I'm hoping that somebody can help me straighten my thinking.

In a Newtonian design reflector (for example), it seems to me that the
secondary mirror is itself an aperture, and should introduce its own
diffraction limiting effects. However, we can't detect them/they don't
matter because the image is essentially already magnified by that point. But
I'm not convinced.

Do secondary mirrors effectively form apertures, if so do they create their
own diffraction effects because of this? What effects can be detected?

(I'm not talking about the effects of spiders or obstructing the primary
mirror in any way).