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    Re: Bris does not work
    From: Greg Rudzinski
    Date: 2012 Jul 27, 09:25 -0700


    Plath and Yrvind got together on the material for the original Bris. The first surface coating is the same technology that Plath uses on the whole horizon mirror. I have a Plath with a whole horizon mirror and it works surprisingly well except at early twilight and for very dim stars. I'm not sure where I read about this. The Plath whole horizon mirror is explained on their web sight as I recall.

    Greg Rudzinski

    [NavList] Re: Bris does not work
    From: Alexandre Eremenko
    Date: 27 Jul 2012 12:00

    > The original Bris
    > has mirrored first surfaces

    Where do you know this from?
    I do not see it on my Bris (which I suppose to be original:
    I bought it is the Cassens Plath office, and the manual has the address
    of the manufacturer. The address is not valid anymore, but this was more
    than 5 years ago).

    It looks like plain glass.

    > so the bright dim pattern would be expected to
    > differ vs. plain glass which is what I use on the camera filter.

    It will also differ if the angles between surfaces are different.
    I suppose that these angles are random (provided they are small enough),
    so each individual Bris has to be graded by observation.

    > The
    > mirrored first surface reflection also gives a true reflection
    > as compared
    > to glass which reflects front and back overlapping
    > reflections resulting
    > in fuzzy asymmetry.

    Could you explain more detail? There are many things which I don't
    understand in this sentence.
    1. First surface. You mean the surface of a glass plate through which
    the light enters?
    2. I suppose that 1 is correct. There will be also a reflection from the
    back surface (inner reflection). However I suppose that the glass plate
    has parallel surfaces. So the influence of the back surface will not
    differ from the influence of the first one. It will reflect parallel rays
    EXACTLY in the same way as the front surface.
    3. Are you really sure that the glasses in the original Bris are
    coated (silvered?)

    If 3 is true then this is indeed an unfortunate design.
    The salt water will certainly penetrate and destroy the silvering.
    Which is almost impossible to clean.

    Ordinary sextant front silvered mirror I can rinse and wipe.
    These surfaces are practically impossible to wipe.


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