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    Re: Sextant Error and Whole Horizon Mirrors
    From: Jared Sherman
    Date: 2002 Jul 9, 21:55 -0400

     I looked at my sextant after reading your message (can only take sights on 
    Canadian Forest Fire Smoke tonight) and realize that although both the index 
    and horizon mirrors do have a "tilt" adjustment, the two have their 
    adjustment screws set at different points (i.e. one at 3 o'clock, the other 
    near 8 o'clock) so that no matter how the two are adjusted, there is really 
    no provision for making them completely co-planar in left-right alignment. (I 
    *think* my geometry and physics are good enough to be sure of that.)
     That would mean that unless you can find a particular model of sextant where 
    the tilt alignment screws are both at 3 or 9 o'clock, you would have to 
    depend on the alignment of the frame as a whole--or use a split mirror, or 
    place a vertical mark on your whole mirror, such that you are only working on 
    one vertical line (i.e. where a split would be) rather than across the whole 
    width of the mirror.
     I would think that a craftsman with very good hands could either adjust one 
    mirror frame, or (safer) shim the mirror out as needed to try making it more 
    fully parallel to the other mirror.
    I wonder if this error is simply inherent to whole mirrors and ignored in 
    them, or if any maker has taken pains to allow it to be corrected out?

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