilizing the fact that HST drifts by
only a very small amount in the 25 seconds between non-destructive reads of
unguided exposures. By shifting the reads and treating them as independent
exposures the full resolution of WFC3 can be restored. We use this "drift and
shift" (DASH) method in the Cycle 23 COSMOS-DASH program, which will obtain 456
WFC3 $H_{160}$ pointings in 57 orbits, covering an area of 0.6 degree$^2$ in
the COSMOS field down to $H_{160} = 25$. When completed, the program will more
than triple the area of extra-galactic survey fields covered by near-IR imaging
at HST resolution. We demonstrate the viability of the method with the first
four orbits (32 pointings) of this program. We show that the resolution of the
WFC3 camera is preserved, and that structural parameters of galaxies are
consistent with those measured in guided observations.
