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    Re: Lunar Distances with Alex's SNO-T
    From: Bill B
    Date: 2006 Dec 04, 17:23 -0500

    Bill, you wrote:
    > "as well as producing a circle to mount on the eye end of the scope
    > with a tiny hole to be sure I am shooting down the center of the
    > scope."
    
    Frank Replied:
    > Do you need that? I never did anything like that in my tests.
    
    I think so, although I may be wrong as usual. It strikes me that for the
    most accurate observations we need to look right down the throat of the
    beast.  When testing be reversing the direction of a light ray, it seems the
    same cautions should apply.
    
    I know experientially that being off center on the horizon glass/mirror will
    affect IE checks with the sun and natural-horizon sights.
    
    When initially playing with the laser line it was easy to center up the line
    in the eyepiece and on the horizon mirror. Height was not a real issue with
    a line.  I was able to get the line projected through the scope and glass to
    remain in the same spot on the wall if the sextant was removed with little
    effort.  I did note that if misaligned the position of the line(s) on the
    wall can move substantially--up to 6" laterally motion at a distance of 15'.
    It seems the distance between the lines varied slightly as well (maybe
    1/32-1/16 of an inch)at 15'.  That could be a substantial shift over 300-500
    feet.
    
    Using a point source made alignment more difficult as now I was dealing in
    three dimensions. I believe the distance between the two spikes seemed to
    vary slightly based on alignment.  To give more precise adjustment control I
    rigged the pointer up to a dial-indicator mag-base.
    
    I also produced a card to sit on the mirror side of the horizon glass to
    indicate dead center (vertically in normal upright sextant position).  This
    idea popped up while trying to determine index-mirror line of sight to
    horizon-glass line of sight distance using George's idea.  Even when the
    sextant was tripod mounted, plumb, and zero-mag sight tube was level my
    measurements ranged from 65mm to 67mm depending on the height of the
    sextant--which was based on eyeballing the perceived vertical center of the
    horizon glass in relation a target line.
    
    You might correctly observe, that 1 mm difference won't matter much over 250
    ft.  Maybe 3".  You would be correct.
    
    You might also point out that knowing the line of sight distance on the
    outset is of no importance as your method will yield that distance at a zero
    IE setting.  I feel it does help to have a starting point to reduce trials
    using your method; especially if working solo at a distance, halting trials
    for foot traffic, or working in the Siberian cold ;-)
    >
    And Bill you wrote:
    > "When matching the predetermined mirror distance, the sextant reading
    > was just a hair below 0.8' on the arc."
    
    Frank Suggested:
    > Sounds good. Try this:
    > Set the sextant micrometer to 0.0'...
    
    Thanks Frank, sounds interesting. Will incorporate it in further tests.  Now
    it is time to get away from the drawing board and see if the refinements
    work in practice.
    
    Bill
    
    
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