Re: [ATM] 16" RCC primary

["vladimir" <svla@isp.com>]

Attila Schné wrote:
>
>      I started a RCC project some month ago.The system includes a 16"
>      f/4
>      primary, which is right now at the state of paraboloid. ...
>      I am not sure that a normal
>      cassegrain would be able to make a coma-free picture in
>      this small area. Maybe a small hyperbolising without any
>      measure would be enough, and than nulling the system by
>      the secondary.

The relatively mild curvatures will require lot of aspherising to correct
aberrations. An ~f/8 RC would require around -1.3 primary and -15 secondary.
Classical Cassegrain would still require -9 , or so, secondary.
It would have about as much coma as an f/5 parabola at 2.5mm off-axis.
Not really bothersome, especially with long exposures, but it depends
on seeing quality to an extent.

If the instrument is intended for imaging, you could consider Loveday's
arrengements (primary reflects light from the secondary back, focusing
through an opening at the secondary), either with paraboloidal or 
hyperbolidal
primary. They would require much less aspherising. With an f/4 paraboloidal 
primary,
an effective f/8.7 system would regure -1.7 secondary conic, with nearly 
half
the coma of the Classicial Cassegrain counterpart.

With hyperbolidal primary (-1.2), the secondary is -2.5 and the system is 
coma free,
with ~2.5 times less astigmatism than comparable RC, and about about twice
milder field curvature. While there shouldn't be much difference in 
performance
for the intended purpose within the given field, the RC mirrors are harder 
to make.

Vlad 

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