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    Re: Back sights
    From: Brad Morris
    Date: 2010 Mar 22, 11:44 -0400

    In answer to my own question, I found the following
    About midway down the page, the author has drawn the optical path for a foresight 
    and a backsight observation.
    It appears that the paths are parallel but not co-linear.  The optical path for 
    the backsight mirror to the index
    mirror is to the inside of, but does not cross the optics of, the foresight horizon mirror.
    Best Regards
    -----Original Message-----
    From: navlist-bounce@fer3.com [mailto:navlist-bounce@fer3.com] On Behalf Of Brad Morris
    Sent: Monday, March 22, 2010 9:27 AM
    To: NavList@fer3.com
    Subject: [NavList] Re: Back sights
    In adjusting the octant or sextant for index error, there is nothing to stop you 
    from adjusting or determining
    it on land and then using that value at sea.  Being as land based as Chauvenet, 
    all my star-star sights are on
    a stable platform.
    There is another dilemma concerning the BackSight Octant that I am attempting to 
    resolve in my head.  The
    octant has only one arc.  That is, the scale on the arc is singular.  There aren't 
    two scales, one for foresights
    and the other for backsights.
    For a foresight, for a given angle, the nonius will  indicate that value.  But the 
    backsight is
    to be used to measure the same angle.  I believe that the index mirror does not 
    move, because the nonius
    itself cannot move, else it would indicate a different angular value.   What 
    therefore, is the arrangement
    of the backsight mirror relative to the foresight mirror?
    The backsight mirror must be parallel to the foresight mirror, in order to obtain 
    the same angular value.
    The optical path from the backsight mirror to the index mirror must then somehow 
    cross through the
    foresight horizon mirror.  Is the backsight mirror slit further away from the plane of 
    the arc, permitting
    the optical path to go through the clear part of the horizon mirror?
    If the paths are parallel but not co-linear, then the index mirror must be larger 
    to support this offset.
    Is there some graphical optical path explanation existent, one perhaps for a real 
    octant we can examine online?
    Best Regards
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