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    Index Mirror Correction
    From: Wolfgang Bosswick
    Date: 2003 May 13, 11:41 +0200
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    Hi, I am a newbe in celestial navigation and currently training with a metal sextant. Since I live quite far from the next usable horizon, I did sun shots using a water filled pan.

    In my first series of shots, I had a quite large error compared to the GPS position (about 20'-25'). Since the reason for the error systematically has been a too large sextant reading (in the arera of 95° to 105° due to the pan method), I checked the index mirror again and gave it a new try to adjust it perpendicular to the arc plane. After that, my shots had been much better in a 2-5' range.

    Has anyone experience with effects of a misadjusted index mirror (and horizon mirror consequently) when measuring large angles? Are there some tricks for practice to easen the adjustment of the index mirror (I found it pretty difficult to adjust to an unbroken arc looking from above through the index mirror). If a few minutes of misadjustment should matter, I think it is quite difficult.

    Any advice would be very much appreciated.


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