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Hot DA white dwarfs have fully radiative pure hydrogen atmospheres that are
the least complicated to model. Pulsationally stable, they are fully
characterized by their effective temperature Teff, and surface gravity log g,
which can be deduced from their optical spectra and used in model atmospheres
to predict their spectral energy distribution (SED). Based on this, three
bright DAWDs have defined the spectrophotometric flux scale of the CALSPEC
system of HST. In this paper we add 32 new fainter (16.5 < V < 19.5) DAWDs
spread over the whole sky and within the dynamic range of large telescopes.
Using ground based spectra and panchromatic photometry with HST/WFC3, a new
hierarchical analysis process demonstrates consistency between model and
observed fluxes above the terrestrial atmosphere to < 0.004 mag rms from 2700
{\AA} to 7750 {\AA} and to 0.008 mag rms at 1.6{\mu}m for the total set of 35
DAWDs. These DAWDs are thus established as spectrophotometric standards with
unprecedented accuracy from the near ultraviolet to the near-infrared, suitable
for both ground and space based observatories. They are embedded in existing
surveys like SDSS, PanSTARRS and GAIA, and will be naturally included in the
LSST survey by Rubin Observatory. With additional data and analysis to extend
the validity of their SEDs further into the IR, these spectrophotometric
standard stars could be used for JWST, as well as for the Roman and Euclid
observatories.
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