Sextant Question(s)

[2nd_apprentice]

Well, one of the things that I considered worth fooling around with has been the art of celestial navigation.
Got everything one needs: tables, calculator, formulas, long term almanach, plenty of sun and last but not least a sextant of course. Dito several books on the topic. Even ordered the "Sextant Handbook" from the Jester recommended reading link (so you gets the credit) but at the moment I'm lost.
Naturally I got myself a second hand sextant though not exactly cheap. It seemed to be in resonable condition - as far as I can tell, at least no obvious faults. Since it's an ex-soviet marine sextant and they're identical with Freiberger (formerly East Germany) Sextants I bought a modern wide vision mirror and fitted it.
That didn't work out quite as fantastic as I had imagined. Never been able to get a consistent index and dip error reading. Finally decided to give it a try and go back to the traditional split mirror and managed to get it right.

I can now rule out perpendiculary errors for frame and index mirror and horizon glass. Adjusted index error but still I'm no happy camper. For one I'm not sure if I got it right with that parallelism thing. If I look through the star scope I definetly don't have a centred view through the horizon glass. It seems to be slightly offset to the left thus diminishing the silvered side to 40% from my view. However there doesn't seem to be anything wrong with the scope itself nor with the mounting bracket. No adjustment screws either.
Since I've also got an astronomical scope (guess that's how it's called) with a handy grid in it I'd prefer working with that one anyway. However the picture it gives is upside down and the left side appears on the right side and vice versa. Bit uncomfy. Even more confusing is that there's no sharply divided picture as with the star scope making it even more difficult to "shoot" anything. It looks almost like looking through a wide mirror instead of a split mirror. Why is beyond me.

Further problem: the index shades impair the vision through the horizon glass while the horizon shades don't fully cover the area viewed through the horizon glass. Almost impossible to shoot the sun with all these odds. What am I missing?

ANY HELP appreaciated!

/forums/images/graemlins/confused.gif
Patric

 

[sarabande]

I'd be concerned about the lack of a consistent index error. It sounds as if the thread and/or clamp is worn; can you detect any backlash (movement in the arm) when locked onto an angle ?

In which case your other problems are irrelevant. Sorry.

 

[2nd_apprentice]

Mhm, don't think so. My guess would be that Freiberger updated the design somewhere during the last thirty years. The index error with the original error can be set quite accurately. No movement/worn thread either.

 

[capnsensible]

Can you borrow another one?? Try using a bowl of water 10 foot or so in front of you as a horizon and bring a street light or something down to it. Do this with both sextants and see what the difference is. It may help to bowl out the snag. Hope this helps.

 

[Guest]

[ QUOTE ]

No adjustment screws either.


[/ QUOTE ]

Patric - just checking - is that a typo ?
Colin

 

[2nd_apprentice]

[ QUOTE ]
Patric - just checking - is that a typo ?
Colin

[/ QUOTE ]

The usual screws are there of course. I was referring to the mounting bracket for the scopes. No possibility to adjust something there.

[ QUOTE ]
Can you borrow another one?? Try using a bowl of water 10 foot or so in front of you as a horizon and bring a street light or something down to it. Do this with both sextants and see what the difference is. It may help to bowl out the snag. Hope this helps.


[/ QUOTE ]

Since I'm on the Seychelles at the moment I can use the Indian Ocean instead of the bowl of water /forums/images/graemlins/grin.gif Problem is however that getting hold of a second sextant might be a bit of a problem...
What puzzles me is that everything looks as if it was meant to be that way. Perhaps I'll manage to take some photos.

Patric

 

[bilbobaggins]

PM sent.

/forums/images/graemlins/smirk.gif

 

[chrisross]

I experienced many similar problems to you I first started with a plastic ebbco which I found to be just a useless piece of uncalibratable junk but enabled me to learn what the various parts and adjustments were. After that I bought a brand new alloy Zeis yacht sextant from Thomas Foules in Leytonstone about 25 years ago. I was so unhappy with it I returned it a couple of days later and swapped it for an old second hand kelvin hughes "Huson" admiralty pattern sextant with a new test certificate. (Probably made in 1950 or so) very heavy and made off brass with a black crackle finish and inlaid silver scale, never been polished (which is always a good sign) also remarkably its new certificate showed negligible errors it was far better than the new Zeis in all respects. The Kelvin Hughes was a revelation, it is a proper scientific instrument, the adjustments all work precisely and seem to "snap" in or out of alignment leaving no doubt. As others have suggested try to borrow a good sextant (difficult as owners of these will be very protective). Taking the sight is relatively simple with a good instrument. then it's all down to the calculations. You really will get frustrated trying to use a poor instrument.

 

[barnaclephill]

If you can't borrow another sextant you could do the calculations from a known site, thus comparing your observed readings and error with the calculated values. This is what I did at a main jetty, and got 4 readings with errors of 4, 11, 8 and 200 miles respectively. Obviously one was way out, but I learnt to do the maths and timing the sights.

Is it possible to reinstall the original scope, or even to eliminate it? If it is showing a reversed image like astronomical telescopes do, it doesn't seem ideal. Also the constant mis-alignment seems a grave issue. Is it possible to shove some layers of plastic sheets behind a mirror to try to pad out the misalignment?

The shading seems very mis-aligned if it blocks one view and pours the sun in to burn you eyes.

I have a plastic Davis Mk3 model, cost Australian $90, and only one book valid for 5 years of data, costing about $60. No multiplicity of books/tables/alamanacs, etc just one book about 2cm thick. Mind you I'm confused by the lunar instructions, the sun instructions I can understand.

If by now, being on this thread one month, it's not sorted, could you post some photos to try to explain it to us?

 

[2nd_apprentice]

Thanks for all the suggestions and help offered! Sent the sextant in for inspection so that might take a while. Somehow I fear that it's unserviceable though in which case I'd have to bite the bullet and get a "new" one. Sigh! Obviously this one hasn't been such a clever investment after all...

 

[AndrewB]

Certification!

The moral is, never buy a second hand sextant unless it has up-to-date certification. (And for good measure get the owner to swear on the Bible that it has never been dropped).

There are far more duds than good instruments on the second-hand market.

Sending it to a qualified adjuster sounds your best option. Hope it is repairable.

 

[2nd_apprentice]

Re: Certification!

You're right of course! On the other hand parting with that kind of money for something that you probably won't use often isn't fun either. If I had to rely on celestial navigation only the best would do but since it's more of a back up one's tempted to try it on the cheap. Obviously not a wise decision. As for emergency navigation: could've easily bought 3 spare GPS units for what a sextant, almanach, tables, scientific calculator etc cost. Go figure!

I'll give it another try though.
Patric