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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
To: NAVIGATION-L@LISTSERV.WEBKAHUNA.COM
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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