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Old 12-03-2009, 22:27   #1
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Astronav. Instrument accuracy and other meanderings.

With a couple of good books, volume 2 and 3 of HO249 tables printed in 1975, and some free time, I've managed to learn how to reduce a sight and plot lines of position for each kind of body, without a calculator. My only electronic aid other than a timepiece has been a computer printer on which I reproduced an awesome sight reduction worksheet (thanks Bluewater Miles). I'm really proud of my new skill and can't wait for the chance to go to sea to try it out.

I've been using a Davis Mk 15 sextant and have yet to plot a fix more than 3 NM from my actual location (excluding fixes which fell outside this range due to arithmetical errors on my part). In fact of the 10 fixes I've plotted over the last week or so, half of them were within 2000 yards of my actual position and again, none were more than 3nm distant. Based on this I feel pretty confident about using astronav for making landfalls, and stowing the GPS under the navstation desk just in case the sextant falls overboard.

My question is: given that the Davis Mk 15 is a plastic "low cost" instrument and that there are other more expensive, and more finely machined instruments available, can I expect to see significantly better accuracy than what I'm already achieving with my current instrument that warrants paying the extra $200-$1800?

I've had my eyes on the half-silvered Astra IIIb and will likely pick one up at some point, if only to experience the "permanence" of a metallic instrument. Folks who I've spoken with who either own one or have used it speak quite highly of it. Therefore this question is not really about whether or not I should spend $2000 on a brand new Tamya Jupiter, but more generally about if the accuracy to money ratio really justifies the investment, in the opinion of members here.

Blue skies and following seas,

Cheers
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Old 12-03-2009, 23:16   #2
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From the perspective of having taught celestial navigation for many years (started teacing the most recent course for my museum just last week), I would think that most of the gains in accuracy you would expect to make will come first from more practise and only subsequently from the superior optics of a better instrument, but any such gains might be negated by conditions. A perfectly adjusted Davis in perfect conditions should produce LOP's consistently within half a mile, while you might target shoot within 200 yards regularly with a good metal sextant. In rough or terrible conditions, there might not be much difference at all, even once you've done it a lot. I think the main differences besides expense lie in the need to check and perhaps correct the plastic sextant almost every time you pick the thing up, and in the satisfaction of using the better instrument, satisfying experience frankly being the main point of the exercise to begin with.
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Old 13-03-2009, 00:18   #3
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...and in the satisfaction of using the better instrument, satisfying experience frankly being the main point of the exercise to begin with.
Practice, sea conditions and accuracy aside, it's heartening to see such weight given to a purely subjective quality. And so far astronav has indeed proven most satisfying.

I'm a rank amateur who likes to read, but I never dreamed that consistently fixing within a quarter mile was within the realm of possibility, let alone fixing within 200 yards! Practice indeed, I have much to do!

Thanks for your insight.
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Old 13-03-2009, 02:56   #4
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Plastic sextants can be very accurate - provided you set them up prior to each sight.

The major difference with the better sextants is their ability to stay accurate.

The major error when using a sextant will normaly come from boat movement, and the use of an incorrect horizon due to the sea state.

It is always nicer to use a great instrument rather than a plastic, but if budget has to be prioritised, there are more important things than this upgrade!

I use a second world war issue sextant, that was issued to my father, and I have had re-silvered.
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Old 13-03-2009, 09:35   #5
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I'm very happy with my Davis Mk 15. I just need to take a series of shots for each object, measure the index error between each shot and guesstimate the actual index error for the sight reduction. I usually pick the shot that lies within the smallest range of index error measurements. On a warm day the index error will deviate by up to +/- 10 minutes of angle in a quarter hour. It's not so bad at twilight. Either way, I'm happy with the accuracy and I'm certain my technique introduces more error into my solutions than the instrument.
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Old 13-03-2009, 11:16   #6
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the real world

