Cosmic 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.

Using multiple exposures also increases the sensitivity to very faint targets similarly to a single longer exposure, but helps counteract sources of noise.