THE PHOTOMETRIC PERFORMANCE AND CALIBRATION OF WFPC2

We discuss the photometric performance and calibration of the Wide Fie ld Planetary Camera 2 (WFPC2) on the Hubble Space Telescope (HST). The stability and accuracy of WFPC2 photometric measurements is discussed , with particular attention given to charge-transfer efficiency (CTE) effects, contamination effects in the ultraviolet (UV), and flat-field accuracy and normalization. Observational data are presented from bot h WFPC2 observations and ground observations using a system similar to that flown. WFPC2 photometric systems are defined both for the ground and flight systems. Transformations between these systems and the Lan dolt UBVRI system are presented. These transformations are sensitive t o details in the spectra being transformed, and these sensitivities ar e quantified and discussed. On-orbit observations are used to revise t he prelaunch estimates of response curves to best match synthetic phot ometry results with observations, and the accuracy of the resulting sy nthetic photometry is discussed. Synthetic photometry is used to deter mine zero points and transformations for all of the flight filters, an d also to derive interstellar extinction values for the WFPC2 system, Using stellar interior and atmosphere models, isochrones in the WFPC2 system are calculated and compared with several observations.