On 08/06/2012 13:53, 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.

There is no image except at the focal plane. The light cone is
converging and provided that the secondary mirror is slightly larger
than the geometrical ray trace requirements to illuminate the final
image plane then there is no hint of it affecting the image.
(beyond that of blocking the centre of the main aperture)

If the secondary is under sized you get vignetting off axis when it
fails to capture the entire light cone.

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

None if the mirror is properly designed and fitted.

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

Undersized you would see vignetting at edge of field and out of focus
stars away from the optic axis would show a partial obscuration of the
aperture which gets worse as you move away from dead centre.

--
Regards,
Martin Brown