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    Re: Bris Sextant
    From: George Huxtable
    Date: 2005 Nov 8, 09:56 -0000

    Frank Reed sent a posting yesterday, most of which I  agree with, but not
    this bit-
    > Also, I notice with my mockup  that the
    > reflected images are all doubled (presumably from the non-zero  thickness
    > of the
    > glass). This effectively "blurs" the reflected image though  with careful
    > observation it should be possible to deal with this.
    Alex got it right in responding-
    >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.
    As long as the two surfaces of each glass are parallel, there should be no
    doubling of the image due to reflections from its fore and aft surfaces.
    Those reflections should coincide exactly. However, if it's low-quality
    glass, with some tapering of its thickness, that would mean the two surfaces
    were non-parallel, and you would see a doubling, exactly as Alex suggests.
    That would show up if you just took a single piece of that glass, and looked
    at the Sun reflected in it. It's the taper, not the non-zero thickness, that
    causes the doubling.
    But there are other possibilities with a Bris device, depending on the
    combination of subtended angles that were chosen. It the two gaps were
    chosen with a nominally-equal angle of parting, then small differences
    between the actual angles would show up as such a doubling. Similarly, if
    angle is nominally (but not precisely) set to be twice the other, then light
    rays undergoing a double-bounce between one pair would almost coincide with
    rays making a single-bounce between the other. No doubt there are many such
    possibilities; which implies that certain angle-combinations (including
    equal, and also 2:1)  are perhaps best avoided, because otherwise confusion
    will result. That must mean avoiding angle-pairs in which n x A1 equals, or
    nearly equals, m x A2, where n and m are small integers and A1q and A2 are
    the subtended angles. In musical terms, that's calling for a discord rather
    than a chord.
    What were the nominal subtended angles, when Frank saw that
    On the question of collimation error, and the necessary rocking to minimise
    it, the three of us (Alex, Frank, George) now seem to be in accord, it's
    pleasing to note.
    contact George Huxtable at george@huxtable.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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