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    Re: Finding IC for A-12 bubble sextants
    From: David Pike
    Date: 2021 Oct 5, 14:17 -0700

    Ed Popko you asked: Can anyone suggest a technique for determining the IE of an A-12 bubble sextant?

    Ed

    The simplest way is taking shots alongside Frank’s GPS Anti-Spoof programme.  Either that or using precalculated shots.  If you do it around meridian passage the body stops within a minute or so for quite a while.  Unlike a Hughes MkIX onwards, you don’t need a correction for every ten-degree slot with an A12.  Any altitude will do to calibrate an A12.  If you know your latitude exactly, and ideally mer-pas time, you don’t really need anything else.  A centre shot on the Sun is fine.  Unless you’re a dab-hand with a Bristol key on the index mirror, be prepared to work with large values of IE.  20 minutes is about the best I’ve managed hand-held.  One day I’m going to make a clamp on a stand.  That might allow me to set the index mirror more accurately, but “hey”; it’s just as easy to add or subtract 30’ as it is 1 or 2’.  We're talking 80 year old aircraft sextants here, not the latest state-of-the-art marine sextants. In all cases, it’s desirable to have an assistant to note the time and record the GPS Anti-Spoof and sextant readings. DaveP

       
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