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Gary LaPook responds:


On May 2, 3:15 am, jean-philippe planas 
wrote:
> Buying a any type of sextant on Ebay is always a kind of lottery.
>
> 1) The A10 or A10As series.
> The main issues with those sextant are:
> -Their bubble systems that are mostly dry and require refill. This operation 
a wristwatch wrench or dedicated tool to remove the bottom lens of the bubble 
assembly. But due to the fact that this refill is not done under a vacuum and 
that modern seals do not perfectly fit it is very difficult to prevent 
subsequent small leaks
> -The fact that the tapered shaft of the main prism is always stuck due to a 
design flaw in the lubrication. It is therefore necessary to take everything 
apart, lubricate the axis and gears.
> -The mirror prisms are often damadged with corrosion spots or crows feet.
>
> The complete overhaul of the A10s does not require  specific  mechanical  
tools or  skills but considering the above comments it may be difficult to 
put it back into working order. Of course recalibration is necessary via a  
trials and error method without a dedicated optical rig.
> I had to buy 4 units on Ebay to finally be able to reconstitute two of them  
in correct working order. When they work the A10s/A10As are a joy to operate 
and the night lighting system is just perfect. The observed altitude is 
easyly red via a very convenient counter.
>




Gary adds:


When reassembling the A-10 the easiest way to align the index prism is
to use the sextant like a normal marine sextant. Set the counter to
zero. Install the index prism and the fixed prism but don't tighten up
the screw holding the index prism shaft, just hold the prism in the
frame with your finger. Don't install the plates that close the inside
of the the A-10 frame as they limit your ability to push the index
prism
out of engagement with the worm.
Go outside and sight on a clear object in the distance, the farther
the
better, by looking around the left edge of the fixed prism. Then move
the index prism up or down by moving it slightly to the left to
disengage it from the worm so that it can be moved up or down. Let it
drop into the worm when you have the same object lined up in the prism
with the object as viewed around the lefthand edge of the fixed prism,
then tighten up the screw to hold the index shaft in.

When reassembling the index prism into the frame put grease not oil on
the tapered shaft because the tapered shaft applies a great deal of
pressure on the mating surface and oil is not strong enough to supply
enough lubrication. If you use oil you will end up with a horrendous
amount of backlash, say about a 30' difference when you bring a star
up
or down..

The trickiest thing about installing the index prism is installing the
spring that goes around the index prism pivot to apply a constant
pressure on the gear teeth against the worm to minimize backlash. You
need a small forceps that lock to grasp the end of the spring and
manoeuvre the end of the spring into the underside of the index prism,
this is a frustrating exercise due to the very limited room for
working
but necessary.

Since the common problem with A-10s on ebay is the index prism tapered
shaft being frozen (requiring disassembly, greasing,
and re=alignment) why not send an enquiry to the seller asking him to
rotate the altitude knob to make sure that it moves before
you bid.

A posibly better choice would be the kollsman periscopic sextant as
these are newer instuments and most on ebay were overhauled in the
'90s so are more likely to be in good shape. They usually go in the
$50 rabge.

