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Dear John,
I will certainly measure the sizes of the glass pieces
in my Bris sext., but I don't see any reliable way
to measure the angles between them. I supose these angles
can be COMPUTED instead, mathematically
from my calibration results.
I will also ask Bill, if he could make good
pictures of my Bris sext. to post them on my web site.

One problem with a Bris sextant is that it cannot be
used for anything, and cannot be calibrated unless you have a
natural horizon. And I don't have it:-(

By the way, I remember about a year ago one member of this
list mentioned successive use of a Bris sextant in a desert.
I wonder what desert was this? You need an extremally flat
desert...

Alex.


On Sun, 30 Oct 2005, John Rae wrote:

> Thanks Alex for your information.
> Yes, I made the test sextant from three pieces of plain glass, and used my 
mountain sunglasses (<7% transmission), but even so, I was careful to block 
the true sun with my hat brim and finger.  This may be OK for a test, but 
obviously not for a working sextant.
> I intend to use a colored glass for one of the elements of the next sextant 
I make.  A local camera store that has an extensive stock of used cameras, 
lenses and accessories sells used filters for about $10 each.  I hope to find 
a suitable filter , and cut a rectangle from the circular part.  The most 
common diameter seems to be 49 mm, but 52 and 55 mm are also available.  You 
might try your local camera store.  I will also try a piece of polaroid with 
the objective of reducing glare at the sea horizon.
>
> I was impressed with your calibration sights using your Bris Sextant, and I 
expect that those might be approximately similar for other instruments, but 
each would have to be calibrated individually.  The rainy season is upon us, 
so I have to wait for a good time, maybe even until next spring before I can 
get a good day.  (And I hope to be tied up to the dock!)
>
> Could I ask if you could measure the dimensions of your Bris Sextant, 
particularly the gap at the top end between the two pairs of glass and the 
length to the bottom apex, so that I could calculate the angle between the 
glass elements?  Your report of 3 bright, 4 dim and 1 very dim suggests from 
my tests that the angles between the two outer glasses are not the same, one 
perhaps twice that of the second angle.

   

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