  We use a newly developed observing mode on the Hubble Space Telescope (HST)
and Wide Field Camera 3 (WFC3), spatial scanning, to increase source sampling a
thousand-fold and measure changes in source positions to a precision of 20--40
microarcseconds, more than an order of magnitude better than attainable in
pointed observations. This observing mode can usefully measure the parallaxes
of bright stars at distances of up to 5 kpc, a factor of ten farther than
achieved thus far with HST. Long-period classical Cepheid variable stars in the
Milky Way, nearly all of which reside beyond 1 kpc, are especially compelling
targets for parallax measurements from scanning, as they may be used to anchor
a determination of the Hubble constant to ~1%. We illustrate the method by
measuring to high precision the parallax of a classical Cepheid, SY Aurigae, at
a distance of more than 2 kpc, using 5 epochs of spatial-scan data obtained at
intervals of 6 months. Rapid spatial scans also enable photometric measurements
of bright Milky Way Cepheids---which would otherwise saturate even in the
shortest possible pointed observations---on the same flux scale as
extragalactic Cepheids, which is a necessity for reducing a leading source of
systematic error in the Hubble constant. We demonstrate this capability with
photometric measurements of SY Aur on the same system used for Cepheids in Type
Ia supernova host galaxies. While the technique and results presented here are
preliminary, an ongoing program with HST is collecting such parallax
measurements for another 18 Cepheids to produce a better anchor for the
distance scale.
