Cometary science benefits from wide-field time-domain optical surveys. Aside from the discovery of new comets, such surveys can provide a better description of known objects. For example, we can quantify intrinsic brightness variation with heliocentric distance and true anomaly (i.e., season); potentially estimate dust-to-gas ratio and its variation with time (when relevant filters are used); and identify cometary outbursts or other interesting phenomenon for follow-up. We describe ZChecker, automated software for finding and visualizing known comets in Zwicky Transient Facility (ZTF) survey data. ZChecker uses on-line ephemeris generation and individual data product meta-data (observation time, image corners, and the world coordinate system) to efficiently identify and download images of targets of interest in the ZTF archive. Photometry of each target is measured, and the images rotated to place the comet-Sun vector at a constant position angle. To help identify comets for follow-up investigations, ZChecker then scales each image of each target to a common heliocentric and geocentric distance, then combines the data into nightly and bi-weekly averages. The difference between the two shows variations in brightness and morphology that potentially indicate, e.g., an outburst of activity or the motion of a precessing jet. Example ZChecker output is presented, including outbursts of 29P/Schwassmann-Wachmann 1 and 64P/Swift-Gehrels, and a curved dust feature at comet C/2017 M4 (ATLAS). Based on observations obtained with the Samuel Oschin Telescope 48-inch and the 60-inch Telescope at the Palomar Observatory as part of the Zwicky Transient Facility project. Major funding has been provided by the U.S. National Science Foundation under Grant No. AST-1440341 and by the ZTF partner institutions. This work is also supported by the NASA Planetary Data System Cooperative Agreement with the University of Maryland.
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