NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Error of Perpendicularity
From: George Huxtable
Date: 2006 Apr 18, 16:33 +0100
From: George Huxtable
Date: 2006 Apr 18, 16:33 +0100
Alex wrote (about aligning the index mirror of a sextant for perpendicularity) - | That's why visual adjustment (without dominoes) is OK too. | Two years ago there was a large discussion on the list about | how exactly to do this adjustment, and the general conclusion | was that dominoes (or visors) are only recommended for sextants | with front-silvered index mirros. | For back silvered mirrors, the usual eye method is OK. I wonder if Alex misremembers slightly, perhaps, about the "usual eye method", which involves squinting at the aec, and its reflection in the index mirror, to check that they align. Just to be a pedant, as I recall, the difficulty is this- it only works if the axis of rotation of the index arm passes along the reflecting plane of the index mirror. Most sextants are arranged so that it does, but a sextant will work just as well if it doesn't. In that case, all that happens is that a slightly different patch of the index mirror is used as the altitude changes, moving toward one edge or the other. It's when you have a sextant, originally designed for a back-silvered mirror, in which the mirror plane aligns with the axis, has its mirror replaced with a front silvered type, that the problem occurs. If the mounting hasn't been redesigned to keep the reflecting surface in the same plane, everything will work just as well, except that the reflection of the arc in the index mirror will no longer appear as a continuation of the true arc. That problem often arises when a rear-silvered index mirror is replaced by a front-silvered one, in manufacture or by the user. But it isn't inherent in the use of a front-silvered mirror, and it may arise also with a rear-silvered mirror, if the usual practice for positioning its plane has not been followed. 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.