Re: [ATM] Stereo view

["Bruce Sayre" <sayre@foothill.net>]

(Long, but you hit my hot button square on.)

<<...why are so many people going through the hassle of 
<< making a complete double telescope, while just a binocular 
<< eyepiece would suffice?

There is, in fact, a reason for this — it's more than just hype from a community of
like-minded Newtonian binocular buffs. While the major benefit of binocular vision is
depth perception, or stereopsis, there is another, more subtle benefit called binocular
summation. Stereopsis plays no role in telescopic observations at all: it's about
summation.

Binocular summation was defined by visual scientists, where studies have shown that visual
perception using two eyes is enhanced over one eye in several ways:

1. Visual acuity 
2. Visual detection at threshold levels, including both absolute light detection and
contrast sensitivity 
3. Brightness perception 
4. Flicker perception 
5. Form recognition 

Experiments have shown that the binocular detection threshold of dim light is 1.4 times
better than for monocular viewing. This has been attributed to two theories, probability
summation and neural summation. 

Probability summation holds that chances are statistically better using two receptors
instead of one. Neural summation hypothesizes that some connection between monocular
neural pathways in the brain might produce a physiological "sum" of the images. Studies
have shown that binocular sensitivity is well in excess of any gain that would be expected
from probability summation, thus supporting the presence of neural summation.

Binocular summation is especially applicable when looking at deep space objects and their
features that are at the thresholds of detection. Obviously, the benefits binocular
summation are totally dependent on either a binocular telescope or binocular viewer. 

I was interested in determining for myself whether binocular summation improved visual
acuity. During an appointment with my ophthalmologist, I tested both eyes without my
glasses, both independently and then together. My right eye tested 20/40 and my left eye
tested 20/30. Using both eyes, however, I tested 20/15 — a definite improvement.

I have no doubt that anyone who invests in a binocular viewer for a large-aperture
monocular telescope will definitely get their money's worth. There is a small amount of
light loss due to the beam splitter, prisms and other lenses, perhaps a half magnitude. 

I suspect the magic of the brain's signal processing actually improves the monocular view
for brighter objects; some argue it helps at all magnitudes. More information on binocular
viewers can be found in Sky & Telescope, Vol. 104, No. 3, September 2002, p. 46 and Sky &
Telescope, Vol. 109, No. 3, March 2005, and in a Cloudy Nights review.

A large binocular, however, has these advantages over a larger viewer-equipped monocular:

1. No light is lost, as is the case with the viewer's extra optical surfaces and prisms
(however well coated) 

2. No Barlow lens is needed, so there is no image magnification (usually at least 20% in a
viewer)
 
3. A binocular can use 2" focusers, so any pair of long focal length 2" eyepieces can be
used, making magnifications below 100x possible — constrained only by the maximum diameter
of the eyepiece and the viewer's interpupillary requirements (e.g., I could use a Tele Vue
41mm Panoptic or 31mm Nagler eyepiece, but many could not) 

These factors point to significantly better low-power viewing with a binocular. I
regularly use 30mm, 80° apparent field 2" eyepieces with my 22" binocular, which yield 95x
magnification (almost as good as a 31mm Nagler). Tele Vue's longest focal length Nagler
with a 1¼" barrel is 16mm. This eyepiece, used with a 1.2x optical compensator Denkmeier
II would yield 209x magnification on the 22" binocular. 

The final reason binoculars may be a better answer is cost. The best measure of power in a
telescope is probably collecting area. I compared a Starsplitter 30", an Equatorial
Platforms 28" and my 22" binocular. I added platforms to the two monoculars, and also the
Denk II binoviewer. 

The cost per square inch of collecting area is $29 US  for the 30", $33 for the 28", and
$26 for the 22" binocular. While an ATM could certainly make a 30" monocular for less
money than the cost of a commercial telescope, he or she could also certainly build a
binocular for less than what I did. 

Any ATM who is considering making a large aperture monocular should give a
smaller-aperture binocular Newtonian serious consideration. While construction may be more
complicated, the extra effort pays off. It's my belief that a binocular's views will
always equal and often surpass those available from a larger monocular built for the same
cost. 

Bruce Sayre
P. O. Box 544
Applegate, CA 95703 USA
mailto:sayre@foothill.net
http://www.foothill.net/~sayre 

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