User`s manual

THE ELEVATION ERROR
When using an angular prism, the built in prism deflects the beam 90°
unregarding the angle of incidence, whitin certain limits. The elevation error
occurs when you rotate the prism. In the picture below, the prism is rotated
180° around the blue dotted line, where you reach the maximum error
(measure A). The closer to the centre of rotation you are able to aim the
beam, the smaller elevation error. With the FD15 you are able to aim within
±0,05 mm (2 mils) eliminating the effects of the elevation error.
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