> ...................Diffraction: light bending around an obstacle in
> its path will produce a pattern of light and dark lines, in our case the
> secondary mirror is the obstacle............


Actually the "obstacle" is the edge of the primary. Light that falls beyond
the
edge of the primary is "blocked" from reaching the image plane. The
secondary
mirror contributes to the basic diffraction pattern in a relatively minor
way, but a
secondary mirror isn't essential for making the diffraction pattern. A
refractor,
with no central obstruction, still makes a bulls-eye-like diffraction
pattern because
lenses too have edges that constitute an "obstacle" to image formation.
That's
why the diameter of the Airy disk (and, therefore, the resolving power of
the
telescope) depends mainly on the diameter of the objective, not the
secondary
(whenever the secondary is much smaller than the primary, as is generally
the case
with reflecting telescopes).

RP

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