The HST See Change Program: I. Survey Design, Pipeline, and Supernova Discoveries – on March 24, 2021 at 7:56 am

The See Change survey was designed to make $z>1$ cosmological measurements by
efficiently discovering high-redshift Type Ia supernovae (SNe Ia) and improving
cluster mass measurements through weak lensing. This survey observed twelve
galaxy clusters with the Hubble Space Telescope spanning the redshift range
$z=1.13$ to $1.75$, discovering 57 likely transients and 27 likely SNe Ia at
$zsim 0.8-2.3$. As in similar previous surveys (Dawson et al. 2009), this
proved to be a highly efficient use of HST for SN observations; the See Change
survey additionally tested the feasibility of maintaining, or further
increasing, the efficiency at yet higher redshifts, where we have less detailed
information on the expected cluster masses and star-formation rates. We find
that the resulting number of SNe Ia per orbit is a factor of $sim 8$ higher
than for a field search, and 45% of our orbits contained an active SN Ia within
22 rest-frame days of peak, with one of the clusters by itself yielding 6 of
the SNe Ia. We present the survey design, pipeline, and SN discoveries. Novel
features include fully blinded SN searches, the first random forest candidate
classifier for undersampled IR data (with a 50% detection threshold within 0.05
magnitudes of human searchers), real-time forward-modeling photometry of
candidates, and semi-automated photometric classifications and follow-up
forecasts. We also describe the spectroscopic follow-up, instrumental in
measuring host-galaxy redshifts. The cosmology analysis of our sample will be
presented in a companion paper.
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