3.7. Accuracy of Tertiary Standards
Hamuy et al. (1992) quote the internal precision of their tertiary spectrophotometric system to be better than 0.01mag at all wavelengths, a claim which is well corroborated by the small statistical errors we find in our own spectrophotometric flux calibration (see below). An additional concern for our purposes, however, is the agreement between the H92 system and the primary calibration for Vega in the spectral range of our observations (approximately the Cousins B-band, 4200-5100Å).
The tertiary system of H92 is calibrated based on the equatorial secondary standards of Taylor (1984), which is in turn calibrated to the primary calibration of Vega by Hayes & Latham (1975). As described in H92, they recalibrate the Taylor (1984) secondary standards to the now widely-accepted Hayes (1985, hereafter H85) calibration of Vega. Details of that process are documented in H92. H92 estimate that their internal consistency in converting Taylor (1984) to the Hayes (1985) Vega system is 0.009 mag in the wavelength range of the Cousins B. They also compare synthetic photometry of the adjusted Vega spectrum with Johnson & Harris (1954) photometry of Vega and find an offset of -0.016 ( ± 0.009) mag at B (in the sense of Taylor minus Johnson). This offset seems to be intrinsic to the Vega calibration, as Hayes also finds that offset between his observations of Vega and the original Johnson observations. We therefore conclude that the statistical accuracy of the tertiary system is roughly 1% and the systematic uncertainty is roughly 1.5%.