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    Re: Sextant "side error" and star-star sights
    From: Alexandre Eremenko
    Date: 2004 Dec 5, 15:59 -0500

    On Sun, 5 Dec 2004, Frank Reed wrote:
    > Sextant users are advised to eliminate "side error".
    > This the little
    > left-right gap that shows up between the direct and reflected
    > image when the  sextant
    > is set to zero. Side error is eliminated when the index mirror
    And also the horizon mirror.
    With my sextant, I find it much harder to adjust the horizon
    mirror perpendicularity than that of the index mirror.
    (Because the screw on the horizon mirror is over-sensitive)
    In practice, I always have a very small side error due to
    the horizon mirror, just because I am unable to adjust
    the horizon mirror precisely.
    Chauvenet has a formula:
    err=-2l^2 sin(1')tan (A/4),
    where err is the error measured in minutes,
    l is the inclination of the index mirror in minutes, and A is
    the measured angle.
    But this formula assumes that the horizon mirror is perfectly
    according to the usual procedure and its inclination is EQUAL
    to that of the index mirror.
    (Which means that the images of two stars would exactly coincide
    when the reading is zero).

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