  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 onboard HST, restoring the importance of ensuring
accurate CTE corrections.
  This paper re-examines 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.

Instrumentation and Methods for Astrophysics