  In the era of modern digital sky surveys, uncertainties in the flux of
stellar standards are commonly the dominant systematic error in photometric
calibration and can often affect the results of higher-level experiments. The
Hubble Space Telescope (HST) spectrophotometry, which is based on computed
model atmospheres for three hot (Teff > 30,000 K) pure-hydrogen (DA) white
dwarfs, is currently considered the most reliable and internally consistent
flux calibration. However many next generation facilities (e.g. Harmoni on
E-ELT, Euclid and JWST) will focus on IR observations, a regime in which white
dwarf calibration has not yet been robustly tested. Cool DA white dwarfs have
energy distributions that peak close to the optical or near-IR, do not have
shortcomings from UV metal line blanketing, and have a reasonably large sky
density ($\simeq$ 4 deg$^{-2}$ at G < 20), making them, potentially, excellent
calibrators. Here we present a pilot study based on STIS+WFC3 observations of
two bright DA white dwarfs to test whether targets cooler than current hot
primary standards (Teff < 20,000 K) are consistent with the HST flux scale. We
also test the robustness of white dwarf models in the IR regime from an
X-shooter analysis of Paschen lines and by cross-matching our previously
derived Gaia white dwarf catalogue with observations obtained with 2MASS,
UKIDSS, VHS, and WISE.
