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    Re: Artificial horizon question
    From: J Parsons
    Date: 2009 Apr 20, 16:45 -0700

    George, GL and Peter,
    Thank you all for continuing to offer helpful advice.  To answer some of 
    George's questions about the horizon I am using, it is the inexpensivce Davis 
    model, and has a sloped-roof type of cover, with solid plastic sides 
    ("gables" if you will) and clear or colored plastic panes.  The panes appear 
    to be quite flat as far as one can tell by looking.  No doubt they are cut 
    from large sheets after emerging from a pair of rollers set to a precise 
    tolerance.  Perhaps in cooling or handling they acquire some irregularity.  
    Moreover, the image is passing through the panes more or less normal to the 
    surface.  It may be refracted a bit going through, but I don't think I'm 
    seeing and extra reflection, as I might if I were reflecting the sun off a 
    stack of horizontal plates.  Again, it may be reflecting from the top and 
    bottom of the pan, so I will try some light-deadening measure on the bottom.
    
    When I see the reflected sun in the pan of mineral oil, it is very distinct 
    and solid, and depending on the cover I have over it, may be bright yellow.  
    As I bring the sun down from the sky, again viewing through various filters, 
    it will appear to the right of the yellow reflected image and will be, 
    perhaps bright green or blue (depends on the filter).  There is no mistaking 
    which is which, and no particular impression that either image is double.  
    They will be separated horizontally anywhere from two to four or five 
    diameters apart, but as I squirm and squint, fiddle with the telescope focus 
    and the micrometer, and try to hold my toes just the right way, they drift 
    slowly together, then apart again. It's much like watching the little 
    floating debris on the surface of your eye; you can sort of chase it in one 
    direction by eye-rolling or will power, but it always wants to slide back.  
    But I digress.
    
    I'll give your suggestions a try next time I'm out.  I'm especially grateful 
    that some of you pointed out the advantage of getting the images edge-to-edge 
    rather than totally coincident.  Let's see if this improves my results!
    
    -John
    
    
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