NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Back sights
From: George Huxtable
Date: 2010 Mar 26, 00:31 -0000
From: George Huxtable
Date: 2010 Mar 26, 00:31 -0000
Brad's diagram, of light paths for fore and back observations in an octant, is most persuasive. I think he has it right, that it would be quite possible to arrange the light paths to be parallel, in the way he has shown, and some octants may indeed be designed in just that way. His diagram implies that it would be necessary, in that case, to use a particularly long horizon mirror, and certain illustrations of back-sighted octants, in Peter Ifland's "Taking the stars", seem to show exactly that. I'm thinking particularly of the Spencer, Browning, Rust octant in fig.50. In which case, I was quite wrong, when considering the light paths between the two mirrors, to insist that there simply had to be an angular difference between those paths, between fore and back observations. In some instruments, the paths may well be designed to be parallel, as shown in Brad's diagram, so I should not have tried to lay down the law in arguing otherwise. However, it's also a perfectly valid design, to have those light paths non-parallel, which was what Bill Noyce was pointing out could be the case. All that that implies is that the sextant frame may be required to nod at a different angle, in the two cases. That diverging geometry was indicated by the diagrams in the Heynau article, that Bill drew to our attention, even if the details in that diagram had not been fully followed-through. In that design, it would be possible to get away with a perfectly ordinary-sized index mirror. In my view, that's how many, perhaps most, backsighted octants were arranged. That view is based only on pictures; not on personal experience, as I haven't handled a backsight octant. So, I suggest that Brad was wrong, if he was suggesting that those two light paths had to be parallel, just as I was wrong, in insisting they couldn't be. Can we agree about that? George. contact George Huxtable, at george@hux.me.uk or at +44 1865 820222 (from UK, 01865 820222) or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK. ----- Original Message ----- From: "Brad Morris"To: Sent: Thursday, March 25, 2010 7:03 PM Subject: [NavList] Re: Back sights Hi George You requested that I sketch an octant for myself, showing the paths. Linked, find just that image. To be clear about the sketch, here are some considerations 1) I made no attempt to optimize for index mirror size. Clearly, I could do so 2) The angles were arbitrarily selected, again, no optimization 3) The horizon, foresight view to horizon and backsight view to horizon are all parallel 4) The view to the zenith for the foresight and backsight paths are parallel. 5) All of the normals are shown, as well as, angles. (this is a sketch, don't expect arc-second accuracy). Conclusion #1: The arrangement I have sketched can be made to work by optimizing angles for index mirror size. Conclusion #2: The paths are indeed parallel. Conclusion #3: I am not "wrong", with all due respect to your opinion. This may be an unusual circumstance in which multiple folks are right without actually agreeing on the details! How? There are many arrangements which can be made to work. There is no one definition of an octant and how the mirrors are arranged. I believe that the main difference is that you required the beam from the object to strike the index mirror in the same spot, and with a differing angle to each horizon mirror. This is not a requirement with a planar mirror, as can be seen in my sketch. Best Regards Brad ---------------------------------------------------------------- NavList message boards and member settings: www.fer3.com/NavList Members may optionally receive posts by email. To cancel email delivery, send a message to NoMail[at]fer3.com ----------------------------------------------------------------