Re: ATM 3-Vane or 4-Vane Spiders

["Dan Chaffee" <dchaffee@blitz-it.net>]

Paul,

>DIFFRACTION IS NOT AN EDGE EFFECT
>DIFFRACTION IS NOT AN EDGE EFFECT
>DIFFRACTION IS NOT AN EDGE EFFECT

I beg to differ. Page 55 in Star Testing Astronomical Telescopes:
"Diffraction is a consequence of a limited aperture."
Also,  "Quasi-static light and dark regions exist because diffraction
can be modeled as a standing wave, with the aperture providing
the boundaries".
Rings resulting from diffraction occur regardless of whether or not an
aperture is centrally obstructed; it's their energy distribution that is
altered.

>What we call 'diffraction' is the observable effect of obstructing part
>of a wave front.

Why don't you consider the edge of the aperture an obstruction of an
unlimited entrance pupil?

>Whether the edge is 'blended', 'graduated', 'matted'
>or whatever is irrelevant.  The obstruction creates the effect

No, it is relevent. The blended or Gaussian filter supresses diffraction
rings and in so doing, increased the size of the diffraction disk, again
the inescapable product of diffraction for the confines of the entrance
pupil.

 >The
>bigger the obstruction the bigger the observed effect, most schemes to
>soften the edge make the obstruction bigger and therefore do not improve
>the situation.

The biggest loss in performance is a result of a smaller entrance pupil, not
just the fact that the central obstruction has increased in percentage.

D Chaffee