A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Frank Reed
Date: 2018 Aug 16, 10:15 -0700
Greg Rudzinski, you wrote:
"Calibrate to Hs then any body from any height of eye can be used if you can see it. This lets you use standard correction tables."
Exactly. That's the safest bet. In your experiments, have you tried getting data on the system's overall accuracy, especially for the strongest reflections? What's the best you can expect with a Bris?
By the way, Tony had mentioned the parallax of the Sun in an earlier message. Just a reminder that any sextant without a telescope cannot "see" the parallax of the Sun. The limiting resolution of the human eye (unaided) is about a minute of arc or slightly better under perfect conditions. The Sun's parallax is 9 seconds of arc at most (when the Sun is low in the sky).
There's another limiting factor for a Bris, intrinsic to the design of the instrument. Since there's neither a scope nor a sight tube, the instrument is subject to collimation error. In a normal sextant, the scope or sight tube is aligned parallel to the frame and the mirrors are carefully aligned perpendicular to the frame. Collimation error is not a real concern with a modern nautical sextant. A Bris sextant is held in the right orientation in a rather approximate way. There is then some residual collimation error that probably seems like random noise. For low altitudes, this is not a problem, but collimation error increases steadily with increasing altitude (if I remember correctly, it's proportional to tan(2x) where x is the measured angle). Greg, this could explain your less successful experiences with higher altitudes.
How can you deal with collimation in a Bris sextant? The minimum angle is found when the line of sight is collimated (this is true with any sextant) and this is also true for finding the vertical orientation of the instrument which we do when we swing the arc. So it's possible (or is it?) to combine two motions and look for the genuinely minimized angle. This is swinging the arc and collimating the line of sight all at once. I experimented with this a few years ago but set it aside.