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    Re: Bris does not work
    From: Alexandre Eremenko
    Date: 2012 Jul 27, 17:11 -0400

    You should have told me earlier!
    I would not clean a coated glass using a needle.
    > Alex,
    > 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
    > Greg,
    >> 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.
    > Alex.
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