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    Re: EBBCO/filters
    From: Michael Daly
    Date: 2007 Nov 21, 02:36 -0500

    Greg R. wrote:
    
    > Do those have the right filters for the damaging rays of the sun (UV,
    > etc.)? I'd be more inclined to just replace the sextant itself rather
    > than take a chance on permanent and irreparable eye damage.
    
    Welding glass is just as safe as eclipse (aluminized mylar) filters or
    steel (or steel/nickel/whatever alloys) coated glass filters used in
    astronomy.  The trick is getting the right welding glass.
    
    A single layer has to be quite dark - #14 for direct observation of the
    sun.
    
    You can use multiple filters of lower density and stack them (most
    sextants have three or four on the index mirror path and three on the
    horizon mirror), but I'm not sure of the way to choose the shade numbers
    (SN) to get the right values.  ISTR that the SN are simply additive
    (e.g. two 7s = one 14), but I can't confirm, so do your homework before
    taking this route.
    
    
    Mike
    
    
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