Skip to main content
U.S. flag

An official website of the United States government

Official websites use .gov
A .gov website belongs to an official government organization in the United States.

Secure .gov websites use HTTPS
A lock ( ) or https:// means you’ve safely connected to the .gov website. Share sensitive information only on official, secure websites.

All-Sky Faint DA White Dwarf Spectrophotometric Standards for Astrophysical Observatories: The Complete Sample



Tim Axelron, Tim Axelrod, Abhijit Saha, Thomas Matheson, Susana Deustua


The purely radiative pure hydrogen atmospheres of hot DA white dwarfs are the least complicated to model, and known to be stable against pulsation. They are fully characterized by two parameters: the effective temperature Te, and surface gravity logg, both of which can be deduced from their Balmer features and line profiles. In turn, the spectra can therefore be used to predict the spectral energy distribution (SED) of the emergent radiation. These objects can thus be self-characterized as spectrophotometric standards, and have been used as the basis of the spectrophotometric flux scale used for the so called CALSPEC system for calibrating data from the Hubble Space Telescope (HST). The flux scale of the CALSPEC system is based on 3 relatively bright DA white dwarfs (11.7 mag < V < 13.4 mag), as described with citations in the Introduction in §1 below. In this paper, we add 32 new fainter (16.5 mag < V < 19 mag) DA white dwarfs spread across the whole sky, that are faint enough to be in the high S/N dynamic range of modern day large telescopes. Using panchromatic photometry with HST/WFC3 from 275nm (F275W) through 1.6 microns (F160W), and ground based spectroscopy to measure their Balmer features, we show that the the observed SEDs of the full set of 35 DA white dwarfs are consistent with their model predictions using the Balmer line profiles (allowing a degree of freedom for interstellar reddening) to < 0.005 mag rms from 2700 Åto 7750 Å, and to 0.008 mag rms at 1.6μm. A new heirarchical modeling process allows determination of passband dependent HST/WFC3 zeropoints with only a single additional measurement that determines the absolute flux scale of our SEDs. This total set of 35 stars is intended for calibrating ground and space-based observatories with unprecedented accuracy, covering the ultraviolet to the near infrared: for instance DECAM, PANSTARRS1, SDSS, and GAIA DR3 and future surveys such as the Vera Rubin Observatory's Legacy Survey of Space and Time and the Nancy Grace Roman Space Telescope (RST).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 and Euclid as well.
Astrophysical Journal


Standards, Cosmology: Observations, Methods: Data Analysis, Stars: White Dwarfs, Surveys


Axelron, T. , Axelrod, T. , Saha, A. , Matheson, T. and Deustua, S. (2023), All-Sky Faint DA White Dwarf Spectrophotometric Standards for Astrophysical Observatories: The Complete Sample, Astrophysical Journal, [online],, (Accessed December 11, 2023)
Created July 1, 2023, Updated October 2, 2023