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    Re: Bris Sextant
    From: Alexandre Eremenko
    Date: 2005 Nov 7, 17:37 -0500

    Dear George and Frank,
    
    > I'm pretty sure there's a real  collimation issue here.
    
    I agree it is similar to collimation in its physical origin,
    but my point is that it does not depend on how the device is
    MADE, it depends on how you hold it. And the analog of
    "roking" I described earlier permits to deal with this
    in a similar manner as in the ordinary sextant we deal
    with another sort of error coming from "non-verticality"
    of the frame plane.
    
    > Also, I notice with my mockup  that the
    > reflected images are all doubled (presumably from the non-zero  thickness of the
    > glass).
    
    Or from prismatic effect. The ray is reflected from BOTH surfaces
    of the glass pane outer and inner surface, so if these surfaces are
    not perfectly parallel you will see a doubling.
    
    Earlier I noticed that my Bris "does not conform to the theory"
    and attributed this to prismaticity of the glass panes.
    Now I think this explanation was wrong, because this
    "prismaticity" will have an effect of doubling which I do not
    see in my device.
    
    So I think that it does not "conform to the theory" because
    the two intersection lines of the planes of the three glass panes
    are not parallel.
    
    This also would explain another "deficiency" the 8 images if Sun
    that I see are not aligned on one vertical line.
    
    However I don't see how it could affect precision, and don't think it
    does.
    
    On my opinion, the only limitation of precision is the absence of
    the scope. If one attaches the scope, the attachment should certainly
    have a collimation adjustmeent (rotating the Bris part around a vertical
    axis). With such adjustment, I do not see why Bris device will be less
    precise than ordinary sextant.
    (Of course I do not mention again its other limitations).
    
    Alex.
    
    
    

       
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