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    Re: Introduction
    From: Gary LaPook
    Date: 2008 May 04, 08:52 -0700
    Gary LaPook writes:

    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.




    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 a common problem with these sextants on ebay is a frozen index prism tapered shaft (requiring disassemly, greasing and re-alignment) why not ask the seller to rotate the altitude knob and determine if it turns freely before you bid.

    The Kollsman periscopic sextants might be a better choice as these are newer instruments and most on ebay have been overhauled in the '90s (ask the seller to look for the overhaul inspection sticker on the instrument before you bid) so they should probably be in better condition tha the WW2 instruments.

    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" <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.
    >
    >
    >
    > >



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