done with Keck. The structure of this paper is as follows. In §2 we give an overview of C3R2 survey strategy. In §3 we discuss the observations and data reduction for the first five nights of observing. In §4 we describe redshift determination and the identification of serendipitous sources. In §5 we present initial results from the survey. In §6 we conclude with a discussion. High confidence redshifts from DR1 are provided in a machine readable table.

C3R2 Survey Overview
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The Keck portion of the C3R2 survey is a joint effort between Caltech (PI: J. Cohen), NASA (PI: D. Stern), the University of California (PI: B. Mobasher), the University of Hawaii (PI: D. Sanders). European participation in C3R2 with the GTC (PI: C. Gutierrez) and VLT (PI: F. Castander), as well as Harvard participation with MMT (PI: D. Eisenstein), will commence in 2017. The first five nights of observing with Keck were allocated by Caltech in the 2016A semester. Here we provide a brief overview of the C3R2 strategy for these observations.

The Self-Organized Map of Galaxy Colors
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In M15 we used COSMOS [@Capak07; @Scoville07; @Laigle16] *ugrizYJH* photometry of $\sim$130k galaxies, closely resembling what will be obtained by 