The Composite Spectrum of BOSS Quasars Selected for Studies of the
  Lyman-alpha Forest
  The Baryon Oscillation Spectroscopic Survey (BOSS) has collected more than
150,000 $2.1 \leq z \leq 3.5$ quasar spectra since 2009. Using this
unprecedented sample, we create a composite spectrum in the rest-frame of
102,150 quasar spectra from 800 \AA\ to 3300 \AA\ at a signal-to-noise ratio
close to 1000 per pixel ($\Delta v$ of 69 km~s$^{-1}$). Included in this
analysis is a correction to account for flux calibration residuals in the BOSS
spectrophotometry. We determine the spectral index as a function of redshift of
the full sample, warp the composite spectrum to match the median spectral
index, and compare the resulting spectrum to SDSS photometry used in target
selection. The quasar composite matches the color of the quasar population to
within 0.02 magnitudes in $g-r$, 0.03 magnitudes in $r-i$, and 0.01 magnitudes
in $i-z$ over the redshift range $2.2<z<2.6$. The composite spectrum deviates
from the imaging photometry by 0.05 magnitudes around $z = 2.7$, likely due to
differences in target selection as the quasar colors become similar to the
stellar locus at this redshift. Finally, we characterize the line features in
the high signal-to-noise composite and identify nine faint lines not found in
the previous composite spectrum from SDSS.
