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    Re: Butterfly Bris Prototype
    From: Frank Reed
    Date: 2009 Jun 18, 21:07 -0700

    Hewitt, you wrote:
    "I've been wanting to try one of these for some time. Are there any
    detailed instructions around?"
    
    They're really simple to make, but I think you should take Greg up on his offer to send you one. :-)
    
    A Bris sextant is a special case of a fixed angle sextant. Those can be 
    instructive to make, too. Take a pair of small mirrors and affix them to a 
    small board or otherwise arrange for them to be perpendicular to some common 
    plane. They'll be angled relative to each other and offset slightly (just 
    picture the mirrors of a normal sextant ignoring the rest of the instrument). 
    Scrape half of the backing off of one mirror to turn it into a horizon glass. 
    Then calibrate it by an observation of a known angle. Note that a fixed angle 
    sextant and a Bris sextant require much the same adjustments as any 
    reflecting instrument. When you take altitudes with a Bris sextant, you have 
    to "swing the arc" and you also have to ensure collimation of the line of 
    sight.
    
    Note that the important advantage of a Bris sextant compared with a basic 
    fixed angle sextant is that it has multiple reflections so you get three or 
    four fixed angles in one small package. The disadvantage is that it only 
    works with the Sun. So picture this: Get a board or a metal plate 4"x24". Get 
    twenty little mirrors about 1" square. Arrange them in pairs up the length of 
    the board (attach a handle on the other side of the board). Each pair of 
    mirrors constitutes a fixed angle sextant which you can "fix" to your 
    favorite observation angles, maybe multiples of six degrees from 10 to 70 
    degrees (you'll fix them approximately upon construction, but the real values 
    of the angles are found by trial). When you're done, you've got more angles 
    than a Bris and the ability to use stars, planets, and the Moon, too. --I'm 
    not seriously recommending such a thing, but it's an interesting exercise in 
    seeing what really matters in a sextant when it comes to measuring angles.
    
    -FER
    
    
    
    
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