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    Pivot Line: was sextant precision.
    From: Robert Eno
    Date: 2005 Jun 23, 09:59 -0400

    George Huxtable wrote:
    
    > 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.
    
    Robert responds:
    
    so far, so good. I got the picture.....
    
    George goes on...
    
    >
    > 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.
    
    Robert comments:
    
    You lost me here George. Can you expand on this idea a bit?  I thought I
    knew what you meant when you referred to the "pivot line" but now I am
    confused. Is the pivot line not a function of the mirror housing?
    
    There are so many conversations on this forum that would be much clearer if
    we could send the occassional diagram with our comments!
    
    Thanks George.
    
    
    Robert
    
    
    

       
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