[ATM] viewing in stereo - a bit off topic, but interesting anyway

[gfbrandenburg at yahoo.com (Guy Brandenburg)]

We got a little demonstration on stereoscopic
binocular modifications last night at the NCA
telescope-making workshop at the Chevy Chase Community
Center in DC. John Smith has been doing various
experiments on this for years, and has made a few
gadgets with inexpensive materials to enable people to
see things that are relatively far away (about 50
feet) with the same degree of stereoscopic vision that
we see at about 6 feet away. He said that at a
distance of 7 feet, our two eyes see things from an
angle of about 2 degrees, which my calculator agrees
with (arctan(3/84) is about 2.045 degrees). His
arrangment of front surface mirrors takes two
different beams of light coming from the relatively
distant object and collimates them into the two
separate oculars of some very inexpensive (~$20)
binoculars from Radio Shack. The optical arrangement
looks a bit like this, schematically:

             you look in here
                || ||      <-- the binoculars
                || ||

  /             /   \             \
 /             /     \             \

.....
        whatever you are looking at is way,
         way far away out here.

The diagonal lines (/ and \) are front-surface mirrors
that he collimated. The whole arrangement is fastened
on a light-weight, but sturdy, wooden frame; he even
has an arrangement wherein the outer mirrors can be
brought closer together if needed. It seemed to me
that the maximum separation was on the order of about
3 feet, which would mean (by my calculations) that at
about 80 feet away, one would get the same
stereoscopic binocular impression of an object that
one would get if one were 7 feet away. 

Smith said that people who have studied how people
perceive depth and 3-dimensionality have discovered
about 10 different clues that we use, and that the
most important of those is the stereoscopic difference
between the image received by the two eyes. Of course,
there is a substantial (10%?) of the population that
does not have stereoscopic vision, for whatever
reasons.

Does anybody recall those stereo viewers from the
1950's and 1960's? I think that those were generally
made with special stereo cameras which were relatively
popular back then. 

His invention is particularly useful for anybody who
wants to see wildlife at a safe distance (safe for
both you and the critter) but doesn't want to have to
suffer from the 'flattening' effect that binoculars
cause. John will readily point out that his
contraption will do you absolutely no good at all if
you wanted to get a 3-d image of the Orion Nebula or
of the craters on the moon. (For the moon, a distance
of 250,000 miles divided by a separation of, say, 4
feet, gives me an angle of about 0.0000002 degrees, or
about 0.0006 arc seconds. He mentioned that people
have produced nice 3-D images of the moon; if I
understood correctly, they did that by taking one
image of the full moon right at dusk - at the equator
- and then taking another image at a different spot on
the equator, at local dawn. Our planet has a diameter
of about 8000 miles, which if compared with the
distance of 250,000 miles, gives you a subtended angle
of about 1.8 degrees, which is just about right. 

By the way, even though all of his materials were
inexpensive, his workmanship was excellent. One of his
gadgets was hand-held, and another one was mounted on
a tripod.

Please forgive the off-topic nature and the
cross-posting. I thought that the fact that many of us
are interested in optics might excuse the intrusion. I
hope I have done some justice to what he said and did.

Guy Brandenburg


=====
Guy  BrandenburgWashington, DChttp://home.earthlink.net/~gfbranden/GFB_Home_Page.html

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