plural -s

Definition of autocollimation 

: the process of collimating an instrument (such as a telescope) having objective and cross hairs by directing it toward a plane mirror and adjusting the lens and cross hairs so that the latter coincide with their own reflected image

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First Known Use of autocollimation

1870, in the meaning defined above

History and Etymology for autocollimation

aut- + collimation

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Time Traveler for autocollimation

The first known use of autocollimation was in 1870

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to make faulty or ineffective

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