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 Nov 6, 15:29 -0800
From: John Rae
Date: 2005 Nov 6, 15:29 -0800
The sun was out today, so I took my home made test
Bris Sextant out. I set the angles at 5 degrees and 7.5 degrees.
Observing the reflected suns, starting from the top, I viewed 2 bright suns, 1
dim sun, followed by 1 bright sun and 3 dim suns. Exactly as predicted by
Alex. The suns were evenly spaced in the vertical direction, but only
three could be viewed at a time without tilting the apparatus.
By tilting the apparatus (around a horizontal axis
in the "pitch" direction) all of the suns could be brought into view.
The apparent height of each sun image in relation to the horizon was not
affected. This demonstrates two factors;
1 The double reflection principle holds true,
just as for a conventional 2 mirror sextant, and,
2 There is no one place for the location of
the observer's eye, it is different for each sun image, but the hand-eye
coordination just seems to come naturally.
I then held the apparatus in front of one side of a
7 x 35 binocular. As proposed by George, the sun's image was much
enlarged, and easy to see, and no doubt would improve the accuracy of reading to
the horizon. The mirror effect projects the sun image to infinity, so
there is no problem with focus. But
the Bris Sextant has to be rotated in the "pitch" direction and moved vertically
up or down to bring the successive sun images into the field of view. So
the binocular adapter would have to be adjustable in two directions, a tricky
design problem. (It might turn out that only tilt is required if only one
sun image, that near the horizon, was needed.)
At this time of year (and weather), the local sun
is very watery, due to light cloud cover, low observed height, etc, so it is not
really bright. My <7% transmission sun glasses seem to be OK, as it is
easy to avoid looking at the sun directly, so all images are reflected at least
twice, and also lose some additional light from passing obliquely through at
least three glass planes. None the less, you are correct that there must
be additional filtration to protect the eye. Pity the poor navigator in
the past who used a cross-staff!
I have learned that ANSI Z87.1-1989(R-1998)
specification covers clear lenses and filters, typically used in welder's mask
windows. The scale runs from clear to 14, with the range 8 to 14
recommended for electric arc welding. (Number 14 is recommended for
children to watch a solar eclipse). The scale used to progressively
increase the density is not the same as the photographic scale. For
instance ANSI 2 seems to be similar to Photo ND1, and ANSI 4 is similar to Photo
ND3.
I am still associated with a pressure vessel
manufacturer, so I intend on Tuesday to talk to the safety supervisor and have a
look at some welding shield window filters. I suspect that this may be an
economical source for good quality filter glass, as any welding supply store
should sell such windows.
I have trouble calling this test apparatus a
"Bris-Sextant" . Can someone please come up with a good generic
name!
