Dennis Persyk posted:

An obstructed scope may offer an advantage in splitting close doubles,
especially those of nearly equal magnitudes. The central obstruction
redistributes the power from the central dot of the Airy disc to the
first ring with respect to the unobstructed scope. This can give the
obstructed scope an advantage when viewing point sources. Of course
there is a corresponding disadvantage when observing extended targets.

The Physics of the obstructed scope is treated in texts like Suiter
and Rutten & van Venrooij.

Well, this might be a bit of an overstatement of any "advantage". While the
diffraction caused by the secondary obstruction does cause a reduction in the
diameter of the apparent "spurious" or Airy disk of a star, the actual amount
of reduction for common central obstruction sizes is slight, and would, of
course, not help with detail in extended objects. It may slightly improve the
ability of the telescope to resolve close double stars but only when the
obstruction reaches a somewhat large size. Indeed, the diffraction disk of a
telescope with a 20 percent central obstruction is only about four percent
smaller than that of an unobstructed instrument. Even a 33 percent central
obstruction would only yield a 10 percent reduction in the Airy disk size, so
for common central obstruction sizes, the "improvement" in effective
resolution would be small at best. Clear skies to you.
--
David W. Knisely
Prairie Astronomy Club: http://www.prairieastronomyclub.org
Hyde Memorial Observatory: http://www.hydeobservatory.info/

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