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    Index error adjustment...
    From: Rick Emerson
    Date: 1999 Jan 26, 11:56 EST

    Russell Sher writes:
     > Hi - With all the talk on sextants, perhaps I can squeeze in a question
     > about index error adjustment;
     > I have an instrument which has two right-angle shaped brackets for assisting
     > with index glass adjustment - With the instrument held horizontally, these
     > calibrating brackets are placed on the arc so that both the reflected and
     > true images are seen. The relative heights of these are adjusted into
     > co-incidence with the adjusting screw to give perpendicularity. When I
     > removed these brackets and eye-balled the arc directly, after performing the
     > adjustment, I noticed that there was still some error - In other words,
     > these two methods are possibly not as reliable as each other- OR is this a
     > case of parallax error, or the way which I am viewing the arc? Celestaire
     > (www.celestaire.com) market calibrating cylinders for the same purpose -
     > does anyone have any experience with these? Any comments etc.
    Without seeing what you're doing with the brackets, it's hard to
    suggest a remedy - if one's needed.  We can talk about this off-list
    if you want.
    As to Celestaire's cylinders, save your money and get a set of dice
    from a toy store.  Dice are machined to surprisingly close tolerances
    (or the results aren't random as they should be).  Made of plastic,
    they don't rust and replacement costs are minimal.  I keep a set in
    the sextant box for use as calibration tools and, if all else fails in
    reducing a sight, as decision-makers... [wink]
    Rick
    S/V One With The Wind, Baba 35
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