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    Re: Buying a sextant- a cautionary tale.
    From: Bill B
    Date: 2006 Apr 30, 17:49 -0500

    Alex wrote
    
    > You can do the same with two stars at small distance if the
    > "off" part of your arc permits. The "off part" of my arc is
    > 5 degrees. So I can practice this method on any pair of stars
    > whose distance is less than 5 degrees.
    
    Yes, I saw that after my first post.  The only difference here being that we
    are measuring the angular distance of the two limbs of the sun (diameter of
    the sun/body) on and off the arc, where with 1 pair of stars we are
    measuring their angular separation (much like a sun where known SD and 4SD
    helps us determine if our observations are close enough to be valued, we
    could use 2X the predicted separation as a sanity check).
    
    I will have to give that a try.  To satisfy my curiosity, what was you IC
    with traditional sun and stars IE checks vs. your current IC using star
    pairs. If I recall about -0.2 with traditional methods and -0.7 with star to
    star?
    
    If we really want to get anal about it, for a one-star IE check we are using
    about the 0 point of the arc and worm gear, and for sun IE checks we are
    using points on and off the arc approx. 32' away from zero point and 32'/28'
    respectively on the worm gear. In our cases we have ruled out significant
    drum/worm gear eccentricity.
    
    That leaves us with a problem with the arc, which I suspect may be largely
    due to transfer of material from the softer worm gear to the arc.  If we
    always do an equal number of clockwise and anti-clockwise checks that
    transfer should be uniform.  If we always do our checks in one direction,
    wear and transfer will not be uniform, and all it took was an almost
    microscopic spore to shift my IE 1.'2 minutes on the Astra.
    
    Bill
    
    
    

       
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