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    Re: Index Error - Real vs. Artificial Horizon
    From: Bill B
    Date: 2013 Feb 28, 17:05 -0500

    On 2/28/2013 4:32 PM, Paul Fithian wrote:
    > For example, if I adjust the horizon mirror so that a near (5' away)
    > horizontal line matches up through the glass and the mirror images,
    > these images don't line up when I view a horizontal line that is several
    > hundred feet away.
    To give you a few parallax examples:
    Lets assume the distance between between the centers of the index mirror
    and horizon glass/mirror is 3 inches. (This is probably not static and
    depends on sextant geometry and front vs. rear silvered index mirror.)
    At 5' (60") using tangent the angle is 2d 51' 45"
    At 300 feet 0d 02' 52"
    At 1 nautical mile it is 0d 0' 9"
    For your immediate needs and the error in the entire system, 1 nm is
    probably sufficient, but not optimal.
    Bill B

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