Re: [ATM] Some questions on Tex versus Sixtests - Hoping I'm done!

["vladimir sacek" <vlad@copper.net>]

Jay Killea wrote:


> Is it fair to say that Tex generally overconstrains the >criteria 
> (especially the Airy disk constraint, since he's >applying geometrical 
> optics to a wave optics >phenomenon)?

It seems to be an approximate empirical criterion, compensating for the test 
accuracy limitations and only applicable to the test results. If it would be 
taken literaly, no mirror would pass it before it is corrected down to ~1/50 
wave p-v s.a. on the wavefront or ~1/25 wave p-v surface.

> Also, I thought the Rayleigh criterion considers two >point sources 
> resolved as distinct objects only when >their diffaction spot centers are 
> more than a whole Airy >disk diameter (not radius) apart.  By that 
> criterion, Tex's >Airy disk radius constraint should really be an Airy 
> disk >diameter constraint.

The Rayleigh criterion considers two point sources resolved when center of 
one pattern coincides with the first minima of the other. Therefor, when 
their centers are
separated by the Airy disc radius (1.22LambdaF linearly, or 1.22Lambda/D 
angularly).

> Finally, in Sixtests is it advisable to put a value in for the 
>  >obstruction diameter in order to get an idea of what the >real-world 
> performance of the mirror will be?  For an 8" >f/6.4 mirror, is 50mm a 
> good value to use (i.e., >representative of a likely diagonal shadow)?  In 
> the case >of my mirror that would be great, significantly improves >all 
> performance parameters.

It is only the wavefront, which benefits from its aberrated central portion 
being blocked by the c.o. But at the same time it causes non-wavefront-error 
related changes in
wave interference at the focus, resulting in transfer of energy from the 
disc to the rings. Suiter's "Star testing..."
can give you a good idea of what the final effect would be  (p198). A 
perfect mirror with 25% c.obstruction would have nearly identical amount of 
energy encircled within the Airy disc (actually within 82% of its radius) as 
an unobstructed aperture with 1/5 wave p-v of spherical aberration (~0.87 
Strehl). Obviously, this effect is much greater than one of improving the 
wavefront quality alone.

Vlad



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