NavList:

>Why alcohol? Usually a solution of ammonia (spirit of Hartshorn) is
>used to clean glass. Also sold as "optical glass cleaner".
>Sometimes they also recommend soap solution. But why alcohol?

Alex,

Alcohol cuts oily films that may coat the surface by air or finger then it evaporates quickly. Ammonia can react to some optical coatings. Usually there are warnings in the instructions if this is the case. Acetone may also react to coatings and should probably not be used as a cleaner.

Greg Rudzinski

[NavList] Re: Bris does not work
From: Alexandre Eremenko
Date: 26 Jul 2012 12:40
Greg,

> The number of visible Sun images is dependent on the amount of shading.

Yes, of course.
Though I cannot say that I completely understand the theory.
3 images are
obtained by 2 reflections.
If you allow 4 reflections, you get 8 ADDITIONAL images.
According to this count, I must have 3 bright and 8 dim images.
While (few years ago) the total number of images was 8. That is
3 bright and 5 dim. (And this is what the manual says: 8 total).

> Your original model may have gained shading properties
> in the built in shade or become opaque with age.

There are two logically possible explanation:
a) the filter became more dense with age, and
b) the reflecting power of the glasses decreased.

I have no clue to any possible physical reason why could a) or b)
happen. The "sextant" was permanently stored in its box since the last
use.

> Try cleaning with alcohol on all the surfaces.
> Use a string soaked with
> alcohol to clean the internal surfaces.

Why alcohol? Usually a solution of ammonia (spirit of Hartshorn) is
used to clean glass. Also sold as "optical glass cleaner".
Sometimes they also recommend soap solution. But why alcohol?

I understand that this kind of cleaning will remedy both a) and b)
IF the reason is dirt. (How could dirt penetrate there, and what
kind of dirt, I have no clue). The outer surfaces, which are frequently
touched with fingers, I usually just wiped with a piece of cloth.
But I don't see how dirt could penetrate inner surfaces.

Technically this cleaning is not simple, because the gap between lenses
in the original Bris is so narrow that no brush can penetrate there.
And I will need to wipe the surface somehow.

But with all my physics background (limited to what I was taught in
high school) I cannot imagine how a dark glass could increase its darkness
with age:-)

That's why I initialy conjectured that the clear glass somehow
lost its reflective power.

By the way, I wrote an e-mail to the address shown in the manual.
My e-mail returned:-(
The manual says: "Copyright 1998" in it.

The YouTube advertisement says that "you can have it for 200 pounds"
but gives no hint where do you get it from...
Searhing on Yrvind on internet gives some guy who explicitly
says that he does not use e-mail:-(

Alex.

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