NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Sextant mirror silvering.
From: George Huxtable
Date: 2006 Nov 8, 17:05 -0000
From: George Huxtable
Date: 2006 Nov 8, 17:05 -0000
I just sent a message, as follows- Frank wrote in [NavList 1642] Re: Don't do this to a sextant | While I'm thinking of it, sextants usually work without serious problems | even with mirrors that have lost 10 or 20 percent of the silvering. Since the | focus is at infinity, a few blemishes eight inches away don't really cause any | problems. I have an octant from c.1830 that works fine despite missing about | 20 percent of the reflective material on both mirrors. I can imagine a potential problem, in that case, when taking altitudes of stars. There may be an intended star, reflected as normal in the index mirror, which may perhaps be confused with a brighter star, rather overhead and somewhat behind the observer's head, seen through the holes in the silvering. Can that happen, in practice? ================== But on second thoughts, (which are usually wiser ones) I now realise that it would be easy to make a distinction. Stars seen by a double reflection would appear to stay in the same place, as the aim of the sextant is altered. Any stars observed by transmission through the index glass would have had only one reflection, and would be very sensitive to any changes in the aim of the sextant. It should be very easy to distinguish between them. George. contact George Huxtable at george@huxtable.u-net.com or at +44 1865 820222 (from UK, 01865 820222) or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK. --~--~---------~--~----~------------~-------~--~----~ To post to this group, send email to NavList@fer3.com To , send email to NavList-@fer3.com -~----------~----~----~----~------~----~------~--~---