rays, highly energetic particles that are abundant in
the area where HST orbits. These particles hitting the camera
detectors causes them to record a bright spot. However,
they fall randomly at different places at different times.
By comparing different exposures of the same target, the
best parts of each image can be used and the cosmic
rays can be ignored.
multiple exposures also increases the sensitivity to very
faint targets similarly to a single longer exposure, but
helps counteract sources of noise.