Precision Spectrophotometry at the Level of 0.1%
  Accurate relative spectrophotometry is critical for many science
applications. Small wavelength scale residuals in the flux calibration can
significantly impact the measurements of weak emission and absorption features
in the spectra. Using Sloan Digital Sky Survey data, we demonstrate that the
average spectra of carefully selected red-sequence galaxies can be used as a
spectroscopic standard to improve the relative spectrophotometry precision to
0.1% on small wavelength scales (from a few to hundreds of Angstroms). We
achieve this precision by comparing stacked spectra across tiny redshift
intervals. The redshift intervals must be small enough that any systematic
stellar population evolution is minimized and less than the spectrophotometric
uncertainty. This purely empirical technique does not require any theoretical
knowledge of true galaxy spectra. It can be applied to all large spectroscopic
galaxy redshift surveys that sample a large number of galaxies in a uniform
population.
