Re: [ATM] Two secondaries, one backwards

[artbianconi@blast.net]

Thanks Dave for the time and effort to help me with this. I received a number of suggestions, some easier than others. 

I  need to sit down with each one and muddle my way through.

Are you interested in seeing some JPG's of the design take from the engineering model?

Regards

Art

Date sent:      	Sun, 25 Jul 2004 17:45:38 -0400 (EDT)
From:           	Woodchuck <djv@bedford.net>
To:             	artbianconi@blast.net
Copies to:      	atm@atmlist.net
Subject:        	Re: [ATM] Two secondaries, one backwards

> On Sun, 25 Jul 2004 artbianconi@blast.net wrote:
> 
> >
> > I recently submitted an idea to friends at S*T*A*R  for a
> > critique that I hoped would solve a problem I have.
> >
> > I would like to be able to move my head slightly and look
> > down through an second eyepiece instead of using a Telrad
> > or similar device.
> 
> The easiest way is to mount a finder scope with a
> right-angle prism in the appropriate place on the outside of
> the tube.
> 
> That said, I commence a ramble...
> 
> > With that in mind, I went to my engineering software and
> > inserted a second secondary (sic!) on the same axis as the
> > first one but aimed towards the sky instead. Then I simply
> > positioned a second eye piece next to the existing focuser
> > to view the sky through that opposed secondary.
> 
> If you do not want any magnification, and do not want a
> cross-hair or other pointing aid in your field of view, then
> the finder eyepiece can be just a hole drilled in the cap of
> a "dummy" eyepiece.  This will then function something like
> the "ghostring" sight used in shooting.  If you need glasses
> for distance vision, and remove them for using the scope
> normally,  you could put a corrector in the eyepiece, maybe
> even trepanned from an old pair of your glasses, or begged
> from an optician. ("Got a piece of -3.5 diopter lens about
> 1/2" across?" would be for me, the near-sighted.)  You want
> the hole in the eyepiece cap to be about the same size as
> your night-adjusted pupil size, call it 5mm.  (this is what
> I would guess, and I hope to be criticized by someone with
> more knowledge of human vision.)  This hole should not be a
> long cylinder, such as is used for alignment, but as thin as
> practical.
> 
> If you want a reticle with cross-hair, then you need to
> design a low-power telescope, with the reticle in the focus
> of its eyepiece. This can be a 1X scope,  perhaps parts from
> a Telrad or a red-dot style pistol scope.  There'll probably
> be room to mount a laser pointer somewhere, too, if you like
> that method.
> 
> > I reasoned that since the second mirror is in the shadow
> > of the first, there would be no additional obstruction of
> > the light going to the primary and I could develop the
> > appropriate lens combination for the second eye piece to
> > help aim the telescope. Perhaps a simple change in lenses
> > would alter the viewing field.
> 
> It's just a refracting telescope you're designing.  Low
> power, but there it is.  The wrinkle is that you have a bent
> or elbow tube. This is like a periscope or those binoculars.
>  A common design is to have the objective right on the main
> mirror's axis, immediately behind this objective is the
> right-angle "second secondary" (prism or mirror), then you
> go "tubeless" up to the eyepiece/focusser. Certainly this
> can all be done in the "shadow" of the main secondary,
> although the mechanics of the mount can get a little bit
> into the "watchmaking" region. In terms of designing, the
> mirror contributes nothing to the optical length, a prism
> would be treated as a plano-parallel of the "obvious"
> thickness.
> 
> I have a vague and dim memory of a rig like this from the
> past, a mechanical tour-de-force, in which the same eyepiece
> was used for both, maybe sliding side to side, or using some
> trick with mirrors.
> 
> > I concentrated on studying the mechanical feasibility of
> > such a design. The inappropriatness of my expectations and
> > my lack of optical knowledge were apparent and my friends
> > were quick to point out as much.
> >
> > Since the attempt is an integral part of an exceptionally
> > light (purposefully!) 6" f-8 open truss "port-a-ball" type
> > telescope, it's important that the solution be of low
> > mass.
> 
> The glass won't be too bad.  I'm thinking of about a 25-30mm
> D objective, a "second secondary" of that same minor
> diameter. This would be easy to obtain with about a 150mm
> focal length or thereabouts (6" in civilized measure), which
> would be about 6X with a 1" eyepiece.
> 
> > What kind of lens(s) must I use to focus on the new
> > secondary, view the sky with low magnification and not
> > obstruct the light path?
> 
> You want an "afocal" system of some kind. (A telescope is an
> afocal system.  Nothing ("air") is also afocal).  If your
> eyepiece focusses on the mirror, then what you'll see will
> be the dust on the mirror.
> 
> Think of yourself using a hand mirror to peak around a
> corner. What optical system would you use for that?   Same
> problem.
> 
> What overall magnification do you want from the finder?
> 
> There may be a worry about orientation of the field.  You
> would be best served if it is the same as that of the Newt,
> else you risk madness.
> 
> Dave
> 
> 


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