DAVIS Mark 3 Sextant - Manual (Page 4)

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MARK 3 SEXTANT ADJUSTMENT
Adjusting your Mark 3 Sextant is easy and should be done each time it is used.
All adjustments are made with the index mirror, the large movable mirror at the
pivot of the index arm (it is not necessary to adjust the small horizon mirror, as
the unit construction makes it impossible to be very much in error). On a correctly
adjusted sextant, the index mirror is perpendicular to the frame and becomes par-
allel to the horizon mirror when the sextant reads zero.
First, adjust the index mirror for “side error” by making it perpendicular to the
frame. Holding the sextant in your right hand, raise the instrument to your eye.
Look at any horizontal straight edge (the sea horizon, for example, or the roof of a
building al least one mile away) and move the index arm back and forth. The real
horizon will remain still while the mirror horizon will appear only when the scales
read close to zero. Line up the mirror horizon and the real horizon so that both
appear as a single straight line (fig. 3).
Now do a vertical adjustment. Without changing the setting, look through the
sextant at any vertical line (a flag pole, for example, or the edge of a building) and
swing the instrument back and forth across the vertical line. If the index mirror is
not perpendicular to the frame, the line will seem to jump to one side as the mirror
passes it. To correct this, slowly tighten or loosen the screw closest to the frame at
the back of the index mirror until the vertical line no longer appears to jump (fig. 4).
Figure 4
Mirror horizon is not aligned with
the real horizon—index arm is not
in proper position.
Index mirror screw
too tight.
Index mirror screw
correctly adjusted.
Index mirror screw
too loose.
Mirror horizon and real horizon
form a single straight line—index
arm is properly positioned.
Figure 3