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    Artificial horizon glass selection
    From: Bruce Hamilton
    Date: 2009 Sep 25, 10:29 -0700

    The most recent Edmund optics catalog just got delivered to me, and I started 
    looking at the first surface mirrors. Depending on the grade, they can be 
    quite reasonable. A 75mm diameter� with a surface accuracy of 4-6 lambda per 
    inch is $24.00 USD.� 
    
    The Freiberger seems to use about a 70 or 80 mm diameter mirror, but i am just 
    guessing by looking at the pictures. If anyone knows the actual diameter , 
    please let me know. Would round or square be better? 
     
    What would be the lowest surface accuracy one could use to have the necessary 
    accuracy while keeping things cheap? I am guessing that first surface mirrors 
    are best, but let me know if anything else would be better. If you can think 
    of an optical item, the Edmund catalog has it.  I remember someone was 
    looking for a penta-prism, and they have these, and the prices are quite 
    good.  They have a web page with a sales area so it looks like you can buy 
    direct.
    
    BTW Thank you for the discussion on GPS time. I was unaware of the difference 
    between GPS time and UCT, and the single occasion this summer when I thought 
    I would use it as my time source, I had a frustrating night where all my 
    sights from my A 12 were off by 25 miles no matter how many times I tried. I 
    took a lot of sights this summer while camping and learned things like never 
    trust a sight that is taken through too many leaves or too close to the top 
    of a hill. All sorts of odd refraction takes place.
    
    Cheers
    
    Bruce
    
    Thanks
    
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