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    Re: Bris Sextant
    From: Frank Reed CT
    Date: 2005 Nov 7, 16:55 EST

    George wrote:
    " It's important, for a  sextant, that both lines of view (to Sun and to
    horizon) are in the plane of its  frame, which is at right angles to the axis"
    
    And Alex you replied:
    "It  is important because you have a frame with the scale attached to it.
    And  measure the angles using this scale."
    
    I'm pretty sure there's a real  collimation issue here. You even mentioned
    the technique for dealing with it in  a post last month. To get the line of
    sight perpendicular to the glass, you  rotate the instrument back and forth around
    the vertical axis. This is the same  procedure that's used with a true
    sextant to detect errors of collimation in the  telescope --but on a radically
    different scale. If you take your Bris "sextant"  and hold it at 45 degrees to the
    line of sight (rotated about the vertical  axis), you should find that the
    angles are very different from their normal  values. That's certainly what I see
    when I play with the Bris sextant mockup  that I assembled... I haven't worked
    the math, but I wouldn't be surprised if  the standard equation for sextant
    telescope collimation error applies without  modification. Since this device
    depends on "hand" collimation, I would expect  errors of collimation of 5
    degrees or so to occur relatively often. A 5 degree  "yaw" yields a 10 arcminute
    error in the measured altitude for an altitude  around 45 degrees. I suspect that
    this is the fundamental limitation of the Bris  "sextant's" performance.
    
    "It seems to me that in Bris sextant NOTHING is  important:-)"
    
    For the most part, I agree with that. There's no index  error, no arc error,
    etc. But there is collimation error. There is probably also  error arising
    from non-uniformity of the glass. Also, I notice with my mockup  that the
    reflected images are all doubled (presumably from the non-zero  thickness of the
    glass). This effectively "blurs" the reflected image though  with careful
    observation it should be possible to deal with this.
    
    "You  just glue three pieces of glass very approximately.
    The only thing that is  important is that the assembly is rigid,
    the glass panes do not move about  each other."
    
    If anyone else is building these, I have a suggestion for  experimentation.
    I've been gluing mine together with water-soluble glue. Makes  re-designs easy.
    My current model has four panes at rather wide  angles.
    
    -FER
    42.0N 87.7W, or 41.4N  72.1W.
    www.HistoricalAtlas.com/lunars
    
    
    

       
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