Time-resolved CCD photometry of an ensemble of stars

Abstract

A technique for obtaining CCD photometry of a large ensemble of stars, with precision limited by atmospheric scintillation and photon statistics is presented. Under bright-sky conditions with a 0.9-m telescope, a precision of about 0.0015 mag relative to an ensemble average standard was obtained for 12th-13th mag stars in M 67 with exposure times of 1 min. The increase in noise level due to variable cirrus clouds is minimal. Effective noise levels for the detection of coherent oscillations with periods of 5-20 min could thus be reduced to about 30 micromag over ten nights of observing for stars of this brightness. A much larger ensemble of faint stars could be followed at lower precision with sky background and photon statistics as dominant error sources. Four probable-cluster DA white dwarfs were detected in the old galactic cluster M 67, using B, V calibration photometry.