NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Bubble sextants on e-bay
From: Geoffrey Kolbe
Date: 2007 Jan 01, 11:02 +0000
From: Geoffrey Kolbe
Date: 2007 Jan 01, 11:02 +0000
To add my experiences to those already voiced, the Link A-12 sextant (or, more correctly, octant) is the most common bubble sextant available on ebay. There is usually one up for auction and they generally go for around $100. Having kept a close eye on sextants on ebay for a while now and having won an A-12 myself for $100, my conclusions are that they generally need taking apart, cleaning and rebuilding. The bubbles (there should be two with each sextant) generally need refilling. Refilling the bubbles needs a "Bristol" headed screwdriver - not generally available, certainly outside the USA - which means making your own. Modern batteries will not fit into the bubble illuminator, which means machining out the bulb holder to accept modern batteries. Not difficult if you have access to a lathe. Impossible if you do not. The bubble reflecting mirror is front silvered on these sextants and they are usually in poor condition after 60 odd years, requiring re-coating with aluminium and preferably overcoating with a layer of quartz so they can be cleaned without damaging the coating. I also have a British Mk IX bubble sextant. I am afraid I do not share Robert Eno's high opinion of this sextant, or its successors, as ".... the best bubble sextants ever made. Period." Personally, I find them clunky and overcomplicated - too much potential for things to go wrong. There is a tendency for the 5 degree increase mechanism to be stuck on these sextants, which means taking the thing apart. Thankfully, people who have gone through this exercise have put their experiences onto websites and full instructions on how to do this can be downloaded. Basically, WWII era bubble sextants are readily available on ebay, but you have to be prepared to take them apart and rebuild them, replacing broken or worn parts. Advantages of the Mk IX and successors is that the altitude is read out directly in degrees and minutes, whereas the A-12 has a vernier with only a two minute precision. Though the Mk IX has a bewildering multiplicity of knobs and levers, they are all easily accessible without having to take your hands off the two handles. The averaging system in these sextants is probably more trouble they they are worth when used in ground based observations. One thing Robert puts great emphasis on is the ability of the Mk IX type sextant to shoot faint (2nd magnitude) stars. He goes on to tell how he was able to take a fix on the Hale Bopp comet and says, "Try that on any other bubble sextant." Well, I suggest Robert tries it with an A-12, which has just a single piece of clear glass between eyeball and star, compared with the Mk IX type which has two mirrors and a piece of clear glass between eyeball and star. Basically, with the A-12, if you can see it, you can shoot it. On a historical note, it seems there is about a one in four or five chance that any given Mk IX sextant was built by Francis Chichester, when he worked at the Hughes factory assembling them. Geoffrey Kolbe --~--~---------~--~----~------------~-------~--~----~ To post to this group, send email to NavList@fer3.com To , send email to NavList-@fer3.com -~----------~----~----~----~------~----~------~--~---