A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Frank Reed
Date: 2018 Aug 18, 14:20 -0700
Tony, regarding collimation, you wrote:
"But is it relevant?"
It is sometimes, yes. As Greg has described, there is a way around this: if the position of the optics remains fixed, like his camera setup, then it doesn't matter, because, as you suggest, it's the same every time.
But consider the more normal Bris sextant. You hold it up in front of your eye and align its slides approximately perpendicular to your line of sight. It's this approximate alignment that would be different every time. There is some "true" axis of collimation in every Bris sextant, and you could determine where it is with the right set of observations. I suppose you could arrange to glue a narrow tube (like a drinking straw) onto the instrument aligned with that axis, but of course that would defeat one of the strengths of the Bris --its extreme compactness. But suppose we just do the usual "eyeball" alignment. There will then be an error that results from the deviation of the orientation away from the true collimation axis. This deviation can be distilled down to a relatively simple equation:
err[m.o.a.] = 1.05 · (inc[deg])2 · tan(alt / 2),
or, in words, the inclination in degrees squared, multiplied by the tangent of half the altitude, and multiplied by 1.05, yields the collimation error in minutes of arc. For a fixed altitude of 45°, if the inclination is 2°, then the error would be 1.7', fairly small, but if the inclination is 8°, which is certainly within range for handheld collimation alignment, then the error is 28', which would generally be considered unacceptable.
Speaking of calibration, we have talked over the years about methods for using one sextant to test the arc calibration of another. Why not do the same thing with a Bris sextant? I haven't thought this through in detail, but it should allow for much faster calibration of a Bris device. Note that this would provide a Bris with "pure" angular calibration (altitude corrections would have to be applied, but that's not tough).