Spectral Classification and Redshift Measurement for the SDSS-III Baryon
  Oscillation Spectroscopic Survey
  (abridged) We describe the automated spectral classification, redshift
determination, and parameter measurement pipeline in use for the Baryon
Oscillation Spectroscopic Survey (BOSS) of the Sloan Digital Sky Survey III
(SDSS-III) as of Data Release 9, encompassing 831,000 moderate-resolution
optical spectra. We give a review of the algorithms employed, and describe the
changes to the pipeline that have been implemented for BOSS relative to
previous SDSS-I/II versions, including new sets of stellar, galaxy, and quasar
redshift templates. For the color-selected CMASS sample of massive galaxies at
redshift 0.4 <~ z <~ 0.8 targeted by BOSS for the purposes of large-scale
cosmological measurements, the pipeline achieves an automated classification
success rate of 98.7% and confirms 95.4% of unique CMASS targets as galaxies
(with the balance being mostly M stars). Based on visual inspections of a
subset of BOSS galaxies, we find that ~0.2% of confidently reported CMASS
sample classifications and redshifts are incorrect, and ~0.4% of all CMASS
spectra are objects unclassified by the current algorithm which are potentially
recoverable. The BOSS pipeline confirms that ~51.5% of the quasar targets have
quasar spectra, with the balance mainly consisting of stars. Statistical (as
opposed to systematic) redshift errors propagated from photon noise are
typically a few tens of km/s for both galaxies and quasars, with a significant
tail to a few hundreds of km/s for quasars. We test the accuracy of these
statistical redshift error estimates using repeat observations, finding them
underestimated by a factor of 1.19 to 1.34 for galaxies, and by a factor of 2
for quasars. We assess the impact of sky-subtraction quality, S/N, and other
factors on galaxy redshift success. Finally, we document known issues, and
describe directions of ongoing development.
