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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






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