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    Re: Finding Distance - The Barr & Stroud Rangefinders by Peter Ifland
    From: Richard M Pisko
    Date: 2007 Jul 01, 12:00 -0600

    On Fri, 29 Jun 2007 05:23:07 -0600, George Huxtable
     wrote:
    
    > The 8-page article Richard refers to is the fourth of a series, by
    > Peter, on
    > rangefinding, in Journal of Navigation (of the Royal Institute of
    > Navigation, based in London).
    > I have scanned and attached extracts which are relevant to Richard's
    > question, which should minimise download time, and I hope will keep
    > within
    > letter and spirit of the rules of copyright. Please tell me if they fall
    > short.
    > Richard wrote-
    >> "Trying to shine a laser through the objective and watching the two
    >> dots converge and diverge on a building half a block away (as I turn the
    >> actuating knob) doesn't work as well as I had hoped."
    >>
    
    > I wonder if he meant "trying to shine a laser through the eyepiece ...",
    > which should provide a useful test, as light will follow exactly the same
    > path, in either direction.
    >
    Thank you George, that is what I meant . . . the laser beam starting at
    the eyepiece and exiting through the two objectives and pentaprisms,
    ending with two spots on the wall.  The distance apart is related to the
    position of the wedge and scale, which is moved by a thumbscrew.  I had to
    flick the laser up and down on the eyepiece to get two alternating spots,
    and move the laser beam left or right a bit to position the left spot on
    the same aiming point on the wall.
    
    
    > I have spent a bit of time studying that diagram and explanation, which
    > has
    > puzzled me, as it seems to have puzzled Richard.
    > At first, I was wrongly thinking of it like a device that follows a small
    > telescope, at the observer's end, just as in a sextant, in which case all
    > the deflecting optics would be handling parallel light-rays, coming from
    > nearly infinity. In which case, because moving the wedge sideways would
    > do
    > nothing to alter the direction of any light, it would have no effect.
    > But it isn't like that, as Richard has probably realised, and as the
    > positioning of the objective on the diagram makes quite clear. The
    > movable
    > wedge system is placed between the objective and eyepiece of the
    > telescope.
    > If we look at Richard's setup, with a laser shining axially through the
    > centre of the eyepiece, along the line of the split between the two
    > views,
    > we can see what happens to the thin laser light-pencil that's shown by
    > the
    > solid line in the right half of Peter's illustration, but with light
    > travelling in the opposite way to what was originally intended. For our
    > purpose, we will here ignore the astigmatisers, that just spread a
    > point-focus into a line.
    > Because the two mirrors, that split the image, are deliberately placed
    > slightly different from 90 degrees apart, the right hand beam
    > intentionally
    > travels a few degrees off-axis, where it meets the wedge. For
    > simplicity, I
    > will assume that the intention is for that ray to be angled to pass
    > symmetrically through the prism, so that it emerges on the other side,
    > now
    > approaching the axis at the same angle. That equality of angles doesn't
    > seem
    > to be crucial, to me.
    >
    I believe the laser beams, or pencils of light from the flag pole, will
    trace parallel lines between the wedge prism and the objective; but will
    follow the same path between the wedge and the eyepiece as the wedge is
    moved.  I also make the assumption that the offset angle of the eyepiece
    prisms is intended to intercept the prism such that the final path is
    close to the center of the objective lens, varying fore and aft according
    to wedge position.  The angle is so small, the wedge seems almost a
    parallel plate.
    
    
    > The light pencil meets the objective at some point across its diameter,
    > and
    > as the wedge moves along its track, that point will shift also.
    > What happens to our light-pencil, as it passes through the objective
    > lens at
    > these different points? To such a narrow ray, the bit of the lens that it
    > crosses looks itself like another wedge of glass, of differing angle. If
    > the
    > ray happened to pass through the pole of the lens (that is, its
    > centre-point) the lens will look like a bit of plane-parallel glass, and
    > the
    > ray will pass through undeflected. If it passes above the pole (as seen
    > in
    > the view of the diagram), then that bit of lens acts as a glass wedge
    > with
    > point-up and base-down, just like the main wedge, and the light will be
    > bent
    > further off-axis. If it passes below (on the diagram) the pole, it will
    > get
    > bent toward the axis. This, I suggest, corresponds to the fanning of the
    > laser light that Richard sees.
    >
    Yes, I think that is the essence of the patent.
    
    I once bought a pair of binoculars for a very low price.  Apparently, one
    of the old fashioned objective lens barrels had been cross threaded into
    the frame.  This resulted in the two eyes looking at different fields of
    view downrange, overlapping so as to enable stereoscopic vision only in
    the center part; and causing a fair amount of eyestrain.
    
    I think that unintentional shift of the center of the objective lens from
    the intended collimation axis in my cheap binoculars is similar in action
    to the designed variable displacement in the Barr and Stroud patent.
    
