Stereoscopic Telescopes

[P3D Peter Abrahams <telscope@xxxxxxxxxx>]

Below is most of an article to be published in the Amateur Telescope Makers
Journal.  Any additional input on the subject is solicited.
STEREOSCOPIC TELESCOPES
In 1893, Ernst Abbe, working for Carl Zeiss, applied for a patent on their
new prism binocular, but it was denied because of the earlier Porro prism
glasses from several European makers.  A revised patent was submitted for a
prism binocular with enlarged objective distance, with the increased
separation between the objectives being the protected feature.  This was
approved, and for 15 years no other optician could make a Porro prism
binocular with objectives more widely spaced than the oculars.  The rapid
development of prism glasses by other quality makers caused the energetic
Zeiss publicity works to seize their unique characteristic and proclaim its
advantages in advertising.  There is a real, if minor, increase in sense of
depth that follows this increase in inter-objective distance, which is
probably perceptible at close focus with standard, hand held binoculars,
although there is wide variation in individual ability in stereopsis.
Zeiss used the term 'plasticity' to describe the enhanced sense of depth,
and it is a very apt term, since nearby objects appear modeled or sculpted.
This characteristic was quantified, with 'specific plasticity' being defined
as objective distance divided by ocular distance, and 'total plasticity' as
magnification times specific plasticity (higher magnification adds to the
effect.)  Increased perception of depth does allow the observer to
distinguish between objects that might otherwise be of very low contrast,
and this advantage was the subject of many studies, papers, advertisements,
and brochures around the turn of the century.
Zeiss also made theater glasses with closely spaced objectives for
portability, and they were not shy about publicizing the advantages of this
configuration.  They claimed that in the theater, diminished depth
perception is useful because the spectator will see the live actor as part
of the painted backdrop.  While these concerns are of minimal import today,
the effects are real, and were a very important part of the introduction of
binoculars to the public.
Date: Wed, 31 Jul 1996 15:02:11 -0500
Errors-To: 3d-moderators@xxxxxxxxxxxxxxx
Reply-To: photo-3d@xxxxxxxxxxxxxxx
Originator: photo-3d@xxxxxxxxxxxxxxx
Sender: photo-3d@xxxxxxxxxxxxxxx
Precedence: bulk
From: photo-3d@xxxxxxxxxxxxxxx
To: Multiple recipients of list <photo-3d@xxxxxxxxxxxxxxx>
Subject: PHOTO-3D digest 1437
X-Listprocessor-Version: 6.0c -- ListProcessor by Anastasios Kotsikonas
X-Comment:   The Stereoscopic Image (Photo-3D) Mailing List  

The Zeiss prism binoculars of 1894 were the first commercially successful,
the first mass produced, and the first high quality binoculars.  At the same
time, Zeiss offered 2 prism binoculars with objectives 12 inches apart (8
power,) and 16 inches apart (10 power.)  A hinge between the oculars allows
them to fold in half, leading to the generic term 'Scherenfernrohr' or
scissors telescope.  These were called by Zeiss, "Relieffernrohre," and were
not successful.   The 8 x 20 model was offered from 1894 to 1906, and the 10
x 25 from 1895 to 1908 and through 1918 for military use.  They give
spectacular views of terrestrial objects, greatly magnifying the perception
of depth in a scene and the appearance of modeled relief in an object.  Here
there is no exaggerating the effect.  They were used as rangefinders in both
World Wars, by several service branches of most of the participants in the
conflict.  Hand held instruments were about 6 x 30, with objectives 18
inches apart, and a folding hinge to reduce the length for transport.
Tripod mounted instruments could have 50mm objectives, for use at dawn and
dusk.  These were used by artillery forces to approximately judge distances.
The smaller sizes were needed for quick judgments on shell bursts, when a
large instrument or more complicated rangefinder could not work quickly
enough.  These 'battery commander's rangefinders' can occasionally be found
at gun shows or military collectors' meetings, and there are a few optical
repair shops remaining that can correct their typical out of collimation
condition.
Truly remarkable instruments were used by the U.S. Navy (among others,) from
prior to WWI through the 1980s, for controlling the large guns of their
ships.  Some of these rangefinders used coincidence sighting, where two
images were brought together in the viewfinder and the distance read off a
scale.  Others were stereoscopic rangefinders that gave a true stereo image
of the target.  A reticle for each eye was fixed in the tube, and formed a
stereo image that appeared to move towards & away from the observer when
optical wedges were rotated.  When the image of the reticles (an arrangement
of diamond shapes,) seemed to be at the distance of the target, the actual
distance to the target could be estimated.
There was extensive research and development on these fire control
instruments during the 1920s, and they were the primary tool used to aim
naval guns through most of this century.  The longest recorded distance for
optical rangefinder controlled gunfire, successfully firing on a moving
target from a moving battleship, is 26,400 yards, achieved in 1940 by the
British.  These rangefinders were designed around a particular gun, and the
distances at which they were accurate were determined by the range of the
gun.  In the U.S. Navy, the Mark 41 (1930s) and Mark 75 (1950s)  had
objectives eleven feet apart, a near focus of 1200 yards, and maximum useful
range of 20,000 yards.  These were made by Keuffel & Esser, weighed about
1200 pounds, and had 147 glass elements, including lenses, prisms, wedges,
reticles, mirrors, and frosted elements.  There were 15 foot models,
weighing about 1500 pounds, in a motorized mount that was connected with
servos to a gyroscope, to maintain the horizon at a level.  The 11 and 15
foot models could be targeted on aircraft, and longer instruments were used
to range ships and targets on shore.  Larger models were made by Bausch and
Lomb, including the 26.5 foot used with the common 16 inch guns.  The Mark
52 consisted of a 25 power system with objectives 46 feet apart, weighing
10,500 pounds and costing about $100,000 during World War II.  Near focus
was 5,000 yards, maximum use at 45,000 yards.  
One interesting aspect of later rangefinders is that they were gas charged
with helium, since it is the only gas with an index of refraction that does
not change in the temperature range encountered by these instruments, and
the extreme length of the rangefinders mandated this stability.  Helium can
leak through steel, and necessitates yet another level of maintenance for
personnel.
These instruments were closely held secrets during their era (still used in
foreign fleets,) and their size and weight ensured their dismantling on
retirement.  Very few persons have had the privilege of viewing through one,
and the effect can only be imagined.  
\\\\\\\\\\\\\\\\\\\\////////////////////
the history of the telescope, the microscope,
   and the prism binocular


------------------------------