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    Re: Some videos about sextants
    From: Bill Morris
    Date: 2011 Jan 10, 13:06 -0800

    Tom Cunliffe's yachting monthly video on the sextant gives advice that is not only individualistic. The advice is plain wrong.

    The order in which adjustments are made _is_ important :
    1) Perpendicularity of the index mirror first, because horizon mirror adjustments depend on it being perpendicular to the arc. This is easier to do without the telescope in place. It has nothing to do with the frame being warped.
    2) Side error next (this places the horizon mirror parallel to the index mirror in one plane).
    3) Index error last(this places the mirrors parallel at a zero reading). This may re-introduce some side error. Correct it if you like, but then re-check index error.

    Side error is relatively unimportant. It is given by the equation:
    error in minutes = 1/6876 x k^2/tan d, where k is the lean of the mirror in minutes and d is the observed angle in degrees.

    Most modern sextants have no means for adjusting telescope collimation. Collimation error has to be fairly gross to cause much reading error, which gets worse as the sextant reading increases. Even so, a lean of the optical axis of the 'scope of half a degree, towards or away from the plane of the arc, with a reading of 80 degrees introduces only 13 seconds error.

    William Henry Simms, the son of the great instrument maker William Simms, gives the error in reading as being too large by an amount = (lean in minutes)^2 x sin 1" x tan 1/2(observed angle in degrees) x 3600.

    Bill Morris
    Pukenui
    New Zealand


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