The SPTpol Extended Cluster Survey
  We describe the observations and resultant galaxy cluster catalog from the
2770 deg$^2$ SPTpol Extended Cluster Survey (SPT-ECS). Clusters are identified
via the Sunyaev-Zel'dovich (SZ) effect, and confirmed with a combination of
archival and targeted follow-up data, making particular use of data from the
Dark Energy Survey (DES). With incomplete followup we have confirmed as
clusters 244 of 266 candidates at a detection significance $\xi \ge 5$ and an
additional 204 systems at $4<\xi<5$. The confirmed sample has a median mass of
$M_{500c} \sim {4.4 \times 10^{14} M_\odot h_{70}^{-1}}$, a median redshift of
$z=0.49$, and we have identified 44 strong gravitational lenses in the sample
thus far. Radio data are used to characterize contamination to the SZ signal;
the median contamination for confirmed clusters is predicted to be $\sim$1% of
the SZ signal at the $\xi>4$ threshold, and $<4\%$ of clusters have a predicted
contamination $>10\% $ of their measured SZ flux. We associate SZ-selected
clusters, from both SPT-ECS and the SPT-SZ survey, with clusters from the DES
redMaPPer sample, and find an offset distribution between the SZ center and
central galaxy in general agreement with previous work, though with a larger
fraction of clusters with significant offsets. Adopting a fixed Planck-like
cosmology, we measure the optical richness-to-SZ-mass ($\lambda-M$) relation
and find it to be 28% shallower than that from a weak-lensing analysis of the
DES data---a difference significant at the 4 $\sigma$ level---with the
relations intersecting at $\lambda=60$ . The SPT-ECS cluster sample will be
particularly useful for studying the evolution of massive clusters and, in
combination with DES lensing observations and the SPT-SZ cluster sample, will
be an important component of future cosmological analyses.
