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This paper describes a set of 35 stars that can serve as precise light standards for modern telescopes. The authors add thirty‑two faint DA white dwarfs (V magnitude 16.5 to 19.8, most near V≈18) to the three brighter white dwarfs that already anchor the Hubble Space Telescope flux scale (CALSPEC). DA white dwarfs are compact stars with nearly pure hydrogen atmospheres. Hot ones (above about 15,000 K) are simple to model and are not variable in brightness, so they make good reference sources for measuring how bright other objects are in physical units.
The team compared detailed model predictions of each star’s spectral energy distribution (SED) — the amount of light emitted at each wavelength — to space‑based and ground observations. Key data came from the Wide Field Camera 3 (WFC3) on the Hubble Space Telescope using five ultraviolet/visible filters (F275W, F336W, F475W, F625W, F775W) and one near‑infrared filter (F160W) collected over several HST programs. For 19 of the stars they also used ultraviolet spectra from the Space Telescope Imaging Spectrograph (STIS). The models are “fully radiative” pure‑hydrogen atmospheres, and the authors fitted those models at the same time as the line‑of‑sight reddening (dust extinction) to the observed photometry and spectra.
The main result is that the model SEDs reproduce the observed broadband fluxes from roughly 275 nm in the near ultraviolet to 1600 nm in the near infrared to within a few parts per thousand. The residuals after fitting have a root‑mean‑square scatter typically about 0.4 percent (roughly 0.004 magnitudes) across the sample. Where STIS UV spectra are available, the comparison supports the models down to about 130 nm for that subset. The authors also note corrections that matter at the long wavelength end, for example a known count‑rate nonlinearity in the WFC3 IR detector that must be handled when using the F160W results.
Why this matters: many modern astronomical projects need photometry (measures of brightness and color) that is consistent across the whole sky and accurate to better than one percent. That is true for precision cosmology, surveys that measure millions of objects, and for combining data from many telescopes. The new network fills a “sweet spot” in brightness for large telescopes and space observatories, and it provides all‑sky coverage that was previously lacking at sub‑percent precision. The authors have also produced synthetic magnitudes for common survey passbands based on these SEDs for practical use.
Important caveats are stated clearly. Full confidence in the entire set will require consistent ultraviolet and infrared data for all objects, not just subsets, so work continues to extend and validate the wavelength coverage. The results rely on the current generation of white dwarf atmosphere models and on dust‑reddening models; future improvements in those models could change the detailed calibration, although the authors report the analysis is robust to changes tested so far. Users must also account for known instrumental effects (for example the WFC3 IR count‑rate nonlinearity) when applying these standards.