Re: ATM 16" F/33 valid design?

[Dominic-Luc Webb molmed <Dominic.Luc-Webb@molmed.ki.se>]

On Tue, 15 Jan 2002, 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.


I should not be counted as anything but a straggler in computer
aided design, especially ray tracing, but I do run equations
based on optics sections of textbooks with paper/pencil from time
to time and at least for simpler scopes, like Newts, I have found
some acceptable ray traces for longer scopes, although, F/70
stretches the limits of my imagination, even after the S&T
article. For a similar magnification 12", this would be F/44.
I could envision a totally acceptable 12" F/11 newt and attempting
a classical cass secondary with an amplification factor of
4, for instance. Perhaps some corrective optics would be
called for, but it seems sane from what I have read in the
typical ATMing books and articles. Not much harm done if it
didn't work. Anyone besides me ever peak at Saturn through
a 12" F/10-12? Images can be quite impressive. In coming days, I
will give consideration to a design using my 18" as a F/10 Newt
and as a classical Cass with amplification of 3. The 12" and 18"
would end up with same focal length as the scope in the S&T
article when used in the Cass configurations mentioned above.



   > 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
   > undesirable image degradation occurs.

The primaries above can easily be parabolized. I think the
respective secondaries could just as well be left spherical.
The Telescope Optics: Design and Evaluation book shows some
graphs for cutoff points when a parabola can be left spherical.
Certainly for testing, one can get a very good feel of system
performance from this. I am more concerned about the amount
of light loss. Hence, I am more interested in persuing this
with my 18" than my 12".


   > In other words,, a primary with a standard parabolic f-6 primary. and a
   > spherical secondary using a off the shelf lens as is. for the cass secondary.
   > how slow can you go before it falls apart.?

Numerically, this could be anything. I suppose that when a ray
trace of the wavelength you are interested in falls significantly
outside the zone it should (relative a hyperbola in this case),
your design criteria has been compromised. I would guess that
when it gets you something like 1/4 wave away from the target
you are definetly in trouble, and we always prefer to be 1/8
wave or better. Obviously, the sphere and parabola get
progressively closer to same curve with increased F/ratio. I
think somewhere around F/10 to F/15 its not much concern,
and above F/15 they might be difficult to separate. Since the
primary has both long focal length AND is parabolic, I think
the influence of the spherical secondary is not bad (amplification
3 to 4).

The Loveday scope is OK too, as near as I can tell.


Cheers,



Dominic

North 59 37' 30"
East  17 48' 10"

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