Parallax of Galactic Cepheids from Spatially Scanning the Wide Field
  Camera 3 on the Hubble Space Telescope: The Case of SS Canis Majoris
  We present a high-precision measurement of the parallax for the 12-day
Cepheid SS Canis Majoris, obtained via spatial scanning with the Wide Field
Camera 3 (WFC3) on the Hubble Space Telescope (HST). Spatial scanning enables
astrometric measurements with a precision of 20-40 muas, an order of magnitude
better than pointed observations. SS CMa is the second Cepheid targeted for
parallax measurement with HST, and is the first of a sample of eighteen
long-period >~ 10 days) Cepheids selected in order to improve the calibration
of their period-luminosity relation and eventually permit a determination of
the Hubble constant H_0 to better than 2%. The parallax of SS CMa is found to
be 348 +/- 38 muas, corresponding to a distance of 2.9 +/- 0.3 kpc. We also
present a refinement of the static geometric distortion of WFC3 obtained using
spatial scanning observations of calibration fields, with a typical magnitude
<~0.01 pixels on scales of 100 pixels.
