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    Re: Index mirror type
    From: Alexandre Eremenko
    Date: 2019 Mar 19, 00:10 +0000

    You noticed correctly one of the advantages of front surface mirrors: they reflect more light.
    Another advantage is that if the two surfaces are not strictly parallel, there will be
    additional distortion in the back silvered mirror as the light might refract 
    when passing the front surface by
    slightly different angles.
    The disadvantage is the vulnerability of the silver coating which is exposed and unprotected.
    Not only to scratches but also to salt water drops.
    On he back silvered mirror the silvering is protected by coating.
    Why do you think that front silvered mirror is more expensive, I do not know.
    It seems equally easy to make.
    To remedy the vulnerability of the front silvered mirror, Soviet SNO-T comes with a spare mirror
    as a standard part of the kit.
    On my opinion, all these things are very minor. When buying an antique sextant on e-bay I always worried
    about the state of the silver coating. But once I tried a sextant whose 
    coating almost completely deteriorated,
    and it worked just fine, with the Sun at least.
    In some old books where they describe re-silvering of old mirrors, I found the advise just to blacken the
    back surface, instead of re-silvering. This should also work fine. (Blackening is desirable to suppress
    the double reflection, from both surfaces. But reflection from the unsilvered 
    front surface is usually sufficient.
    From: NavList@fer3.com [NavList@fer3.com] on behalf of Igor S. ["NoReply_IgorS."@fer3.com]
    Sent: Monday, March 18, 2019 6:16 PM
    To: eremenko@math.purdue.edu
    Subject: [NavList] Index mirror type
    Hello everyone,
    I don't know if somebody have noticed that index mirror in different sextants 
    are of different types namely either first or second surface mirrors. I've 
    made some comparison photos (Tamaya vs. Freiberger sextants) from where it is 
    clearly can be seen. As we see Tamaya has second while Freiberger (and also 
    Russian sextants) firs surface mirrors.
    First surface mirror reflects a bit more light which is a slight advantage for 
    star sights but it is more prone for scratches and is more expensive.
    Any more advantages or disadvantages of either?
    kind regards,
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