  We calibrate spectrophotometric optical spectra of 32 stars commonly used as
standard stars, referenced to 14 stars already on the HST-based CALSPEC flux
system. Observations of CALSPEC and non-CALSPEC stars were obtained with the
SuperNova Integral Field Spectrograph over the wavelength range 3300 A to 9400
A as calibration for the Nearby Supernova Factory cosmology experiment. In
total, this analysis used 4289 standard-star spectra taken on photometric
nights. As a modern cosmology analysis, all pre-submission methodological
decisions were made with the flux scale and external comparison results
blinded. The large number of spectra per star allows us to treat the
wavelength-by-wavelength calibration for all nights simultaneously with a
Bayesian hierarchical model, thereby enabling a consistent treatment of the
Type Ia supernova cosmology analysis and the calibration on which it critically
relies. We determine the typical per-observation repeatability (median 14 mmag
for exposures >~ 5 s), the Maunakea atmospheric transmission distribution
(median dispersion of 7 mmag with uncertainty 1 mmag), and the scatter internal
to our CALSPEC reference stars (median of 8 mmag). We also check our standards
against literature filter photometry, finding generally good agreement over the
full 12-magnitude range. Overall, the mean of our system is calibrated to the
mean of CALSPEC at the level of ~ 3 mmag. With our large number of
observations, careful crosschecks, and 14 reference stars, our results are the
best calibration yet achieved with an integral-field spectrograph, and among
the best calibrated surveys.
