NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Bubble sextant collimation
From: Ken Gebhart
Date: 2015 May 20, 21:48 -0500
From: Ken Gebhart
Date: 2015 May 20, 21:48 -0500
David,
I'd like to comment on your post, but only from the standpoint that I only receive maybe 20% of all Navlist posts. Who knows why? Except for your first sentence, you are absolutely correct. For the first sentence though, the airplane does not make rapid changes in heading nor speed, particularly while the navigator is taking a shot, so the bubble does not "race across the field of view". It does wander about though because of the aircraft's natural oscillation (commonly called dutch roll), and from small inputs from the pilot (or autopilot) in attempting to keep the heading constant. The period of such oscillations are from one to two minutes of time, thus accounting for the one or two minute averagers. These times are not picked out of thin air. In other words, they are not averaging random motions. Of course in turbulent air things get dicier, but most navigators wait for a period of smooth air if possible. I only post this because most of my navigation life has been spent in the air with a bubble sextant, and I like to keep the record straight when it does not match my experience.
Ken
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.htmlTalking 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 RuizAndrés. 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
