Why do reflecting telescopes usually have a secondary mirror in addition to a primary mirror?
A) to increase the light-gathering power
B) to make the telescope shorter
C) to increase the focal length
D) to combat chromatic aberration

Question

Quizplus · Accepted Answer

The secondary mirror is added in reflecting telescopes to make the telescope shorter. This is because the light has to travel a longer distance in a refracting telescope due to the bending of light as it passes through lenses, but in reflecting telescopes, the mirrors reflect the light, reducing the length of the telescope.

Why Do Reflecting Telescopes Usually Have a Secondary Mirror in Addition