  Free of any atmospheric contamination, HST provides the best available
spectrophotometry from the far-UV to the near-IR for stars as faint as V~16.
The HST CALSPEC standard star network is based on 3 standard candles: the hot,
pure hydrogen white dwarf (WD) stars G191B2B, GD153, and GD71, which have
Hubeny NLTE model flux calculations that require the atomic physics for only
one atom. These model flux distributions are normalized to the absolute flux
for Vega of 3.46x10^{-9} erg cm^{-2} s^{-1} \AA^{-1} at 5556\AA using precise
Landolt V band photometry and the V bandpass function corrected for atmospheric
transmission by M. Cohen. The 3 primary WD standards provide absolute flux
calibrations for FOS, STIS, and NICMOS spectrophotometry from these instruments
on the HST. About 32 stellar spectral energy distributions (SEDs) have been
constructed with a primary pedigree from the STIS data, which extends from 1150
\AA for the hot stars to a long wavelength limit of 1\mu m. NICMOS grism
spectrophotometry provides an extension to 1.9\mu m in the IR for 17 of the HST
standards and longward to 2.5\mu m for a few of the brighter stars. Included
among these HST standards are Vega, the Sloan standard BD+17$^{\circ}$4708, 3
bright solar analog candidates, 3 cool stars of type M or later, and 5 hot WDs.
In addition, 4 K giants and 4 main sequence A-stars have NICMOS
spectrophotometry from 0.8-2.5\mu m. The WD fluxes are compared to their
modeled SEDs and demonstrate an internal precision of 1-2%, while the A-stars
agree with the Cohen IR fluxes to ~2%. Three solar analog candidate stars
differ from the solar spectrum by up to 10% in the region of heavy line
blanketing from 3000-4000 \AA and show differences in shape of ~5% in the IR
around 1.8\mu m.
