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    Mirrored Artificial Horizon
    From: Kieran Kelly
    Date: 2003 Nov 2, 10:51 +1100

    For many years I have used a Zeiss Artificial Horizon in the bush for
    celestial navigation and position fixing. It has a standard three- legged,
    adjustable mount and came supplied with a dark, machine-ground glass
    reflector, for sun observations. Needing to do star observations I had made
    a front-silvered mirror from a piece of float glass. This was obviously not
    machined but as the glass was floated when molten, it should have been
    reasonably accurate due  to the force of gravity on a liquid. To do star
    sights I simply removed the dark glass plate from the frame and inserted the
    front silvered mirror. This arrangement has given great service over the
    years and I am normally not out by more than 2 nautical miles from known
    positions on land (arithmetic calculation errors being the exception).
    However, I have often wondered just how flat and accurate is the front
    surfaced, glass mirror. I recently took it to an optical engineering
    workshop here in Sydney who assessed it as being accurate to 7 wavelength's
    of light. They advised that while this was not perfectly level anything
    under 10 wavelengths of light is so accurate that it would not add any
    meaningful error to that already produced by someone holding a hand held
    sextant. It was their belief that even after index error and instrument
    error was accounted for the, minute deficiencies in the sextant would still
    produce greater error than the 7 wavelengths of light in the horizon. Also
    it is clear that the levelling process for the horizon itself i.e. defects
    in the bubble levels that I use and operator error eg in reading the bubbles
    would produce error.
    Does anyone on the list have a comment. What is the error in a ground piece
    of glass? The engineering workshop told me they could get it down to
    practically zero wavelengths. What is a 7 wavelength's error? Can this be
    translated into minutes of arc? Can the error in the horizon mirror be
    eliminated through adjustments in the sight reduction process?
    Your advice would be appreciated.
    Kieran Kelly

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