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 ( T _ { \mathrm { eff } } > 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 ( T _ { \mathrm { eff } } < 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 .