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    Re: How does the AstraIIIb split mirror work?
    From: Ken Muldrew
    Date: 2004 Apr 23, 09:54 -0600

    On 23 Apr 2004 at 9:01, Jim Thompson wrote:
    
    > 4a. Joel and George proposed that the sun's image could be reflecting
    > off the clear portion of the split mirror, since it grows dimmer
    > toward the left side of the field of view.
    
    If this was the case, you would see something entirely different. At
    the moment that you rotated the reflected image off the mirror, you
    would then see two equally bright reflected images that were both
    much, much dimmer than the mirror reflection. With a 5mm thick
    glass/mirror, these two images would only partly overlap and you
    would have no chance using the sextant properly because you wouldn't
    know which one to place on the horizon (or whatever object you're
    bringing them toward). I'm not saying these reflections don't exist,
    they certainly do. For example, if you're out in the dark taking a
    sight and somebody suddenly switches a porch light on behind you,
    then you will suddenly see a bright reflection of your telescope and
    face in the clear horizon glass (two reflections, actually, you can
    try it and see) and your observation will have to wait until the
    light is switched off. But normally, the glass reflections have a
    brightness that is only a few percent of the mirror reflections, so
    you don't notice them at all. If you did, the sextant would be a far
    more difficult instrument to use.
    
    Ken Muldrew.
    
    
    

       
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