Re: [ATM] secondary size dillemma

["vladimir" <svla@isp.com>]

Hi John,

> Does going from 19% to 16% really make enough 
>difference to the diffraction to give a noticeable contrast 
>increase? 

C.obstruction "c" in units of aperture reduces central intensity 
of diffraction disc to (1-c^2)^2. This also closely
indicates amount of energy lost to the rings (similar to the
Strehl). The above obstructions would cause reduction
of peak intensity to 0.95 and 0.93, respectively. The
difference is similar to that between 0.95 and 0.93 Strehl,
or 1/8.3 wave and 1/7 wave p-v of spherical aberration.
Not much to talk about.

I wouldn't quite rule out the psychological factor: people
expect better contrast with smaller obstruction, so they
tend to "see" it. 

Vlad
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