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    Error of Perpendicularity
    From: Robert Eno
    Date: 2006 Apr 17, 13:25 -0400
    I recall asking this question before so if I did, I apologize for repeating myself. It seems to me that I never received a definitive answer.
    What is the practical consequence of the index mirror not being perfectly perpendicular to the frame of the sextant? I have dozens of navigation texts, some of them dating back quite a few years and while all of them direct the navigator to check for it, none of them reveal the consequences of not having the index mirror perpendicular to the frame of the sextant.
    Furthermore, I know of a few techniques for adjusting the index mirror for perpendicularity, including the eyeball method and placing objects (such as a pair of dominos) of exactly the same height at opposite ends of the sextant limb but it seems to me that these methods are rather coarse and not nearly as precise as those for reckoning index and side errors.
    Any comments on this most fundamental topic are welcome.
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