Q:

How can diffraction occur in a reflecting telescope?

It's understandable how diffraction occurs in refracting telescopes, because the lens itself is a small aperture for light to pass through.
But what about reflecting telescopes? Since light is reflected, and a mirror is not exactly an 'aperture'? How can light diffract in a reflecting telescope?

A:

Any obstruction in the optical path from the target to the telescope's focus will cause diffraction effects. In the case of reflecting telescopes, almost all designs require supports for the secondary mirror and these supports give rise to diffraction effects. See this Wikipedia article for more details.

