Re: ATM more on off-axis telescopes

["Andrew Schott" <aschott@cfl.rr.com>]

No problem understanding your english. At least at some point in your life
you learned it.  I was born in Kentucky and we never learnt no good
language.

Since you have some experience with off-axis optics, maybe you can peer into
your crystal ball of knowlege and tell me if I'm totally off track or maybe
close to some point of sanity with my idea.  I'd like to make a unobstructed
binocular with about 5" dia objective mirrors and about f/10.  If I
construct a 10" f/5 parabolidial primary and trepann or cut the two 5" disks
from it for my binoc's, would that provide the correct figure for them to
work properly?  Would there be any consideration to the separation of the
disks since your head would basically be placed in between them when viewing
and your eye/eyepiece would be at prime focus?  In other words, would I have
to grow the diameter of the primary to include the interocular distance of
the finished binoc array?  10" places the two binoc primaries' inner edges
in contact and in coincidence with the center of the original mirror.  This
basically makes a wedge out of both mirrors with the center of a parabola at
the edge.  See, I can't talk either!

I realize the problems of first fabricating a mirror from glass then
chopping it into pieces.  This will probably ruin the figure or at least
produce edge effect from the cutting which would have to be masked
off....BUT... theoretically it should produce the desired mirrors for the
binoc's to work.  And it seems to be the main focus of what you're talking
about below.


----- Original Message -----
From: "Dennis Nikitin" <ndenis@sarkor.com>
To: <usc@sover.net>; <atm@shore.net>
Sent: Friday, December 28, 2001 7:20 AM
Subject: ATM more on off-axis telescopes


>
>
> ----- Original Message -----
> From: Joseph Johnston <usc@sover.net>
> To: Dennis Nikitin <ndenis@sarkor.com>; <atm@shore.net>; James Lerch
> <jlerch1@tampabay.rr.com>
> Sent: Thursday, December 27, 2001 11:32 PM
> Subject: Re: ATM stopped down apat. to 18 inches? comments?
>
>
>
>
> Hey Joseph,
>
> >      I understand completely what you were explaining, but the problems
> you mentioned
> > shouldn't be relevant now, correct?
> >
> >
>
> Ok, I'll try again. Actually I admit that my poor English is the reason
why
> you don't understand what I want to say.
> Sorry if so - my native language is Russian. When you get this message let
> me know if my English is understandable. I really don't want to fuck
peoples
> minds as they don't understand me.
>
> I insist that this project won't work as is, i.e. with diameter and focal
> ratio you choose(18" and f/10). Please face it! The reason why I pay
> steadfast attention to your project is that I failed with similar scope in
> 1988. I don't need OSLO to evaluate your design. I wanted to make off-axis
> scope with spherical primary. Normally if you want to make compact
off-axis
> scope with reasonably fast focal ratio your primary should be off-axis
> parabola. This kinda figure, as some other list members mentioned,
requires
> extraordinary figuring skills. I wish I could make one.
>
> Now I'll try to explain some theoretical statements. The first aberation
> which should be killed in any system is spherical one.
> Your mistake is that you think your  18" f/10 scope will have spherical
> aberation like 18" f/10 Newtonian with spherical primary. But the cruel
> truth of life is that your 18"er will have spherical aberation like 48"
> f/3.75 Newtonian also with spherical primary. Now simulate 48" f/3.75
> spherical mirror with your ray tracing software. How do you think what
would
> be the result?
>
> One can take real (not virtual) 48" spherical mirror and put a mask on its
> face. The mask should have 18" off-axis hole. Optically and mathematically
> that case and your case are absolutely equal. Then one can measure
spherical
> aberation of unmasked 48"er, then aply the mask and measure
> spherical aberation of 18" portion and compare. It will be one and the
same
> spherical aberation. Now do you understand why your project won't work?
> Why you'll have impermissible spherical aberation?
> Astigmatism and coma also will be like in 48" f/3.75 Newtonian.
>
> There are several ways out:
>
> 1. Decrease f. ratio of imaginary 48"er to f/15 may be more I don't have
> time to calculate exactly . Your 18" primary will have roughly f/45 then.
> You will have very very long tube.
>
> 2. Leave everything as is and consider Maksutov's corrector. Maksutov
> invented his corrector in 1941. In 1942 he suggested all possible systems
> (Newtonian, Cassegrian, Gregory and your system) with meniscus correctors.
> The corrector also should be a portion of big imaginary 48" corrector.
Note
> that it's difficult to make 18" corrector.
>
> 3. Consider Yolo design.
>
> 4. Consider off-axis parabola. It's very difficul to make, I would say
> impossible, tons of experience required. It's true that professional
> off-axis scopes have very
>  good images because they have no shadowed primary and no spider. Such
> scopes are preferable for solar, lunar and planet observations. But they
all
> have off-axis parabola.
>
> 5. Forget about this project. Guys, I just want to save you of
> disappointments.
>
> 6. Ignore my comments and go ahead with your scope.
>
>
> Good Luck,
> Dennis Nikitin.
>
>
>