  Large surveys of galaxy clusters with the Hubble and Spitzer Space
Telescopes, including CLASH and the Frontier Fields, have demonstrated the
power of strong gravitational lensing to efficiently deliver large samples of
high-redshift galaxies. We extend this strategy through a wider, shallower
survey named RELICS, the Reionization Lensing Cluster Survey. This survey,
described here, was designed primarily to deliver the best and brightest
high-redshift candidates from the first billion years after the Big Bang.
RELICS observed 41 massive galaxy clusters with Hubble and Spitzer at 0.4-1.7um
and 3.0-5.0um, respectively. We selected 21 clusters based on Planck PSZ2 mass
estimates and the other 20 based on observed or inferred lensing strength. Our
188-orbit Hubble Treasury Program obtained the first high-resolution
near-infrared images of these clusters to efficiently search for lensed
high-redshift galaxies. We observed 46 WFC3/IR pointings (~200 arcmin^2) with
two orbits divided among four filters (F105W, F125W, F140W, and F160W) and ACS
imaging as needed to achieve single-orbit depth in each of three filters
(F435W, F606W, and F814W). As previously reported by Salmon et al., we
discovered 322 z ~ 6 - 10 candidates, including the brightest known at z ~ 6,
and the most distant spatially-resolved lensed arc known at z ~ 10. Spitzer
IRAC imaging (945 hours awarded, plus 100 archival) has crucially enabled us to
distinguish z ~ 10 candidates from z ~ 2 interlopers. For each cluster, two HST
observing epochs were staggered by about a month, enabling us to discover 11
supernovae, including 3 lensed supernovae, which we followed up with 20 orbits
from our program. We delivered reduced HST images and catalogs of all clusters
to the public via MAST and reduced Spitzer images via IRSA. We have also begun
delivering lens models of all clusters, to be completed before the JWST GO call
for proposals.
