NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Introduction
From: Jean-Philippe Planas
Date: 2008 May 2, 14:12 -0700
From: Jean-Philippe Planas
Date: 2008 May 2, 14:12 -0700
I apologize but I have to re-edit this post to correct a number of misspellings.
JPP
JPP
Buying any type of sextant on Ebay is always a kind of lottery.
1) The A10 or A10As series.
The main issues with those sextants are:
-Their bubble systems that are dry most of the time and require refill. For this operation a wristwatch caseback wrench or dedicated tool is necessary 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 frozen due to a design flaw in the lubrication of this mechanism. It is therefore necessary to take everything apart and lubricate the shaft 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 easily red via a very convenient counter.
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 easily 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 dimensions 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 it 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 always 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 too 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 requires 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. The last two are working to some extend.
3-The British MKIX/MKIXAs series
These sextants are excellent although relatively heavy (specially the MkIXA) due to the presence of the clockwork averager. Every main subsystems are rather easily 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 frozen 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 may also be frozen 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 and corrosion requiring them to be resilvered.
-During the overhaul process, pay attention not to disturb the sealed screws providing calibration in order to keep the instrument calibrated.
So, most likely, any significant work on these instruments has to be performed by a manually 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" <glapook@pacbell.net> 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-Army_W0QQitemZ250241094048QQihZ015QQcategoryZ37971QQssPageNameZWDVWQQrdZ1QQcmdZViewItem
http://cgi.ebay.com/US-Army-Airforce-Bubble-Sextant-in-Original-Case_W0QQitemZ190217401830QQihZ009QQcategoryZ66528QQssPageNameZWDVWQQrdZ1QQcmdZViewItem
http://cgi.ebay.com/vintage-ansco-air-force-u-s-army-aviators-sextant_W0QQitemZ140228089165QQihZ004QQcategoryZ66528QQssPageNameZWDVWQQrdZ1QQcmdZViewItem
http://cgi.ebay.com/ARMY-AIR-FORCE-SEXTANT-BUBBLE_W0QQitemZ250241982466QQihZ015QQcategoryZ4721QQssPageNameZWDVWQQrdZ1QQcmdZViewItem
http://cgi.ebay.com/vintage-ansco-air-force-u-s-army-aviators-sextant_W0QQitemZ140228089165QQihZ004QQcategoryZ66528QQssPageNameZWDVWQQrdZ1QQcmdZViewItem
http://cgi.ebay.com/US-Army-Airforce-Bubble-Sextant-in-Original-Case_W0QQitemZ190217401830QQihZ009QQcategoryZ66528 QQssPageNameZWDVWQQrdZ1QQcmdZViewItem
http://cgi.ebay.com/A-10-Sextant-Manual_W0QQitemZ250243064536QQihZ015QQcategoryZ37971QQssPageNameZWDVWQQrdZ1QQcmdZViewItem
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.
>
>
>
> >
Be a better friend, newshound, and know-it-all with Yahoo! Mobile. Try it now.
Be a better friend, newshound, and know-it-all with Yahoo! Mobile. Try it now. --~--~---------~--~----~------------~-------~--~----~
Navigation List archive: www.fer3.com/arc
To post, email NavList@fer3.com
To , email NavList-@fer3.com
-~----------~----~----~----~------~----~------~--~---