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In most observations one uses a filter in front of the index glass.
It certainly absorbs much more light than you can save by silvering, so the difference between
silvering front or back surface is really non-significant.

The only case when a filter is not used is observation of a faint star.

Alex.
________________________________________
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.
https://en.wikipedia.org/wiki/First_surface_mirror
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,
Igor
File:

[http://fer3.com/arc/imgx/Tamaya_index_mirror.jpg.thumb.jpg]
[http://fer3.com/arc/images/dl-icon.png]
File:

[http://fer3.com/arc/imgx/Freiberger_and_Tamaya.jpg.thumb.jpg]
[http://fer3.com/arc/images/dl-icon.png]
File:

[http://fer3.com/arc/imgx/Freiberger_Index_mirror.jpg.thumb.jpg]
[http://fer3.com/arc/images/dl-icon.png]

   

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