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    Re: Index Error and Micrometer Drums
    From: Greg R_
    Date: 2011 Jan 3, 19:34 -0800

    On 1/3/2011 5:14:52 PM, Lu Abel (luabel@ymail.com) wrote:
    > To ensure good readings one should always bring the body to the horizon in
    > a consistent direction (either up or down) AND one should measure Index
    > Error exactly the same way (ie, if you measured IE by bringing the
    > reflected horizon down to the viewed one, you should always bring bodies
    > down to the horizon).
    > If your observation choice is to bring the body down to the horizon and
    > you somehow overshoot, bring it back up to well above the horizon and then
    > bring it back down again to insure consistency.
    Good advice, though when I was first learning how to use my sextant (Astra
    IIIB) a few years ago I did a series of test shots (on several different
    days and locations) to see which technique would yield the best results -
    bringing the body down to the horizon, bringing it up, and also just for
    fun - "rocking" the drum back and forth to bring it to the horizon.
    I expected the "rocking" sights to give the worst results, but I really
    wasn't able to find any meaningful difference between the techniques (using
    a GPS position as an AP and the intercept computed by the Navigator software
    to score the accuracy of the sight), all of the errors were between a few
    miles and a fraction of one (with no real discernable pattern as to which
    one was most accurate).
    Though there also may be too many variables at play here to draw any
    meaningful conclusions (the geartrain parameters on mine might not match
    those of someone else's, sharpness of the horizon on a given day, other
    random personal errors from sight to sight, etc.).
    I eventually settled on a technique that Greg Rudzinski described: Initially
    bringing the body down to just slightly above the horizon, and then letting
    the Earth's rotation bring the body to the horizon - when I'm "in practice"
    that usually yields average accuracies of a mile or less when shooting from
    a dry land location with a sea horizon.

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