Re: ATM very fast dob (f/D<4)

[Aplanatic@aol.com]

Stuart Field wrote:

> As an example, I tried out the numbers for a 30" f/3 scope.  The closer the
>  plate is to the mirror, the lower the astigmatism (but the greater the
>  hyperboloid).  Thus I placed it right in front of the diagonal as a 
> reasonable
>  compromise, which I took as 24" from the focal plane.  This leads to a 
> mirror
>  with a Schwartschild coefficient of -1.72, that is, 72% deeper than a
>  paraboloid.  The plate would be about 8" in diameter and probably could be
>  figured using the vacuum pan technique.  On axis there is a little SA, but
>  that's presumably because the specs were figured out from 3rd-order Seidel
>  analysis (yes, I finally figured this out!).  It is always possible to 
cure 
> the
>  SA of the mirror by tweaking the plate.  In OLSO LT I couldn't figure out 
> how
>  to optimize the aspheric coefficients on an element.


Interesting work, Stuart.  Can you try a 42" f/3.3 Newtonian using this coma 
corrector technique?  This may be a better answer than the 4-element coma 
corrector that my friends are considering.  Also, can you tell me the 
corrector surface profile coefficients for the 30" f/3 example that you have 
now?  In other words, if the corrector surface profile is given by Z=Ax^2 + 
Bx^4 where x is radial distance from center, what are A and B?  I can then 
work out whether a vac pan technique is suitable.  What is the diameter of 
the corrector plate for the 30"?  I assume that it causes no additional 
vignetting.

So, you figured out the Siedel normalization nightmare?  Someday I would like 
a tutorial.

Thanks,

Dave Rowe
Torrance, CA