A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: David Pike
Date: 2021 Oct 5, 14:17 -0700
Ed Popko you asked: Can anyone suggest a technique for determining the IE of an A-12 bubble sextant?
The simplest way is taking shots alongside Frank’s GPS Anti-Spoof programme. Either that or using precalculated shots. If you do it around meridian passage the body stops within a minute or so for quite a while. Unlike a Hughes MkIX onwards, you don’t need a correction for every ten-degree slot with an A12. Any altitude will do to calibrate an A12. If you know your latitude exactly, and ideally mer-pas time, you don’t really need anything else. A centre shot on the Sun is fine. Unless you’re a dab-hand with a Bristol key on the index mirror, be prepared to work with large values of IE. 20 minutes is about the best I’ve managed hand-held. One day I’m going to make a clamp on a stand. That might allow me to set the index mirror more accurately, but “hey”; it’s just as easy to add or subtract 30’ as it is 1 or 2’. We're talking 80 year old aircraft sextants here, not the latest state-of-the-art marine sextants. In all cases, it’s desirable to have an assistant to note the time and record the GPS Anti-Spoof and sextant readings. DaveP