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    Re: Determining tangency for moon & sun sights
    From: Lu Abel
    Date: 2011 Nov 7, 21:24 -0800
    Randall:

    Is your Davis one with split mirror or full horizon mirror?  

    I've tried both and, despite all the claims that full horizon mirrors are superior, I find it much harder to determine tangency with one of them.   The horizon seems murky.

    Lu Abel

    From: "Randall.F.Morrow@kp.org" <Randall.F.Morrow@kp.org>
    To: NavList@fer3.com
    Sent: Monday, November 7, 2011 2:13 PM
    Subject: [NavList] Determining tangency for moon & sun sights


    Can anyone offer advice on determining the exact point of tangency when doing sun sights, moon sights and sun index error sights?

       My results with sun and moon sights are very inconsistent and I suspect it is my trouble deciding just what 'tangency' looks like through the Davis telescope.   After adding a darker filter to reduce brightness looking into a mirror a.h., there is still a distortion of the lines between the direct and reflected bodies.   Without the extra shading the brighter image washes out the edge of the dimmer.    The drawing below illustrates what I have been doing.   I watch for the slight color change in the overlap between the yellow and orange discs of the direct and reflected images.   This lighter colored sliver gets smaller and smaller, and when I perceive it has disappeared I call that 'tangency'.   The distortion in true roundness of the images as they near tangency makes this seem like the right thing to do.  If I reverse direction and slowly close the gap between the circles, the point of tangency is lost in the distortion in roundness I mentioned.  
    Is this a useable method?
    Is the quality of the optics a factor in seeing the point of tangency?  
    What about magnification?
    Should I bring the images together or separate them as I mentioned above?




    Randall F Morrow PT
    Kaiser Permanente - Kern County - Bakersfield

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