[ATM] Classical Cassegrain Design Tradeoff Help Needed

[atmmessages at infinichron.com (Gerald Boutin)]

Thanks, Mike. That is exactly the kind of feedback I was looking for.

The field curvature issue was more or less the last concern I had. I've
pretty much satisfied myself on pretty much everything else. I just wanted
to make sure that I wasn't going to end up with bloated stars.

So, about f/4 for the primary and onto banging my head against the wall
trying to figure out what to do about baffling and keeping the obstruction
down. Fortunately, I have an easy to use spreadsheet for calculating
baffling parameters. Hmm, that's yours! Thanks, again.

--
Gerald Boutin

> -----Original Message-----
> From: atm-bounces@atmlist.net 
> [mailto:atm-bounces@atmlist.net] On Behalf Of Michael Peck
> Sent: January 29, 2004 4:38 PM
> To: The Amateur Telescope Makers List; ATMList
> Subject: Re: [ATM] Classical Cassegrain Design Tradeoff Help Needed
> 
> 
> At 12:52 29-01-04 -0700, Gerald Boutin wrote:
> >I am (slowly) moving ahead on design of a Classical 
> Cassegrain telescope.
> >The design is for a 350 mm diameter F/14.5.
> 
> 
> 
> 
> >What it boils down to is that I don't have a good feel for 
> what effect
> >varying the flatness of the focal plane has on visual 
> observing. In other
> >words, how flat of a field curvature do I need to get nice 
> sharp points over
> >the maximum 1/2 degree field? Although I have done a bit of 
> Googling on eyes
> 
> 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.
> 
> Keep in mind too that a not so well corrected eyepiece has 
> substantial 
> astigmatism and field curvature in general.
> 
> I'd make design decisions on other factors than field 
> curvature, if the 
> telescope is going to mostly for visual use.
> 
> Mike Peck
> 
> 
> _________________
> 
> Michael Peck
> email mpeck1@ix.netcom.com
> Wildlife photography page http://home.netcom.com/~mpeck1/index.html
> Amateur telescope making 
> http://home.netcom.com/> ~mpeck1/astro/astro.html
> 
> 
> 
> _______________________________________________
> ATM mailing list http://www.atmlist.net/
>