Re: ATM more on off-axis telescopes

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

Many thanks to all who've responded to my brainstorming.  More arrive as I
write this!  Most seem to kringe at the thought of running a saw through a
perfectly good 10-13" mirror and making little ones out of it.  I too would
have a hard time doing this.  The only possible way I would ever think of
engaging this project would be to start with pieces, like discs trepanned
from the coffee table mentioned before and casting them into a large disk
for grinding.

As a long time 3-D enthusiast and photographer, I know the extra added
benefits of providing two live independent images to the eyes.  You're brain
functions much better when it has two images to process.  Since this extra
added benefit works best with the sharpest image attainable, and can
overlook the problems of brightness and contrast to some extent, I went with
the unobstructed design.  I should stop calling them that and refer to them
by their official names.  Is it the Gregorian that has no secondary?
Herschiellian?  Which one actually uses the off axis paraboloid?

Several responders have mentioned considerations I have to make for
secondaries or diagonals.  Since I'm going to all the trouble of building a
pair of off axis scopes, I don't intend to use any diagonals, secondaries or
anything else.  I want my eyepieces to have clear, unobstructed views of
their respective primaries.  Nothing in the way of either.  No spider, no
star prisms, nothing.  The light makes one bounce then it's in the eyepiece.
You stand between two 6" tubes that look back over your shoulders into
space.  The eyepieces are situated in such a way that the starlight
silhouette of your head barely grazes the primary's inside edge on either
mirror.

If anything, this project would be nothing more than a novelty to drag out
to star parties just to have something different to play with.  But that's
how my 6" project got started and I'm very happy with the way it turned out.

Thanks again for the response!

Andrew Schott W.O.T.S.

Leftfieldstar wrote:
I don't see any advantage to using off-axis segments of a larger mirror to
build a binocular scope. Is not binocular vision dependent on overlapping
fields of view to produce real 3D effects? It also seems that a binoviewer
used with a 13" parent mirror would produce a larger field and brighter
views
with higher net contrast.

Would not the total time to fabricate such an off-axis system exceed the
effort to produce two 10" on-axis systems?

Saaaaaay. Are we starting one of those end of year dreamsheet episodes?

Happy New Year Gang!
Dominic DiLeo


Steve Fejes wrote:
  Your idea is a good one, but you need to add the center spacing where
the secondary mirror would be.  So, for a 5" f/10 off axis mirror you
will need to cut it from a mirror larger than 10" diameter, say a 13"
diameter mirror.  The full telescope would then be a 13" f/3.9.  This
would be a challenging mirror to figure because it is so fast.  And once
you got this mirror finished it would seem a shame to cut it up.  A 13"
mirror could give better images than a 5" off axis binocular scope
could.  Plus there is the possibility that the glass could be distorted
when you cut a section out of the big mirror.  <snip>

John Sherman wrote:

Yes, it would. Your plan will produce two off-axis sections of a paraboloid,
it
will work (neglecting the other problems you already mentioned). And since
your
binocs are two separate scopes then you don't have to worry about adding the
interocular separation to the 10". But you do have to remember the secondary
will block part of the 10". In your case, the secondaries will block part of
each 5". But that is not a problem.
My only thing is that I think a 10" Newt would provide a much better view
than a
5" 'noc. And be easier to make. But you probably have three of them sitting
around, already...

Have fun,

John