Re: ATM Figuring/Testing my 8" f8

[Aplanatic@aol.com]

Paul,

> Hmm.. did you run some math to get that 3 arcsecond number?  It seems to
>  contradict the theory I have been following, and if the theory is
>  faulty I would like to know.
>  
>  A parabola can be thought of as an ellipse whose far focus is infinitely 
>  distant.  It is mathematically possible to determine how near the far
>  focus can be and still give excellent images.  Focault did it.  And it
>  was written up in a Sky & Telescope article in January, 1979 page 92.

You are right, I misread your focal length, using f/6 not f/8, sorry.  

As John Upton has commented, if you figure the mirror perfectly using this 
test it will have a P-V wavefront error of 0.25 waves and will have a Strehl 
Ratio of 0.8.  It does eliminate the need for a Foucault tester, and a null 
test is probably somewhat more accurate (and certainly more aesthetically 
pleasing) than a Foucault test.  The trouble I see is that you will be 
leaving a bit of resolution on the table, even if you make the mirror so that 
it nulls exactly in your test.  If it ends up a little off in the wrong 
direction, you will have a mirror that may not perform up to your desires.  
It's sort of like aiming at the edge of the Bull's eye and not the center.

Sorry for the confusion, and best of luck!
Dave Rowe.