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Alex,

In answer to your question,  yes it is possible to purchase a working
aircraft bubble sextant on e-bay, just as it is possible to flip ten coins
into the air all at once and have them all come up heads. I am being
somewhat facetious of course. Buying a sextant on e-bay is a crap shoot. I
have purchased an astro-compass on e-bay and was quite pleased. I am still
waiting for my Creaver Lifeboat Sextant so time will tell how that turns
out.

If you are looking for a bubble sextant, there are only two that I would
recommend without hesitation and in this order:

British MK IX A

No question in my mind that these are the best bubble sextants ever made.
Period. When they work, they work well. Re-filling the bubble chamber is not
all that complex and worth the effort. The MK IX is accurate, easy to use,
comfortable to hold and has an outstanding illumination system for nighttime
viewing. I mean outstanding. You can even nail 2nd magnitude stars with it.
(Several years ago, I took a series of observations of the comet Hale-Bopp).
Try that on any other bubble sextant. There are two shortcomings: 1) you
cannot determine instrument and index errors without special calibration
equipment. Your only other option is to take lots of observations from a
known location and try to reckon an approximate error based on these
(notwithstanding the lengthy discussion that previously ensured regarding
the accuracy of LOPs from sextant observations. 2) the fine tune mechanism
can get gummed up, making it hard to rotate. This can be remedied but is
complicated -- at least for me.

C.Plath Bubble attachment -- adjustable bubble.

This is the next best item. It replaces the telescope on the sextant frame.
It is easy to use, easy to repair, robust and bullet proof. I have two of
them: one which I keep for spare parts. I have had mine for over 10 years --
I purchased it second hand from Ken Gebhart -- and have had no problems,
even at minus 40 degrees. Drawbacks: not as accurate and easy to use as the
MK IX A. A really crappy illumination system for night time observations
(you cannot beat the MK IX's illumination system) and finally, the unit adds
considerable weight to the already heavy sextant.

It is compatible with C.Plath (pre-and Cassens & Plath. Cannot tell you if
it is compatible with other sextants but I am sure Ken Gebhart can answer
this question.

Other than the above, my advice is to ensure that whatever bubble sextant
you choose, the bubble should be adjustable and the unit should be
relatively easy to repair and especially the bubble chamber should be easy
to re-fill should it run dry.  You should purchase it from a reputable
dealer who knows sextants and who has hopefully overhauled the unit you
intend to purchase.

There are a lot of MK IXAs out there yet. It is just a matter of finding one
that functions properly.

As for where to find them, there are several dealers out there. Two come to
mind: our own Ken Gebhart  and Robert E. White &Sons of Boston
Massachusetts. If I can think of any other dealers who are likely to have
these items, I will post them here.

Hope this helps you out.


Robert


----- Original Message -----
From: "alex" 
To: "NavList" 
Sent: Sunday, December 31, 2006 3:02 PM
Subject: [NavList 1927] Bubble sextants on e-bay


>
> Dear list members,
> I need an advise about bubble (aircraft) sextants.
>
> (I need one to measure altitudes from my balcony.
> The balcony is too shaky to use the ordinary liquid art. horizon.
> The "professional bubble attachment" from Celestaire is too
> expensive, so I am looking for an aircraft bubble sextant;
> they are typically sold on e-bay for about $100 or less.
> Seems to be a good value if it is usable).
>
> I know little about bubble sextants (what I know mostly comes
> from Ifland's book "Shooting the stars" and from the info available
> on the Internet. So I will be grateful for any advise how to choose
> one.
>
> The sellers of all air sextants on e-bay say explicitly that they do
> not
> understand how the thing works, so they are unable to tell you whether
> it works at all.
>
> One major potential problem (to my understanding) is the bubble.
> I know the guy who bought a British Mk IXA, it had no bubble at all,
> and the guy inquired on this list (few years ago) how to restore it.
> He has a web site where he explains how he filled a bubble chamber
> with hexan and so on... and had to seal and reseal it, and it
> is still leaking, etc. This seems too complicated to me.
>
> So my specific question is: is it possible to buy a WORKING aircraft
> sextant
> on e-bay, and what is the best way to do it. And what models to look
> for.
>
> Alex.
>
>
> >


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