RE: [ATM] Re: A new kind of folded Newtonian

["Tony Dunlap" <tdunlap@adelphia.net>]

What I was looking for was a comparison. Does the effect of an
inperfection change depending on where in the lightpath that
imperfection exists? My sense is telling me that a flat before the
primary doesn't need to be as flat as one after the primary. My
reasoning, perhaps flawed, is that the parallel light rays (before they
hit the primary) hitting the big flat of, say, 1/8 wave accuracy, will
be changed less than would converging rays (after hitting the primary)
hitting a smaller flat of the same 1/8 wave accuracy.

> -----Original Message-----
> From: atm-bounces@atmlist.net 
> [mailto:atm-bounces@atmlist.net] On Behalf Of Vladimir Sacek
> Sent: Monday, April 19, 2004 4:38 PM
> To: atm@atmlist.net
> Subject: [ATM] Re: A new kind of folded Newtonian
> 
> 
> I did quick calculation, and seems that smooth surface error 
> on the diagonal would result in nearly three times the 
> wavefront error of astigmatism. In other words, a 1 wave 
> surface error would result in 2.7 waves of astigmatism (keep 
> in mind that 1 wave of astigmatism compares to 2/3 wave of 
> spherical aberration contrast-wise). So if you'd like to 
> limit astigmatism of this origin to 1/4 wave (that comapres 
> to 1/6 wave of spherical, contrast-wise),  the diagonal 
> should be better than 1/10 wave surface.
> 
> You might want to consider variation that doesn't have a 
> diagonal flat. The flat is tilted just enough to receive 
> light passing by the primary (less than 10 degrees with an 
> f/5 mirror). Such flat causes much less astigmatism (a very 
> shalow sphere up to a few wavelengths deep would do just 
> fine), it is lighter than diagonal, and there's no need for 
> an elliptical hole in it (although you's probably have to 
> fiddle a bit with its back opening, if you want evenly 
> illuminated outer field).
> 
> Vlad
> 
> _______________________________________________
> ATM mailing list http://www.atmlist.net/
> 

_______________________________________________
ATM mailing list http://www.atmlist.net/