Instrument Science Report 99-04:
Time Dependence of CTE from Cosmic Ray Trails
A. Riess, J. Biretta, and S. Casertano
December 23, 1999
We have developed a method to measure the counts in cosmic ray trails (CRTs), which result from imperfect charge transfer efficiency (CTE). Like previous measures of CTE, the counts in the CRTs are a strong function of the number of serial and parallel transfers, background, source counts, and lifetime of the instrument. Analysis of the CRTs reveals that some charge is trapped and released on short timescales (~seconds or less), although the charge in CRTs does not account for all observed CTE losses. The time dependence of Y-CTE and X-CTE are shown for WFPC2 and STIS using the STScI archive of dark calibration frames. All show a steady growth with time since the installation of the respective instruments. There is also evidence for a mild increase in the growth rate (i.e. acceleration), of the WFPC2 Y-CTE. The results indicate that this method may provide a precise way to monitor CTE with greater time sampling than is currently feasible and without the cost of additional pointed observations.