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

Alex.

> 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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