gl


> 2) The A12
> -The bubble assembly(ies) always need refilling. But it is a very easy task 
to be performed provided you've got the correct tools (Bristol wrench). 
Thereafter the refill must be redone every now and then to keep the bubble 
the correct size due to undetectable small leaks.
> -The shades tend to separate from their mounts due to the cement ageing. If 
the shades are not lost they are easysy glued again in their mount. If they 
are not present in the box, one has to find a substitute (not easy to find)  
to be cut to dimension and glued in place.
> -The battery cannister allowing the night shots can be missing preventing 
any nignt observation, or it might be stuck in place. Then it is very 
difficult to extract without damaging eather the sextant or the battery 
cannister.
> -Sometimes the reflecting mirror located in the bottom of the mirror tube requires re-silvering.
> -The main issue with Ebay A12s is that they almost often feature damages on 
the edges of  the sector gear and on the anti-backlash mechanism due to 
mishandling or inappropriate use of the instrument. If to many teeth are 
missing the instrument is useless and nothing can be done.
>
> The A12 is a light rugged instrument very simple to operate and maintain. 
The altitude is measured via a vernier which require a magnifier to read to 
two minutes precision. The night lighting sytem definitly does not provide 
good performance.
> I had to buy four of them to get two in excellent shape and working order 
with the complete set of accessories along with a good calibration and the 
last two are working to some extend.
>
> 3-The British MKIX/MKIXAs series
> These sextants are excellent although relatively heavy for the MkIXA due to 
the presence of the clockwork averager. Every main subsystems are rather 
easyly accessible for overhaul.
>
> -The bubble assembly will most probably be dry and require refill. This is 
certainly the most tricky part of their overhaul process because it requires 
desoldering a metal seal, drilling a small hole without sending metal chips 
in the system, refilling the right quantity of hexane (almost impossible to 
find) or xylene, and resoldering the system. All these operations are not so 
easy to perform and require some trials and training and some dedicated 
equipment (soldering iron, very small drills...)
> -The small +5/-5 mirror tapered shaft will most likely be stuck and will 
require to be extracted (sometimes the 3 fitting screws are so tight that 
they are almost impossible to remove) and relubricated.
> -The main mirror will may also be stuck or at least be sticky requiring to 
be partially taken apart and re-lubricated to enable the anti-backlash system 
to operate.
> -Both mirrors may feature spots  requiring them to be  resilvered.
> -During the overhaul process, pay attention not to disturg the sealed screws 
providing calibration in order to keep the instrument calibrated.
>
> So most likely significant work to be performed by a manually rather gifted 
operator. Some specific tools must also be machined on purpose to ease the 
operations.
> When they work MKIXs are good sextants (although rather bulky) with an 
outstanding bubble system and lighting for night shots. The altitude is very 
clearly red via a counter to the minute. I bought two of them to finally 
reconstitute one in outstanding condition and the other one in working order.
>
> The conclusion is that, depending on luck, you'll have to buy several units 
(on Ebay) to be able to eventually get one in good working order. But it is 
also part of the adventure as it is nothing more than a hobby.
> Finally, for the ease of use and overhaul simplicity I would recommend the A12.
>
> JPP
>
> "Gary J. LaPook"  wrote: Gary LaPook writes:
>
> Don't just restrict yourself to A-12s as there are plenty of other
> aviation sextants available. I have attached links to current ebay
> sextants. Look at the A-10 (Ansco, Fairchild) which is a light hand held
> sextant and you can use the natural horizon by looking around the left
> edge of the index prism.
>
> The Kollsman periscopic sextants (used in B-52, KC- 135s, etc.) was the
> culmination and perfection of aviation sextants and are very accurate,
> plus or minus one or two minutes. You can't use this type with a natural
> horizon. It look s awkward at first sight but becomes very easy to use
> with a little practice. If you do not plan to take sights in flight you
> can remove the averager mechanism (the rectangular box like structure on
> the back of the instrument, just remove four screws) and it then becomes
> much lighter and even easier to use.
>
> Another very light and easy to use sextant is the A-7 and is the
> lightest of all of the aviation sextants, and can be used with a natural
> horizon.
>
> I have used all of these in flight and all are accurate instruments. I
> am particularly partial to the A-10 since that is the type I used when
> ferrying small planes across the ocean.
>
> The  Kollsman periscopic sextant  extends through the top of the
> fuselage in B-52s, C-130s, other military planes and also in Boeing
> 707s, and DC-8s. Airline Flight Navigators used celestial for oceanic
> flight up until the early '70s and the military used celestial routinely
> through the  '90s ( they figured the Soviets would turn off their radio
> navigational aids in the event of war.) It is still in the current Air
> Force navigation manual,  AFPAM 11-216.
>
> Here is a link to more information about aviation sextants:
>
> http://home.earthlink.net/%7Es543t-24dst/airnav/index.html
>
> Here is a link where you can download the current (2001) Air Force
> Navigation manual.
>
> http://www.e-publishing.af.mil/shared/media/epubs/AFPAM11-216.pdf
>
> 
http://cgi.ebay.com/Vintage-Fairchild-Aviation-Sextant-Air-Force-US-A...http://cgi.ebay.com/US-Army-Airforce-Bubble-Sextant-in-Original-Case_...
>
> http://cgi.ebay.com/vintage-ansco-air-force-u-s-army-aviators-sextant...
>
> http://cgi.ebay.com/ARMY-AIR-FORCE-SEXTANT-BUBBLE_W0QQitemZ2502419824...
>
> http://cgi.ebay.com/vintage-ansco-air-force-u-s-army-aviators-sextant...
>
> http://cgi.ebay.com/US-Army-Airforce-Bubble-Sextant-in-Original-Case_...
>
> http://cgi.ebay.com/A-10-Sextant-Manual_W0QQitemZ250243064536QQihZ015...
>
> gl
>
>
>
> Michael Dorl wrote:
> > Bruce wrote:
>
> >>3) I have looked, and bid on, several a-12 air sextants on e-bay. Has
> >>anyone ever had a good result with an e-bay A-12?  Are they fairly
> >>rugged? I hate buying a pig in a poke, but celestaire is not cheap (like
> >>me).
>
> > A few years ago, I was able to buy a an5851 aircraft sextant for about
> > $150 on ebay.  Mine is in pretty much mint condition and has the
> > averaging mechanism and original Bakelite case.  This is a fairly
> > heavy instrument  but does have a ring for suspending it in your
> > aircraft  navigation dome. This sextant can both be used with the
> > bubble or the horizon and is equipped with various filters and an
> > stigmatizer. Don't expect more than a minute or so of precision though.
>
> ---------------------------------
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