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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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