[Raytrace] Re: ATM 16" F/33 valid design?

[Michael Peck]

At 21:26 1/15/2002 -0600, Joe Mayenschein wrote:
>I agree f-70 would be crazy,, that's where I was hoping  some of the ray trace
>guru's could play with it some.  See with that say 10" f-6 primary,  how 
>"Fast"
>you could make it with simple "Edmund" Specials lenses used as the convex
>secondaries (spherical of course)  how fast (low of a f- ratio) can it go 
>before
> > > {snip}
> > > Please check out the below page of a project of a EXTREME long F-Ratio
> > > scope I was thinking of trying.  what's everyone think?
> > >
> > > http://www.qsl.net/wb9sbd/path.html

I'm going to crosspost this to the raytrace list for the sake of generating 
some traffic there. I've set up one example for you, ignoring the flats.

The particular example I tried has a 250mm f/6 primary, secondary 
magnification of 10 and paraxial focus (for the straightened out light 
cone) at the primary vertex. With those parameters your cassegrain 
secondary would end up positioned about 136mm from primary focus (probably 
farther away than you want it judging from your diagram), the required 
diameter would be just about an inch, and the radius of curvature about 
-303mm. If you wanted the secondary positioned closer to prime focus you'd 
need either to make the secondary magnification larger or move the 
cassegrain focus forward (or both).

One thing that surprised me a little bit is that you could leave the 
secondary spherical with a secondary magnification as low as 10 and still 
get diffraction limited performance. In fact you'd be diffraction limited 
over an entire 2" diameter field, with a P-V wavefront error on axis just 
slightly better than 0.1 wave, deteriorating to 0.2 wave at the edge (note 
to those who object to P-V errors: the axial wavefront error is all 3rd 
order spherical aberration). With a system focal length of 15000mm the 
maximum angular field available with a 2" eyepiece would be about 11 
arc-minutes, and the magnification with a 55mm eyepiece (the practical 
minimum) would be 270x.

To answer the question posed on the web page, changing the secondary 
spacing would have some zoom effect, and a huge effect on the position of 
the focal plane. Decreasing the primary-secondary distance by 1mm would 
move the position of best focus out by about 113mm and change the effective 
system focal ratio to about f/64. This seems like more of a problem than an 
opportunity to me, since it means you're going to have to pay special 
attention to maintaining repeatability of secondary position and focus will 
still tend to wander significantly during the course of a night.

Anyway, with the caution that I haven't considered the flats I'd say that a 
design like this might be feasible. I wouldn't say it's a good idea. The 
one potentially sensible application I can see for this would be if you 
wanted to do diffraction limited ccd imaging of small, bright targets (i.e. 
planets). If I wanted to undertake a program like that I'd just dedicate a 
telescope to that purpose and forget about convertibility. If you do that 
you can make it a DK cass, which with this extreme focal ratio would be 
nearly indistinguishable from an aplanatic design.

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