  Hubble Space Telescope (HST) fine guidance sensor observations were used to
obtain parallaxes of eight metal-poor ([Fe/H] < -1.4) stars. The parallaxes of
these stars determined by the revised Hipparcos reduction average 17% accuracy,
in contrast to our new HST parallaxes which average 1% accuracy and have errors
on the individual parallaxes ranging from 85 to 144 microarcsecond. This
parallax data has been combined with HST ACS photometry in the F606W and F814W
filters to obtain the absolute magnitudes of the stars with an accuracy of 0.02
to 0.03 magnitudes. Six of these stars are on the main sequence (with -2.7 <
[Fe/H] < -1.8), and suitable for testing metal-poor stellar evolution models
and determining the distances to metal-poor globular clusters. Using the
abundances obtained by O'Malley et al. (2017) we find that standard stellar
models using the Vandenberg & Clem (2003) color transformation do a reasonable
job of matching five of the main sequence stars, with HD 54639 ([Fe/H] = -2.5)
being anomalous in its location in the color-magnitude diagram. Stellar models
and isochrones were generated using a Monte Carlo analysis to take into account
uncertainties in the models. Isochrones which fit the parallax stars were used
to determine the distances and ages of nine globular clusters (with -2.4 <=
[Fe/H] <= -1.9$). Averaging together the age of all nine clusters, leads to an
absolute age of the oldest, most metal-poor globular clusters of 12.7+/- 1.0
Gyr, where the quoted uncertainty takes into account the known uncertainties in
the stellar models and isochrones, along with the uncertainty in the distance
and reddening of the clusters.
