Re: [ATM] Two secondaries, one backwards

[Woodchuck <djv@bedford.net>]

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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