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ATM Re : Diffraction limited spheres


Jim,

great stuff ! Finally something to back up my long battle in trying to
convince people that 6" f/9 left spherical ISN'T piece of junk, but in
fact a fine performer. I am surprised that noone thought about looking
at it from this angle before (I guess I'm not surprised that _I_ didn't 
think of it before).

Thanks for the insight.

That table again :

Minimum f-ratio for a spherical mirror to
satisfy 1/4 wavefront (~1/14 RMS)

<html><pre>
  D(in)   D (mm)      f/     

  4       102         7.1   
  6       152         8.2   
  8       203         9.0   
  10      254         9.7   
  12      305        10.3  
</pre></html>

or,  F/D (f/number) shoud be greater than  1.5256*D^(1/3)   (D in mm)

Note that these spheres do NOT satisfy simultaneously Danjon & Couder
criteria 1 and 2. But this merely shows us how strict D&C criterion is,
and that mirror that _does_ satisfy it is going to be a stellar performer
indeed.

(and yes Tom, this does mean that parabolizing 4.25" f/11 is an overkill :-)
Nice experience though.)

Bratislav