NavList:

JPP,
I don’t know that, but whatever it was- a newly manufactured sextant probably met the requirement with ease. The problem occurred over time (not usage) when the lubricant absorbed into the pewter-like material of which the axle was made.  This left a semi dry surface which the (too) small spring holding the worm gear against the sector gear was unable to overcome.  The A-10/ A-10A sextants became notorious as the “backlash” sextants.  When they were used often, they were great sextants. But if they sit around for a few years they will begin to freeze.  Needless to say, almost every sextant for sale on ebay , or a private seller has been subject to this condition.

Backlash on any sextant should not be a big problem if you always remember to take the reading in the same direction.  However it is a bother to do.

Ken


On 1/2/07 3:53 PM, "jean-philippe planas" <jeanphilippeplanas@yahoo.com> wrote:

Just a question for Ken. Does he know what the initial A-10 or A-10A backlash specifications were? Or what is acceptable for a rebuilt instrument?
  
JPP

Ken Gebhart <gebhart@celestaire.com> wrote:
  

Alex and others,

I agree with Robert about the MK IX being good. We used to sell them for a
while. My high school aged son overhauled them without any problems. There
were 2 models: an A model which had no averager, and a B model which had a
large cylindrical averager sticking out the forward end. The wind-up
averager is really of no use unless you are shooting from an airplane, so we
just sold the A model. When these ran out, we simply lopped off the
averager of a B model with a hacksaw, and voila- it became an A model We
probably have 50 to 100 of these left over in a self storage unit. I would
be glad to sell some of these for, say- $50 each (which is mainly for the
effort of digging through them to find the decent ones). For that matter,
we also have piles of A-10s, A10As and AN 5851-1s too. I probably will never
do anything with them, so if someone wants to come to Wichita with a
trailer, I can make you a deal on the lot.

The A-12 sextant used C size batteries whose covering was a cardboard sleeve
(during the war). The design was to remove the sleeve and insert the
batteries. You can still peel off the jacket of modern C cells, and insert
them. They won't look pretty, but they work ok. If you like you can cover
them with masking tape.

Do not under any circumstances buy an A-10 or A10A on ebay (or from anywhere
else for that matter). Not only are the bubble chambers probably dry and
hard to fill, but 99% of them are frozen internally, due to poor lubricating
design. Total disassembly is required to fix this problem. Even if you get
the 1% that works, it will probably have horrible backlash.

The AN 5851-1 (also known as the Navy mark V or the A-14) has the raduim
lighting mentioned earlier, but the paint containing the radium has
deteriorated over the years such that the radium light cannot get out very
well. Most of them have 50% deterioration requiring well dark adapted eyes
to shoot stars at night.

Lastly, Alex, you prefaced your analysis of CN usage with the phrase "I
think" Therefore, I can't say you are wrong. However, if you could stand
in our booth during a boat show, and hear the favorable comments from
passersby (in contrast to 10 years ago), or see the many people that come to
our seminars, I think you would feel differently. Celestialis


On 1/1/07 1:11 PM, "alex" wrote:

>
> Dear Robert,
> Thank you very much for a lot of useful info.
>
>> 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
>
> Do you have any experience with AN 5851-1 (a.k.a. A14) recommended
> by others? And its development A15 with variable-time averager?
>
>> You should purchase it from a reputable
>> dealer who knows sextants and who has hopefully overhauled the unit you
>> intend to purchase.
>
> I have not seen an e-bay dealer yet who sold an air sextant without a
> disclaimer
> that he does not know how to use it. My experience with e-bay so far
> confirms
> your example with 10 coins falling heads.
> Of the 8 items I bought on e-bay so far, except books, 3 had hidden
> defects and 3 were
> badly damaged in the process of delivery (and I was never able to
> collect any insurance).
>
>> 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.
>
> Robert White's web page does not seem to mention any air sextants.
> And I did not know that Ken sells any used equipment that is not
> listed in Celestaire catalog.
> Ken, how to find out what antique items you have for sale?
>
> Alex.
>
>
>

JPP

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