stereo telescopes (long message)

[T3D Peter Abrahams <telscope@xxxxxxxxxx>]

Tech 3d'ers; here is the article that will appear in the next Amateur
Telescope Maker's Journal, posted after clearing with moderator, and hoping
it doesn't cause any anemic e-mail software to choke.

RANGEFINDERS AND 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.
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.  

Bernard Merems of Patagonia, Arizona, is an ambitious ATM who is
constructing a binocular refractor with a widened base between the
objectives.  At Riverside '96, Bernie described his half-finished project.
Two B & L telephoto lenses, 5 inches in diameter, and 40 inches in focal
length, are mounted onto a prism housing so that the objectives are 16
inches apart.  Light from the objectives enters the housing and strikes
first surface mirrors, mounted at 45 degrees from the lens' optical axis, to
converge the light into prisms at the center.  The first prisms are standard
90 degree reflecting prisms, to direct the light back towards the oculars.
These prisms will serve as collimators, by rotating around a vertical axis,
and are an unfinished aspect of the instrument design at this point.
Collimation of twin telescopes is quite difficult, and it is likely that
adjustment about a single axis will not suffice to correct all collimation
errors.  
The light exits these prisms in the same horizontal plane that it entered
the instrument, having been reflected twice, and giving a correctly oriented
image.  However, it was desired to direct the oculars downwards, at 60
degrees to the horizontal, to allow comfortable viewing of the sky.  This
created many complications in anticipating final image orientation.  Bernie
finally consulted R. Buchroeder of Tucson on the subject, and was advised to
purchase two toy periscopes, that allow rotation between the two mirrors.
When held vertically, in using position, and the upper half rotated 360
degrees to scan the entire horizon, the image rotates to upside down when
pointing backwards and back to right side up, when pointing forwards again.
Continued perusal of this phenomenon is thought to give an intuitive grasp
of the complexities of designing image erecting systems.  The final image
erecting design, if first light does not force any revisions, adds a
deflecting prism with two silvered surfaces, with the light exiting at 60
degrees to the horizontal.  A total of four reflecting surfaces in this
orientation should give an inverted and reversed image.  For terrestrial
use, part of the assembly will be replaced with a single prism or flat.  
Inter-ocular distance will be adjusted by mounting all four prisms and the
oculars, as a pair of assemblies onto a sliding track, with right and left
handed threaded rods to change separation.  Two inch oculars will be used.
The 16 inch objective separation places a distant limit on the close focus
of the instrument, for viewing nearby objects would require the telescopes
to swivel inwards towards each other.  This reduces the required eyepiece
travel, for they will not have to rack out for focusing on nearby objects.
Many such details must await final assembly of the instrument.
Telescopes are typically used to increase resolution, contrast, and light
gathering ability.  Their potential for enhancing stereoscopic perception of
depth is a fascinating and overlooked subject.  Any input on the topic is
welcome.

Peter Abrahams

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