Re: ATM 3-Vane or 4-Vane Spiders

[PK6811S@ACAD.DRAKE.EDU]

D Chaffee wrote:

>I beg to differ. Page 55 in Star Testing Astronomical Telescopes:
>"Diffraction is a consequence of a limited aperture."

"limited", "obstructed", whatever.  'You say tomayto' I say tomahto'...

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

Yup, we are in agreement.

Me: >>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.

It is relevant only to the distribution of the energy, it is irrelevant
to the amount unless it changes the size of the obstruction.  As I said
before you can remove 'spikes' by spreading the diffraction around, but
you can't throw the energy they contain out of view (this idea is filed
for future research :-)

>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.

I'm sorry, I must have missed the introduction of this theme in the
previous discussion.  There were a lot of doubly and triply repeated
messages that I deleted without reading.  I was only responding to a
remark that diffraction could be removed by 'blending' the edge.

Paul Kline
pk6811s@acad.drake.edu