[ATM] Classical Cassegrain Design Tradeoff Help Needed

[chaosopher23 at yahoo.com (Kevin Whitefire)]

Which performance are you referring to?  The narrow
f/D cone will give a certain type of performance,
where the f/3.9 cone will give another type of
performance.  The 3.9 is like a sports car to film,
while the 14 is more like a Cadillac for the eyes and
eyepieces.

Personally, I'd rather have it easy on the wallet and
the eyes when it came to high power views.  I can
always put a telereducer in the light path of a long
cone if I really need a wider field...  but I like
high power and small details, so I won't telereduce. 
Moon, planets, doubles, and even galaxies can use the
longer f/D.  

I think my next project will be a 16" binocular
cassegrainian or gregorian.

Kevin
--- Frank <frank@katestone.com.au> wrote:
> Hi All
> 
> Are we comparing a F/14.9 Cass with a F/3.9 Newt ???
> 
> I don't think this is a valid comparison as the FOV
> of the Newt
> would be much bigger and it would definitely have
> more coma.
> 
> We should compare a F/14.7 Cass with a F/3.9 Newt in
> which case
> the Newt ( I believe) would perform better.
> 
> Cheers
> Frank Q
> 
> ----- Original Message ----- 
> From: "Michael Peck" <mpeck1@ix.netcom.com>
> Subject: Re: [ATM] Classical Cassegrain Design
> Tradeoff Help Needed
> 
> 
> >
> > Gerald:
> >
> > My 12.5" (31cm) cassegrain has virtually the same
> parameters as what
> you're
> > proposing: the primary is just under f/3.9 and the
> system is about f/14.7.
> > That makes the net field curvature just slightly
> greater than your f/4 -
> > f/14.5 design.
> >
> > With my 55mm Plossl I can just perceive that stars
> aren't quite pinpoints
> > at the edge of the field if I look there. With my
> attention at the center
> > of the field the edge looks sharp to me. Basically
> I find the off-axis
> > performance of a classical cass to be much more
> esthetically pleasing than
> > a newtonian with the same OTA length, which is
> going to have considerable
> coma.
> >
> 
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