Filters are a popular tool in the field of solar and planetary observation. They increase the contrast for certain details that are poorly or not at all to see without a filter. Basically, there are two problems with the observation: (1) over-exposure, the boundary between two regions of the observed object with different brightness "fraying" or simply blurred because the eye is overwhelmed with the contrast in bright light; and (2) adjacent areas have similar coloring, but only small differences in intensity. Both effects cause that the combination of eye-brain can no longer separate details and thus tries to show both objects as a single one.
- Solar Astronomy