A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: David Pike
Date: 2015 May 20, 13:35 -0700
In general, the available literature on bubble aircraft sextants avoids the observation of the limb of the Sun or Moon; the semidiameter correction. Handbooks of bubble sextants like A-5, A-7, A-12, and general documents like AFPAM11-216, AFM 51-40 (ED082051), also http://home.earthlink.net/~s543t-24dst/airnav/index.html
Talking about collimation — the correct alignment of the images of the bubble of a sextant and the object being observed —, with my A-12 using the technique described in "Bubble sextant collimation.pdf" I obtain very good results when the center or the UL/LL is observed, immediately checked with:
I will be pleased hearing other points of view, other techniques for UL or LL observations (SD).
Andrés. You’ve got to remember that these devices were originally designed for use in moving aircraft where a tiny change in heading or speed will send the bubble chasing across the field of view; hence the averaging mechanism. In such circumstances, it’s hard enough keeping the celestial body roughly in the centre of the bubble let alone trying to put an edge in the centre. On the ground for an instantanious shot, it might be easier, but why bother? I would have thought it’s easier to judge when a circle’s central within a circle than judge when an edge is in the centre of a circle. There might be a case for using SDs when attempting to shoot certain views of the moon. To air navigators the moon at night is a bit of a nuisance. You never seem to get good results from it, and it makes nearby stars harder to see. Dave