    
    > Here, the diagram is somewhat deceptive, maybe even wrong. It shows, as a
    > solid line, a ray passing off-centre of the objective, but which appears
    > to
    > be undeflected in angle by the lens. Maybe that's because the deflection
    > would be small, but as it seems to play an important role, it may have
    > been
    > better to play-up that bending.
    >
    I wish they had done so on the first patent drawing I saw; I may have
    satisfied myself that I understood the optics a bit earlier.
    
    > Of course, we have been looking at the path of just one single ray, and
    > in
    > reality the situation is more complex, forming an image by light from
    > every
    > point of the scene, occupying the whole area of the objective. I haven't
    > really attempted to explain the action of the shifting wedge in that
    > real-life situation.
    >
    The field of view is quite limited through the right eyepiece, the one
    which merges the top half of the image from one objective with the bottom
    half from the other, due to the high magnification of the optical system.
    
    > There's another aspect that puzzles me, here. Next, the light passes
    > through
    > a pentaprism, which deflects it through another 90 degrees. But if you
    > look
    > at the way that on-axis light, shown by the dotted line, passes through
    > the
    > prism, and look at the way in which the off-axis laser beam, shown by the
    > solid line differs from it, if that solid line is to be deflected
    > through 90
    > also, it would emerge from the pentaprism diverging from the left-hand
    > beam,
    > not converging as shown.
    >
    Yes, that is true, and I noticed it myself.  The original patent drawing
    showed a plane mirror set at a 45 degree angle, and for that setup the
    solid line was drawn to converge as shown; as the total of the angle of
    incidence and the angle of reflection angle would be less than 90
    degrees.  Plane mirrors were much too sensitive to position errors and
    motion of the frame due to differential heating, the pentaprism was very
    reliable.  They did poorly in one test, the second, by trying to make
    their modified model cheap enough for the military to buy; using plane
    mirrors as I understand it.  Worked fine on the overcast morning,
    performance fell apart in the bright sun.  From then on they ignored the
    competitors pricing, and simply made the best they could.
    
    If a series of parallel lines are drawn from the wedge to the objective
    lens, and proper allowance is made for the bending at the objective, I
    think you will find that the convergance / divergence fan through the
    pentaprism shows upas being reversed to that reflecting off a plane
    mirror.  This doesn't really matter, just mark the scale to read the other
    way, or invert the deflection prism to retain the direction of the scale
    movement.  If necessary, a simple fixed corrective wedge can be placed
    somewhere in the optical train, but I think repositioning the eyepiece
    prisms slightly would do the job.  In practice, for my model, a Zero on
    the moon or on a distance board is obtained by an adjustment at the left
    side, near or on the objective lens.  There is a dark filter at the
    eyepiece to allow the sun to be used for the infinity calibration (zero),
    but I don't trust it.
    
    
    > My final quibble is in the view of the flagpole. Presumably the lenses of
    > the telescope make up a proper high-magnification inverting telescope,
    > not a
    > Galilean with its inherent limits. In which case the image should be an
    > inverting one, which would not affect the operation of the instrument.
    > But
    > the view of the flagpole would be upside down. Am I right about those
    > assumptions? Richard, with his rangefinder, may be able to tell us.
    >
    On mine, there are a series of prisms between the eyepiece and the wedge.
    These also change the angle of the eyepiece so the observer would be
    looking down and forward at about a 45 degree angle to avoid neck strain
    in the infantry and artillery models.  The result is that my view is
    upright and correct from left to right.  On another model, the top half is
    a mirror image of the bottom half, and this is accomplished by either
    leaving out an inverting prism or by adding another; I'm not sure which.
    
    
    > So, all in all, I can't claim to understand all of Peter Ifland's
    > diagram.
    > Far from it. So where does that diagram come from? His caption states
    > "Redrawn from Moss and Russell, 1988, and Anonymous, 1917", and in his
    > references he cites-
    > Moss & Russell's book "Range and Vision - The first Hundred years of
    > Barr &
    > Stroud " (1988), which Richard has already seen in a library.
    > There are two references listed as "Anonymous (1917)", which are-
    > Description of the Barr & Stroud 9 foot horizontal base Self-Contained
    > Rangefinder Type F. Q. 2 No 1905 of May 16y, 1911. Government Printing
    > Office, washington DC.
    > Handbook of Range-Finders, 70CM. and 80 CM.Base For Use Of Infantry and
    > Cavalry. No 1797 of December 9, 1915, Government Printing Office,
    > Washington
    > DC.
    > I hope this, with its admitted gaps, helps Richards quest for
    > understanding.
    > George.
    >
    Thank you very much.  Since your line of reasoning seems to agree with
    mine, and the limited testing doesn't seem to contradict us; I will file
    that little puzzle as mostly solved.
    
    I am sending this after a hasty check.  I can't seem to find the drawings
    I did of the wedge / objective / pentaprism paths; but they helped my
    reasoning.
    
    --
    Richard . . .
    Using Opera's e-mail client since Dialog, "the Dog", died.
    
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