  A first experimental test of tracking relativistic charged particles by
`drifting' Cherenkov photons in a water-based optical time-projection chamber
(OTPC) has been performed at the Fermilab Test Beam Facility. The prototype
OTPC detector consists of a 77~cm long, 28~cm diameter, 40~kg cylindrical water
mass instrumented with a combination of commercial $5.1\times5.1$~cm$^2$
micro-channel plate photo-multipliers (MCP-PMT) and $6.7\times6.7$~cm$^2$
mirrors. Five MCP-PMTs are installed in two columns along the OTPC cylinder in
a small-angle stereo configuration. A mirror is mounted opposite each MCP-PMT
on the far side of the detector cylinder, effectively doubling the
photo-detection efficiency and providing a time-resolved image of the Cherenkov
light on the opposing wall. Each MCP-PMT is coupled to an anode readout
consisting of thirty 50 Ohm microstrips. A 180-channel data acquisition system
digitizes the MCP-PMT signals on one end of the microstrips using the PSEC4
waveform sampling-and-digitizing chip operating at a sampling rate of
10.24~Gigasamples-per-second. The single-ended microstrip readout determines
the time and position of a photon arrival at the face of the MCP-PMT by
recording both the direct signal and the pulse reflected from the unterminated
far end of the strip. The detector was installed on the Fermilab MCenter
secondary beam-line behind a steel absorber where the primary flux is multi-GeV
muons. Approximately 80 Cherenkov photons are detected for a through-going muon
track in a total event duration of ~2 ns. By measuring the time-of-arrival and
the position of individual photons at the surface of the detector to
$\le$100~ps and a few mm, respectively, we have measured a spatial resolution
of $\sim$ 15~mm for each MCP-PMT track segment, and, from linear fits over the
entire track length of $\sim40$~cm, an angular resolution on the track
direction of $\sim60$~mrad.
