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Alex,

On this topic, you noted the following: "it seems that the ideal solution would be a front silvered half mirror. But I have never seen such arrangement, and I do not know why." Such an arrangement actually exists on the Freiberger Yacht Sextant (https://www.fpm.de/index.php?option=com_virtuemart&view=productdetails&virtuemart_product_id=32&virtuemart_category_id=2&Itemid=264&lang=en). Newer models have a front-silvered horizon half-mirror, and I'm a big fan of that layout (I've been told that older models of this sextant didn't have front-silvered mirrors, but that was apparently changed several decades ago). I've taken numerous sights with mine, including lunars and sights at sea, with excellent results. I haven't seen any drawbacks of it having a half-mirror versus a traditional split horizon mirror: Even without a clear-glass half, the view through the telescope shows partially overlapping views of the horizon and celestial object. And althought it has a slightly smaller radius than a full-size sextant and a slightly lower theoretical precision, I haven't seen any actual impact on accuracy in practical use.

Denis