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    Re: sextant precision.
    From: Bill B
    Date: 2005 Jun 23, 16:14 -0500

    >> I am coming around to your way of thinking on sextant mirrors.
    >
    > How gratifying it is to find a sinner who sees the light by thinking about
    > it and convincing himself...
    
    Careful George. "If you make people think they are thinking, they will love
    you; but if you make people really think, they will hate you."   I do not
    know the source of the quote, but have always liked it and valued people who
    do not resent being made to think.
    
    > As a general rule the mid-point of the reflecting plane of the index mirror
    > is indeed aligned with the pivot axis of the index am. But it's in no way
    > crucial that the axis should be placed exactly there, in the reflecting
    > plane or across the middle of the mirror's area.. If it wasn't, then as you
    > shifted the index arm, that mirror would be displaced sideways a bit, and
    > in the end you might lose a bit of light because one edge would move in and
    > shrink the width of the light-path to the horizon mirror. But it wouldn't
    > alter the sextant readings, not a jot.
    
    Understood, and a nice breakthrough in my thinking.  Somehow I felt it had
    to be that way because traditional sextants were designed so the axis was in
    the center of the mirror vertically AND in line with the rear-silvered
    surface.  So I was always suspect of front-silvered mirrors in the same
    mount as rear-silvered, but took it on faith until now.
    
    > Alex Eremenko has commented that in his Russian SNO sextant that is indeed
    > the case. The pivot line lies behind the front-silvered face of his index
    > mirror, not on it, as I recall. And the only disadvantage that has resulted
    > is that with such a geometry, a favourite trick for checking
    > perpendicularity of the index mirror doesn't work. That trick is to look at
    > at the continuity of the direct and reflected arc as seen in the index
    > mirror. It only applies if the effective reflecting plane is exactly
    > aligned with the pivot. Because, in the past, all sextants were made that
    > way, that restriction on the use of the method was never made clear. Alex
    > has to use a different method to check perpendicularity.
    
    My Astra IIB Deluxe, like Alex's SNO-T, has a front-silvered index mirror.
    Like Alex I have to use gauge blocks to align the index mirror.
    
    Thanks again
    
    Bill
    
    
    

       
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