NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Bris Sextant
From: John Rae
Date: 2005 Oct 30, 16:10 -0800
From: John Rae
Date: 2005 Oct 30, 16:10 -0800
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.