Spectroscopic Needs for Calibration of LSST Photometric Redshifts
  This white paper summarizes the conclusions of the Snowmass White Paper
"Spectroscopic Needs for Imaging Dark Energy Experiments" (arXiv:1309.5384)
which are relevant to the calibration of LSST photometric redshifts; i.e., the
accurate characterization of biases and uncertainties in photo-z's. Any
significant miscalibration will lead to systematic errors in photo-z's,
impacting nearly all extragalactic science with LSST. As existing deep redshift
samples have failed to yield highly-secure redshifts for a systematic 20%-60%
of their targets, it is a strong possibility that future deep spectroscopic
samples will not solve the calibration problem on their own. The best options
in this scenario are provided by cross-correlation methods that utilize
clustering with objects from spectroscopic surveys (which need not be fully
representative) to trace the redshift distribution of the full sample. For
spectroscopy, the eBOSS survey would enable a basic calibration of LSST
photometric redshifts, while the expected LSST-DESI overlap would be more than
sufficient for an accurate calibration at z>0.2. A DESI survey of nearby
galaxies conducted in bright time would enable accurate calibration down to
z~0. The expanded areal coverage provided by the transfer of the DESI
instrument (or duplication of it at the Blanco Telescope) would enable the best
possible calibration from cross-correlations, in addition to other science
gains.
