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TM 9-254
10-10.
Lens Bench - Continued
(9)
Eccentricity. A lens is said to be concentric when the geometrical or mechanical axis of the lens
coincides with optical axis. When a lens is concentric, there will be no perceptible movement of the
image when the lens is rotated about its geometrical axis. Conversely, when these two axes do not
coincide, the lens is eccentric, and the amount of eccentricity is proportional to the movement of the
image when it is rotated about its geometrical axis. When the optical bench is used to measure the
eccentricity of a lens, the regular chuck in the nodal slide is replaced by a special chuck, which will
permit the lens to be rotated freely on an axis parallel to the ways of the bench and coincidental with the
axes of the collimator and microscope. When a lens is mounted in the chuck and is rotated through 360
degrees, the image of the intersection of the cross lines will trace a circle against the target, as seen in
the observing microscope. The radius of this circle is directly proportional to the eccentricity of the
element under observation. A convenient target for measuring the actual amount of eccentricity
consists of a group of twelve concentric circles whose centers are at the center of the target and whose
radii will indicate a series of deviations from 1 to 12 minutes for the particular focal length of the
collimator being used.
10-13/(10-14 blank)
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