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    Question about horizon mirror unsilvered half
    From: Joe Wong
    Date: 2024 Jul 9, 02:43 -0700

    Hi there folks, as a landsman, I've owned and operated a few sextants in recent years after I picked up celnav as a hobby back in 2019. One thing that baffles me is the different horizon glass configurations seen on various types of sextants, which leads to my question: Is the unsilvered half on horizon mirror really a necessity? 

            The Tamaya sextant that I own does not have such transparent half at all (only a black shade fitted on top of the horizon mirror to block sun's excessive glare) and I always get satisfying LOPs after careful observation (LOPs fell within 1 nm every time when used in conjunction with the AH,sometimes intercept could come as small as 0.1 nms, Actual GPS coord. used as assumed pos.)

            By my own experience,I would assume that having or not having a transparent half on horizon mirrors would not affect actual operation and accuracy at all. 

            I also owned a well-used Astra,this one has a circular frame to secure the horizon mirror,  the transparent half is present on the horizon mirror. Another thing that makes the Astra and Tamaya different would be the location of the horizon mirror adjustment screws. On the Astra, the adjustment screw is at the very top of the transparent half, while on the Tayama, the screw is fitted on the top edge of the horizon mirror itself, closer to the sextant frame.

              It appears that horizon mirror with an unsilvered half would a least benefit in adjusting side errors, since an adjustment screw fitted farther from the frame would allow a finer adjusting movement.

    Quite recently I acquired a Francis Barker box sextant(see attached pics), it has some issues which I would elaborate on a later thread. The box sextant is essentially a miniature but still, a full-fledged instrument with every part you would expect to see on a standard mariner's sextant. 

            Returning to the issue, the box sextant has a full horizon mirror with the unsilvered part present. Yet the horizon mirror is frameless, no adjusting srew is visible over the mirror, the side error is then adjusted and eliminated through rocking or tilting the entire horizon mirror assembly at the base(the assembly has two protruding nibs at the bottom act as pivots) 

           So what would be the logical explanation as to why certain sextant models decided to retain the unsilvered half even though it does not seem to benefit anything (like the box sextant)? Much obliged.



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