G'Day Skookum,

We cruised with celestial for a couple of years, and had adequately accurate results to fond harbours and not hit anything hard. Also, I must admit that landfalls now with GPS are not nearly as exciting or as satisfying as those early ones were. We started out with a Davis Mk 25, and eventually took advantage of a spike in the exchange rate with the pound to get a Zeiss sextanat from a vendor in London. I frankly didn't find that my results were very much better with the much better instrument. The reason is that taking sights from the deck of a small yacht at sea introduces errors that generally outweigh instrument errors. The very low height of eye coupled with a lot of motion and a wavy horizon kept us from acheiving superb accuracy most of the time. But in practical navigation at sea, who cares? If you can get within a couple of miles consistantly, your Mk one eyeballs will do the rest! The one advantage of the Zeiss was that the better optics allowed sights under worse visual conditions. Plus, of course, better mechanical stability...

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Old 13-03-2009, 12:21   #7
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Have seen this discussed many time in many places over the years, and if you are getting 3NM positions now on the land--stick to whatever you have and whatever you are doing.

There are some atmospheric distortions over/near land, and if you are at sea on a moving small craft 2NM seems to be the best that anyone will hope to regularly attain, so you're already pretty damn close to 'as good as it gets'.

Having said that...each sextant is different, the same way that sports cars differ in rider and handling. The telescope on a Freiberger Yacht Sextant (nice light weight) adjusts from the front, so your eyeball doesn't change the focus as you rotate against it. But on a way more expensive Plath Horizon Ultra...the 'scope is adjusted the opposite way and your eye will take it out of focus. While your arms notice the double weight.

Then again, the Ultra has some nice extras, like a green cast in one half of the image versus a red cast in the other, so stars "pop into white" as well as getting sharper when they are exactly aligned.

I'd say to try finding some meetings of other sextant users (planetarium or museum), try looking at some hands-one either new or in consignment shops, and see if any of the extras really say "Take me home!" to you.

Meanwhile...3NM would be outstanding precision at sea, there's no hurry to replace anything.
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Old 13-03-2009, 13:29   #8
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I have the Davis and the MK 15 and the Astra IIIB. The biggest difference is the Davis tends to warp a little when the temperature changes. This causes the mirrors to get out of adjustment. This doesn't seem to be as noticeable with the Astra.

If your getting 3 mile accuracy your doing great. My first sights were off by about 30 miles.

Tim
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Old 14-03-2009, 11:55   #9
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I reduced one of yesterday's afternoon sights and loe and behold, the line of position lay 250 yards to the south of my position. 300 yards!

I plotted another sight from that session and its line of position lay 250 yards south of me!

Squarerigger, you are wise. That Davis Mk.15 was the best investment I ever made! Thank you.

Fair winds and following seas.
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Old 14-03-2009, 15:54   #10
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Congratualtions! Now, did you even get quite that same feeling of acomplishment from pushing the button on a GPS?
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Old 14-03-2009, 21:43   #11
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The only feeling I experience while operating a GPS is impatience while I wait for it to acquire satellites. While it may take me half an hour to reduce two or three sights and plot a fix, the only person responsible for that delay is me. Me and my time are not at the mercy of an indifferent "thinking machine."

On that note, I'm reading my first Hiscock adventure: "Beyond the West Horizon." Cracking.
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Old 14-03-2009, 21:51   #12
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skookum, a modern GPS should give you a lock and fix in under one minute assuming it hasn't been jetting around since the last use. Honest, it can be worth updating.

But am I the only one who has had a giggle fit realizing that "GPS time" may be off by 15 seconds or so--but it is still IN MY HAND giving me the time averaged from 12-16 atomic clocks each with a value in six or seven figures?

World's most expensive pocket watch?
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Old 15-03-2009, 01:01   #13
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a modern GPS should give you a lock and fix in under one minute
I was tongue and cheek of course. But a minute is a minute. It's the principle of the matter.

Quote:
World's most expensive pocket watch?
...that's still off by up to 15 seconds.
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Old 15-03-2009, 11:03   #14
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Not really OFF, you just have to know what time zone it is reporting.(G)

IIRC if they have been alllowed to run for 15 minutes 'recently' then most/many of them also pick up the correction for the system and your display will be sub-second accurate at that point. (It is transmitted in the data stream, about once every 15 minutes.)
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