Re: ATM 18" f4.6 with a 2.6" diagonal

[Marinus van der Lugt <marinus.lugt@dss.dornier.dasa.de>]

Richard Schwartz wrote:
> 
> > A smaller diagonal will not lower coma-
> 
> I think  you are wrong about this.  A small enough diagonal will prevent
> light from the edge of the primary from reaching the focal plane.  This
> is the area that contributes the most to coma.  You can get the same
> effect by using a smaller mirror.  In fact, a small diagonal can cure
> turned down edge!

On-axis you are right. Off-axis (where the coma becomes noticable) the
edge of the mirror will be used, even with a small diagonal. I did'nt do
the math's, but it seems likely that with a small diagonal the effect
will
be worse compared to a 'normal' sized diagonal. The area of the 'good'
part (of the mirro) will decrease, whereas the area of the 'bad' part
will
increase.

This also implies that you cannot cure a turned down edge with a small
diagonal. You will need a mask to cure it.

Rien.