Re: ATM more on off-axis telescopes

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

I'm not sure what you mean by a split secondary or putting the eyepieces at
the back.  My intention was to avoid not only the obstruction of a
newtonian's diagonal, but also the additional optical surface of a secondary
all together.  I want my eyepieces looking directly at the primaries, if
possible.  Another concern is the orientation aspect.  If one image is
somehow reversed, rotated or angled in relation to the other it will be
impossible or at least very uncomfortable for the images to merge.

My reasoning comes from the illustration of taking a healthy 10" f/5 system
and sawing it in half.  Move the two halves about 7" apart and stick your
head in where the secondary/focal plane would be.  Basically I'm cheating
physics with a cheap trick, hoping that half of a parabolic mirror will
produce one eye's share of a pair.  (Oh-oh, here we go again with the
language problem:)  It seems I have read -somewhere- about off axis and
cutting a smaller mirror out of a larger one to make the off axis paraboloid
necessary.  It seems though, from what I remember, each mirror must include
a portion of the center of the mirror to be correct.  This means I couldn't
get two 5's out of a 10 as there is only one central region.

I like your idea of creating the two blanks prior to grinding and cementing
them together in a large disk of dental stone.  That might just put this
within my reach to fabricate.  I can make the disk, then cast a tool....
That will give me the excuse I need to cut up that old coffee table top I've
got out back.  I didn't think dental stone would hold up to such grinding
activity.  Tools made with the stuff are usually faceted with bathroom tile
or ceramic in order to produce a hard, glass like surface to compete with
the hardness of the mirror being ground.

I haven't "ray-traced" this idea out at all and I'm probably waaaaaay out in
left field with my expectations but it's fun to brain storm.  My cousin has
been flogging me with tons of questions about binoc systems ever since he
ordered two of the little 3" jobs from SurplusShack so I'm just thinking out
loud to myself.  Thinking about it doesn't require any cerium oxide or pitch
at all :-)


----- Original Message -----
From: "Frank Ward" <thewards@mindspring.com>
To: "Andrew Schott" <aschott@cfl.rr.com>
Sent: Friday, December 28, 2001 4:28 PM
Subject: Re: ATM more on off-axis telescopes


> Andrew,
>
> Why not cut the disks first and make up a dental stone disk
> with them incorporated (the way diagonals are made.) You could
> grind and polish and figure the "Disk" and then hammer away the
> dental material.
>
> When you place the disks in their tubes you could just mark the
> central core and place that up. The diagonals would be at the edge
> of that mark with the eyepieces above them....or you could make them
> into a Gregorian Binocular and put the eyepieces at the back with the
> split secondary between the tubes.
>
> Frank
>
> ----- Original Message -----
> From: "Andrew Schott" <aschott@cfl.rr.com>
> To: <atm@shore.net>
> Sent: Friday, December 28, 2001 3:09 PM
> Subject: Re: ATM more on off-axis telescopes
>
>
> >
> > 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.
> > >
> > >
> > >
> >
> >
>
>