NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Bris Sextant
From: Doug Royer
Date: 2005 Nov 7, 16:00 -0800
From: Doug Royer
Date: 2005 Nov 7, 16:00 -0800
Dear George and Frank, > I'm pretty sure there's a real collimation issue here. I agree it is similar to collimation in its physical origin, but my point is that it does not depend on how the device is MADE, it depends on how you hold it. And the analog of "roking" I described earlier permits to deal with this in a similar manner as in the ordinary sextant we deal with another sort of error coming from "non-verticality" of the frame plane. > Also, I notice with my mockup that the > reflected images are all doubled (presumably from the non-zero thickness of the > glass). Or from prismatic effect. The ray is reflected from BOTH surfaces of the glass pane outer and inner surface, so if these surfaces are not perfectly parallel you will see a doubling. Earlier I noticed that my Bris "does not conform to the theory" and attributed this to prismaticity of the glass panes. Now I think this explanation was wrong, because this "prismaticity" will have an effect of doubling which I do not see in my device. So I think that it does not "conform to the theory" because the two intersection lines of the planes of the three glass panes are not parallel. This also would explain another "deficiency" the 8 images if Sun that I see are not aligned on one vertical line. However I don't see how it could affect precision, and don't think it does. On my opinion, the only limitation of precision is the absence of the scope. If one attaches the scope, the attachment should certainly have a collimation adjustmeent (rotating the Bris part around a vertical axis). With such adjustment, I do not see why Bris device will be less precise than ordinary sextant. (Of course I do not mention again its other limitations). Alex.