Contrast is the difference in brightness between the bright objects in your field of view and the background (when compared to the background). Good contrast is needed for seeing fainter objects and discerning subtle visual details.
Maximum image contrast is desired for viewing low-contrast objects such as the Moon and planets. Newtonian and catadioptric telescopes have secondary (or diagonal) mirrors that obstruct a small percentage of light from the primary mirror. This degradation is only significant if more than 25% obstruction is present.
To calculate the secondary obstruction, use the formula πR2 to calculate the area of the secondary and primary mirrors, then divide to find the percentage of obstruction. For example, an 8 in telescope with a 2.16 in secondary obstruction has a 7.3% secondary obstruction:
Primary radius is 4”: πR2 = 50.27
Secondary radius is 1.08”: πR2 = 3.66
Percentage = 3.66/50.27 or 7.3%
For a given object, a telescope’s design (including central obstruction), coatings, optical quality, cleanliness, and collimation affect contrast. Important external factors can also affect contrast, including seeing (air turbulence) and air quality.