A Revised Characterization of the WFPC2 CTE Loss

Charge transfer loss on the Wide Field Planetary Camera 2 (WFPC2) onboard the Hubble Space Telescope is a primary source of uncertainty in stellar photometry obtained with this camera. This effect, discovered shortly after the camera was installed, has grown over time and can dim stars by several tenths of a magnitude (or even more, in particularly bad cases). The impact of CTE loss on WFPC2 stellar photometry was characterized by several studies between 1998 and 2000, but has received diminished attention since ACS became HST's primary imager. After the failure of ACS in January 2007, WFPC2 once again became the primary imaging instrument on board HST, restoring the importance of ensuring accurate CTE corrections. This paper reexamines the CTE loss of WFPC2, with three significant changes over previous studies. First, the present study considers calibration data obtained through 2007, thus increasing the confidence in the reliability of the CTE corrections when applied to recent observations. Second, the change in CTE loss during readout is accounted for analytically. Finally, a reanalysis of the CTE dependencies on counts, background, and observation date was made. The resulting correction is significantly more accurate than that provided in the WFPC2 Instrument Handbook (Dolphin 2002 and updates through 2004), resulting in photometry that can be enhanced by over 5% in certain circumstances.