The extragalactic distance scale key project. V. Photometry of the brightest stars in m100 and the calibration of WFPC2

Abstract

We present multiepoch VI observations of the brightest stars in the Virgo spiral galaxy M100 obtained with the WFPC2 aboard the Hitbble Space Telescope. Particular attention has been given to the WFPC2 calibration process, and a new calibration is presented here. We find that a calibration based on short exposure (t ≤ 60 sec) observations of the galactic globular clusters ω Cen, Pal 4, and NGC 2419 yields results that are in excellent agreement with the calibration of Holtzmann et al. However, a calibration based on longer exposures (t ≥ 1300 s) yields magnitudes that are systematically brighter by 0.05 mag, on average, in both V and I. We have independently employed both the DAOPHOT/ALLFRAM..