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    Re: IE determination Freiberger Yacht Sextant.
    From: Peter Monta
    Date: 2021 Sep 26, 16:54 +0000
    Hi Herman,

    Now I got one other problem, the reflected sun had a little shifted ghost image near it.

    This could be due to non-parallelism of either a second-surface index mirror or a second-surface horizon mirror.  As you mention, you could substitute a bit of front-reflective, first-surface mirror in either or both locations to see if the ghost disappears.

    The ghost image is faint at normal incidence but can get bright and annoying at large angles of incidence (as would certainly happen at the index mirror when the sextant is near its maximum angle).  "Fresnel equations".

    I suppose for symmetry both mirrors must be front or rear reflective.

    I don't think there's any particular relationship---either mirror can be of either type.

    Or is the sextant designed for rear reflective mirrors?

    Or is the little curved solarfilm (see photo) the cause of double sun.

    There's no optical-design requirement for rear-reflective mirrors, although before vacuum coatings were available, that type of mirror was less expensive and more robust.  (Front-surface silver coatings on glass were possible since the mid-1800s courtesy of the Foucault chemistry, but they would have been susceptible to corrosion in a marine environment, I guess.  Another option would have been a metal mirror rather than a glass one, e.g. speculum metal, but those are not as reflective as silver or amalgam and the corrosion problem remains.)

    So a practical problem that more navigators must have solved?

    Probably solved by selecting second-surface mirrors that were parallel within close tolerances.  The test would be as follows:  collimated point source; test mirror at near-grazing incidence; shop telescope.  If I had to guess, I'd say maybe your mirror was replaced at some point.

    By the way, there's no harm in using the sextant even with the ghost image as long as you're consistently using the same image with which you measured the index error.


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