On Fri, 8 Jun 2012 22:53:45 +1000, "Peter Webb"
wrote:
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).

....and then we also have the even smaller lenses in the eyepieces to
worry about.... :-)