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These very
accurately machined cylinders are used to remove the error of perpendicularity
of the index mirror of the sextant. By placing the cylinders on the arc
of the sextant while laying on its side, and bringing them into conjunction
through the optical system, the index mirror can be adjusted to make it
perfectly perpendicular to the arc.
Sextants
having front surface silvering on the index mirror (such as C. Plath, Cassens
& Plath, Freiberger, and Astra IIIB Deluxe) benefit most from these
cylinders. This is because the silvering is not exactly on the axis of
rotation, and requires that the line of sight be parallel to the sextant
frame when adjusting the index mirror.
The cylinders
come in a velour pouch and can be readily carried in the sextant case.
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