Constraints on Photometric Calibration from Observations of   High-Redshift Type Ia Supernovae
  The good match of the type Ia supernova (SNIa) Hubble Diagram to the prediction of a not-unreasonable cosmological world model shows that measurements of standard stars and their comparison with point sources down to m=25 mag is good to better than +/-0.5 mag over an 11 mag range. It also shows that the true spectral energy distribution (SED) shapes of standard stars are known to better than +/-0.5 mag over an octave in wavelength. On the other hand, the SNIa argument for an accelerating Universe assumes that the magnitude system is good to much better than ~0.1 mag over the 11 mag range, and that SED shapes are known to much better than ~10 percent over an octave in wavelength. There is no independent empirical evidence for these plausible assumptions. 
David W. Hogg (IAS)