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We report on the methodology and first results from the Deep Lens Surveytransient search. We utilize image subtraction on survey data to yield allsources of optical variability down to 24th magnitude. Images are analyzedimmediately after acquisition, at the telescope and in near-real time, to allowfor followup in the case of time-critical events. All classes of transients areposted to the web upon detection. Our observing strategy allows sensitivity tovariability over several decades in timescale. The DLS is the first survey toclassify and report all types of photometric and astrometric variabilitydetected, including solar system objects, variable stars, supernovae, and shorttimescale phenomena. Three unusual optical transient events were detected,flaring on thousand-second timescales. All three events were seen in the Bpassband, suggesting blue color indices for the phenomena. One event (OT20020115) is determined to be from a flaring Galactic dwarf star of spectraltype dM4. From the remaining two events, we find an overall rate of \eta = 1.4events deg-2 day-1 on thousand-second timescales, with a 95% confidence limitof \eta < 4.3. One of these events (OT 20010326) originated from a compactprecursor in the field of galaxy cluster Abell 1836, and its nature isuncertain. For the second (OT 20030305) we find strong evidence for an extendedextragalactic host. A dearth of such events in the R passband yields an upper95% confidence limit on short timescale astronomical variability between 19.5 <R < 23.4 of \eta_R < 5.2. We report also on our ensemble of astrometricallyvariable objects, as well as an example of photometric variability with anundetected precursor.

The Deep Lens Survey Transient Search. I. Short Timescale and Astrometric Variability