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    Re: Index Error and Micrometer Drums
    From: Lu Abel
    Date: 2011 Jan 3, 17:14 -0800
    One thing to remember is that any sextant (but particularly the plastic ones, I haven't experienced the problem with metal sextants) is the problem of "lash" in the micrometer and teeth of the mechanism.   The practical implication of lash is that one may get slightly different readings bringing the body up to the horizon vs bringing it down. 

    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.


    From: Greg Rudzinski <gregrudzinski@yahoo.com>
    To: NavList@fer3.com
    Sent: Mon, January 3, 2011 4:23:07 PM
    Subject: [NavList] Index Error and Micrometer Drums

    The recent ex-meridian/ LOP exercise which had all my LOP's away from GPS had me back out taking Sun lines to investigate a possible explanation. What I discovered was the bad habit of heavy fingers on the micrometer drum during the index error zero process. My advice to all is to use light fingers when operating the micrometer drum (especially for plastic sextants). Sometimes this is easier said than done when conditions are windy and the deck is tossing. The next best thing to do is to preset the sextant to a close altitude and let the body set or rise to the horizon.

    Greg Rudzinski
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