4.2.4. Cosmic Ray Detection
If low-energy cosmic ray events were left unidentified in WFPC2 data, we would unintentionally include the signal from those events in the inferred background level. Figure 6 shows the total number of cosmic ray events as a function of total flux, peak flux, and affected pixels per event. In each histogram, the number of cosmic ray events falls off sharply well before cosmic rays would become difficult to detect: both peak and total fluxes for cosmic ray events are significantly higher than the mean level of the background ( 10 DN), and the total number of pixels affected by a single cosmic ray event rarely drops below three. We therefore conclude that the cosmic ray removal process detects virtually all cosmic rays which hit the chips.
Figure 6. Histogram of cosmic ray events as a function of the number of affected pixels (top panel), total flux (middle panel), and peak flux (bottom panel). The number of events falls off in each plot before an event would become difficult to detect in these data.