A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Philippe Posth
Date: 2019 Mar 19, 03:12 -0700
Theorically, the advantage of the first surface mirror is to avoid the light to pass through the glass and avoid one risk of refraction error.
Practically, I'm not sure that the difference is visible for a "normal" user.
However a more important thing is to verify if the silvered surface is exactly at the same place than the alidade axis. If not the difference can generate some little errors in measures.
Some years ago, the index mirrors of Astra sextants was silvered on first surface. This is not the case anymore.