Techniques and Review of Absolute Flux Calibration from the Ultraviolet to the Mid-Infrared

Abstract

The measurement of precise absolute fluxes for stellar sources has been pursued with increased vigor since the discovery of the dark energy and the realization that its detailed understanding requires accurate spectral energy distributions (SEDs) of redshifted Ia supernovae in the rest frame. The flux distributions of spectrophotometric standard stars were initially derived from the comparison of stars to laboratory sources of known flux but are now mostly based on calculated model atmospheres. For example, pure hydrogen white dwarf (WD) models provide the basis for the HST CALSPEC archive of flux standards. The basic equations for quantitative spectrophotometry and photometry are explained in detail. Several historical lab based flux calibrations are reviewed; and the SEDs of stars in the major on-line astronomical databases are compared to the CALSPEC reference standard spectrophotometry. There is good evidence that relative fluxes from the visible to the near-IR wavelength of ∼2.5 µm are currently accurate to 1% for the primary reference standards; and new comparisons with lab flux standards show promise for improving that precision.

Spectroradiometry with space telescopes

“…Figure 9 illustrates the change with time in responsivity that the HST instrument STIS has experienced in orbit: an initial improvement of efficiency and later decay of efficiencies, as well as the occurrence of a data gap. 22 The description by Bohlin et al (2014) of the HST calibration begins with an in-depth discussion of the comparison of stars to "Laboratory Flux Standards", i.e. "laboratory-pedigreed" stellar irradiances.…”

“…Rather than describing the extended efforts that went into modelling stellar atmospheres, particularly those of white dwarfs with hydrogen atmospheresexploiting non-LTE effects, determining effective temperatures of stellar atmospheres from line-profile analysis, and deriving stellar gravity via stellar-diameter measurements-we refer to Bohlin et al (2014).…”

“…3, all archived data were recalibrated by use of the pure hydrogen WD G191-B2B. 48 The IUE flux reference deviates by 6 % from the current HST CALSPEC reference (see Bohlin et al 2014, and references therein).…”

“…3. An overview of the standards used in the HST CALSPEC procedures is given by Bohlin et al (2014). Electronic versions of the UV spectra of all HST calibration standards as well as overviews over the instruments and their documentations and the proceedings from the calibration workshops can be found on the STScI Web pages.…”

“…The need for precise spectroradiometric calibration has been demonstrated once more by the fact that the uncertainties in the spectral irradiance (often called "flux distribution" in the references) of stellar standards are the dominant systematic error in measuring relative irradiances (often called "relative fluxes" in the literature) of redshifted supernovae Ia and, thus, in determining the nature of the dark energy that is driving the observed accelerating cosmic expansion (Sullivan et al 2011;Bohlin et al 2014).…”