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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