[ATM] Classical Cassegrain Design Tradeoff Help Needed

[chaosopher23 at yahoo.com (Kevin Whitefire)]

I have been playing with OSLO-LT for awhile now,
trying to get refractor views from a mirror
combination.  I feel I have succeeded, but I need to
put it to glass to figure out if my computer's been
lying to me or not.

It involves a Gregorian arrangement with a 22%
diametric obstruction and a corrector lens that
changes the f/D only slightly, but alters the
wavefront to the extent that I can get the stars into
the airy disk size at 3/8 degree.

I may need to build a focuser that can compensate for
such a field.

Kevin of Eastern Iowa
Seeker of the Darkness


--- Michael Peck <mpeck1@ix.netcom.com> wrote:
> 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
> 
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