diffraction limit of secondary mirror?

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).

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.... :-)

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

In article ,
"Peter Webb" writes:
In a Newtonian design reflector (for example), it seems to me that the
secondary mirror is itself an aperture,

It isn't typically an aperture (or "stop") as the term is usually
used in optical design. The main effects of diffraction depend on
the aperture stop, not the primary. Usually the aperture stop is the
primary or near it, but in a Schmidt design, for example, the
aperture stop is the corrector. In other designs, the aperture stop
may be at other surfaces, or there may be multiple stops.

In most telescopes, the secondary is oversized, and no ray that makes
it through the aperture stop is vignetted by the secondary. In these
circumstances, diffraction at the secondary has very little effect.
One way of looking at it is that the wave amplitude is negligibly
small outside the radius of the secondary, and therefore whether a
mirror is present or not has little effect. In unusual systems with
multiple aperture stops, each one has its own diffraction effects.
These are very complicated to calculate. Basically, one has to do a
full wave-optics calculation through the whole system. Breault
Research is one company that makes a living doing just that (no doubt
among other types of calculations they do).

Even in common telescopes, there are second-order wave optics effects
because of the finite secondary. For standard imaging telescopes,
though, they are usually negligible. They may be non-negligible when
stray light is a concern or if the exact form of the point spread
function is important. I don't think there is any simple description
of these effects, and as noted above, calculating them is
non-trivial.

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