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    Re: Logic of shades
    From: Francis Upchurch
    Date: 2018 Oct 27, 07:43 +0100

    Re slide rules,
    My homemade cylindrical Fuller can do 4-7 decimal places and I have excellent 
    results in CN. My Bygrave  gives very good results for Alts and great circle 
    distances but is not a general use slide rule like the Fuller.
    I can provide more details on request.
    -----Original Message-----
    From: NavList@fer3.com [mailto:NavList@fer3.com] On Behalf Of alexander duytschaever
    Sent: 26 October 2018 17:16
    To: francisupchurch---.com
    Subject: [NavList] Logic of shades
    I'm a n00b user/amateur astronav that dived into astro navigation after 
    stumbling over a Bris sextant and wondering if one could ever perform LOP 
    calculatons using a slide rule (got a few of those) :-) (*) So I soon 
    acquired a small shiny King Hughes 1917 replica and recently a Davis Mk 15, 
    and I can happily live with their shortcomings (found an article on how to 
    deal with those shortcomings, and I still have to calibrate these three).
    My question is: what is the logic behind the different shades? I understand 
    that one needs more shades for the index mirror (4 vs 3 for the horizon), but 
    why were those colours (blue, blue, orange, grey) chosen? I suspect that the 
    light grey colour can be used for moderate lighting or to increase contrast, 
    and it also reminds of lunar light conditions; but the two blue glasses look 
    similar (I guess a main function is to block out UV), but are there intended 
    ways to combine those glasses?
    (*) for now, my answer is that a SR can serve for interpolations; it doesn't 
    have enough digits to provide sufficient accuracy. or maybe with A LOT of 
    sightings, who knows...
    [plain text auto-generated]
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