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    Re: Vernier sextant adjustment?
    From: Bill Noyce
    Date: 2003 May 8, 13:58 -0400

    First of all, understand what is meant here by "the plane of the
    instrument."  It's the plane containing the circle through which
    the index arm moves.  Both the index mirror and the horizon mirror
    are adjusted to be perpendicular to this plane (that's a separate
    topic...).  If the telescope points toward this plane, or away from
    it, then its field of view will move less than it is supposed to
    when the index arm is moved.  Thus, we need ot adjust it to be
    parallel to the plane.
    I haven't seen the special "telescope containing the two parallel
    wires," but from the description I think this is how it works.
    The wires are parallel to the plane of the instrument (i.e. they
    are vertical when the sextant is held in the normal way).  One
    is well to the right of center in the field of view, and one is
    an equal distance left of center.  Viewing the pair of stars
    at the right-hand wire is the same as looking through a telescope
    that points too far to the right (or to the left, taking into
    account the telescope's reversal of an image?).  Similarly for
    the other wire.  Each ought to make the measured angle too large,
    since both sights are slightly out-of-plane.  Whichever one makes
    the measured angle larger is the one that's more out-of-plane,
    showing which direction to adjust the mounting.
    I assume you read the measured angle so that if the other sight
    shows a displacement, you can move the arm to figure out which
    star is which (i.e. whether to increase or decrease the angle
    to make them match).
            -- Bill
    -----Original Message-----
    From: Royer, Doug [mailto:doug.royer@REMEC.COM]
    Sent: Thursday, May 08, 2003 1:13 PM
    Subject: Vernier sextant adjustment?
    I was looking through an old navigation book last night and came to a
    discriptive paragraph of telescope adjustment for a sextant.This book was
    written in 1918 and all the illustrations of the sextants were of the
    vernier type.I was hopeing some of you who are familiar with older
    instruments can understand what the instructions mean and can explain them.I
    will write it directly from the book.
    From "Elements of Navigation and Pilotting" by Lt. W.J. Henderson, A.M. :
    section IV. The line of sight of the telescope must be parallel to the plane
    of the instrument."Screw in the telescope containing the two parallel
    wires,and see that they are turned untill parallel with the plane of the
    sextant;then select 2 stars,at least 90* apart,and make an exact contact at
    the wire nearest the plane of the instrument,and read the measured
    angle.Move the sextant so as to throw the objects on the other wire,and if
    the contact is still perfect,the axis of the telescope is in its right
    situation and the telescope adjustment is correct.If the images have
    seperated,it shows that the object end of the telescope droops toward the
    plane of the sextant,and if the images overlap, it proves that the object
    end of the telescope points away from the plane of the instrument.This will
    be rectified by the screws in the collar of the sextant.A defect in the
    telescope adjustment always makes angles too great"(Patterson)
    What are the wires he talks about?What is accomplished by reading the
    measured angle?

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