Ground-Based Submillimagnitude CCD Photometry of Bright Stars Using Snapshot Observations

We demonstrate ground-based submillimagnitude (< 10(-3)) photometry of widely separated bright stars using snapshot CCD imaging. We routinely achieved this photometric precision by (1) choosing nearby comparison stars of a similar magnitude and spectral type, (2) defocusing the telescope to allow high signal (> 10(7) e(-)) to be acquired in a single integration, (3) pointing the telescope so that all stellar images fall on the same detector pixels, and (4) using a region of the CCD detector that is free of nonlinear or aberrant pixels. We describe semiautomated observations with the Supernova Integrated Field Spectrograph (SNIFS) on the University of Hawaii 2.2 m telescope on Mauna Kea, with which we achieved photometric precision as good as 5.2 x 10(-4) (0.56 mmag) with a 5 minute cadence over a 2 hr interval. In one experiment, we monitored eight stars, each separated by several degrees, and achieved submillimagnitude precision with a cadence (per star) of similar to 17 minutes. Our snapshot technique is suitable for automated searches for planetary transits among multiple bright stars.