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    Re: Bris Sextant
    From: George Huxtable
    Date: 2005 Nov 7, 21:53 -0000

    Alex wrote-
    
    > On Mon, 7 Nov 2005, George Huxtable wrote:
    >
    >> It's important, for a sextant, that both lines of view (to Sun and to
    >> horizon) are in the plane of its frame, which is at right angles to the
    >> axis
    >
    > It is important because you have a frame with the scale attached to it.
    > And measure the angles using this scale.
    > It seems to me that in Bris sextant NOTHING is important:-)
    >
    > You just glue three pieces of glass very approximately.
    > The only thing that is important is that the assembly is rigid,
    > the glass panes do not move about each other.
    >
    > Then they will deflect the Sun rays by some fixed angles,
    > you do not care much what these angles exactly are.
    > (You will just measure them when you graduate the device).
    >
    > Then the only point is that these angles remain fixed. And that's it.
    > As I said in my previous message, my own Bris is made in a very sloppy
    > way.
    > The beauty of the invention is that IT DOES NOT MATTER,
    > and does not affect precision.
    
    ==============
    
    Reply from George.
    
    I wonder...
    
    I don't think collimation error, for a sextant, has anything to do with the
    fact that the angles are measured againnst a frame in a particular plane.
    
    Here's a thought-experiment, which I don't propose to put into effect. You
    could modify a sextant by gluing its index mirror in place,   with, say, 15
    degrees between the two mirrors. In that  case it could only measure one
    altitude, of 30 degrees.  And you could then saw off the telescope, the
    index arm and the arc, which would now be redundant, and nearly all of the
    frame; just leaving enough in place to locate the two mirrors to each other,
    glued in place. Somewhat like a Bris, it would be, then, wouldn't it?.
    
    And yet we know, for a sextant, that collimation error arises unless the
    view line from the eye is parallel to the frame carrying the arc. Now, we
    have no such frame or arc, just two mirrors fixed with respect to each
    other. But it's still the same instrument, except that it can measure only
    the one altitude of 30 degrees. By sawing off the frame, we have done
    nothing to make it insensitive to collimation error, which occurs when the
    viewline from the eye is tilted. Tilted with respect to what, though? All
    that's left is two mirrors, defining a line in space where their planes
    would intersect. The observer's viewline must be in a plane, perpendicular
    to that intersection line. Otherwise, collimation error will result. It's
    related to the geometry of the light-paths; nothing to do with the arc and
    frame.
    
    Now, how does that situation differ from a Bris device measuring a similar
    angle by reflections between its glasses? In principle, not at all. That,
    too has a plane, which is defined to be at right angles to the line of
    intersection.
    
    Looking at the Sun via a reflection in the Bris glasses,  put that
    intersection line parallel to the horizon. Now, yaw the instrument about, so
    that first its right-hand edge, then its left, is closest to the eye. Only
    at one position, when it's exactly at right angles, so the edges are
    equidistant from the eye, will the viewline be in exactly the right
    direction to avoid collimation error. What I ask is this; in use, is that
    optimum orientation obvious to the observer?
    
    In a normal sextant the eye viewline is always constrained, by a telescope
    or at least a peep. Not so, for the Bris. So how does the observer find the
    appropriate yaw-direction in which to hold those glass plates?
    
    It may be that it doesn't really matter, for the following reason.
    Collimation error comes in, in quite a big way, when measuring large
    altitudes with a sextant. For low altitudes, it's rather negligible. We have
    learned, from recent correspondence, that the Bris device is useful over
    only a small range of low angles. Perhaps over those angles, collimation
    error presents no problem.
    
    What I am really arguing against is Alex's offhand dismissal of the
    possibility of the Bris instrument being affected by collimation error.
    
    George
    
    contact George Huxtable at george---.u-net.com
    or at +44 1865 820222 (from UK, 01865 820222)
    or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
    
    
    

       
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