Sextant, what to buy?

[Snowpetrel]

A side question, but those of you with frieburger type alloy sextants, what oil do you use to lubricate them. I Just pulled it out of it box after reading this thread and noticed my index screw is getting a little stiff. The book specifies non acid instrument oil. I havent used it in any salt spray yet, only in calm conditions so it shouldnt have salt in it unless the PO didnt clean it properly. Cheers

[banjoship]

Originally Posted by Snowpetrel

A side question, but those of you with frieburger type alloy sextants, what oil do you use to lubricate them. I Just pulled it out of it box after reading this thread and noticed my index screw is getting a little stiff. The book specifies non acid instrument oil. I havent used it in any salt spray yet, only in calm conditions so it shouldnt have salt in it unless the PO didnt clean it properly. Cheers

I use singer sewing machine oil. It is basically a fine machine oil, doesn't gum up and easily removable. I use it to coat all my tools etc. Cheap at Woolworths

I have recently come across Boeshield-9, still experimenting with it, results are promising. Yet to apply it to my sextants...which are SNO-T and Heath Hezzanith. Sno-T is alloy similar to Freiburger (Russians took Freiburger in WW2), Hezzanith is bronze.

Boeshield-9 is hard to find in Aus....ebay and some bike shops, or direct import from US. Relatively new product from Boeing. It does leave some residue.

[Snowpetrel]

Thanks Banjo, singer sewing machine oil it is then, I also just rang Welsh optical, and the microscope repair guy there said exactly the same thing. Got to love these old school shops that have people that actually know stuff!

Cheers Banjo, and I will keep my eyes out for that Boesheild.

Sent from my SM-G930F using Cruisers Sailing Forum mobile app

[TeddyDiver]

Originally Posted by rockDAWG

What happens if the whole GPS Satellite system goes down in a flash? Well I will die along with thousands of other who are foolish didn't know how to use a sextant or forget to carry one on the plane.

You don't need a sextant. Tables, sunrise and sunset would be enough.
In case in the arctic summer stay between the coast and ice..

BR Teddy

[Jim Cate]

Boeshield-9 is hard to find in Aus....ebay and some bike shops, or direct import from US. Relatively new product from Boeing. It does leave some residue.
banjoship is online now Report Post

True, not often seen in Oz, but surely not a recent product. I was using it in California in the early 70's, and was not aware of it being new then!

Good stuff, too, for applications where the waxy coating it leaves isn't a problem. don't think it would be so good for sextant lube, though. If long term protection is needed, I'd look more at things like CRC Longlife, which leaves a more oily residue... easier to remove when wanted, will not gum up the fine threads and gears on the sextant.

Jim

[banjoship]

Originally Posted by Jim Cate

True, not often seen in Oz, but surely not a recent product. I was using it in California in the early 70's, and was not aware of it being new then!

Good stuff, too, for applications where the waxy coating it leaves isn't a problem. don't think it would be so good for sextant lube, though. If long term protection is needed, I'd look more at things like CRC Longlife, which leaves a more oily residue... easier to remove when wanted, will not gum up the fine threads and gears on the sextant.

Jim

This website suggests 1982 Our Story | Boeshield T-9®
It has been slow to penetrate the market (pun intended) and has really only been evident in Aus in the last 10 years or so. Ferociously priced,too.

[SeeLevel]

Originally Posted by hellosailor

Seelevel-
I would say "wait."
Get hold of Mixter's "Primer of Navigation" which is exactly that, (...snip)
There are a lot of subtle differences that don't get mentioned. With some, as you swing the sextant to check for level, your eye will turn the focu sof the telescope. Duh? With others, the "turning" is done from the front and your eye can't mess that up.
With the Plath Ultra, if you take a star sight you will see a slight red cast to one image, a slight green cast to the other--and they snap into white as they align, making the alignment VERY exact and easy. As far as I know, that's not documented anyplace.
A lot of features and variations don't make any difference when you are reading about sextants, but may make the difference in your preference once you've met them.

And that's ignoring the larger question of whether to buy one at all. (...snip)

Just things to consider. Take your time, what's on the market will still be on the market, no fire sales needed.

Awesome advice and thank you for the 'take your time' recommendation. Primer of Navigation is on its way, I'll begin with that book.

Can you elaborate on the less desirable models, "With some, as you swing the sextant to check for level, your eye will turn the focus of the telescope. Duh?"
With such a wide range of prices, $100 for plastic to nearly $2K at the high end, I'd like to scratch these off my list. And yes, I take your point in making certain I will use it rather than owning just another box that sits on a shelf.

[SeeLevel]

Originally Posted by lesterbutch

I have an Astra III B, That is brand new, never been out of the box. I simply never had the urge to get it out and use it for real. I had a Davis as well and did use it a bit. If your interested in the IIIB you can have it for $250 plus shipping. You can get in touch with me at the following:
W. "Butch" Lester at:
wle0423@live.com

Seems a generous offer, thank you. I will research further on this model and get back to you. PM sent.

[SeeLevel]

So, a couple of questions. . .

What is the light option used for?
Does a sextant come ready to use or are additional filters possibly required?
If using (practicing) on land should I be sure to find a model which allows an artificial horizon attachement?

While I appreciate the subjective nature of purchasing a sextant, other than wear and tear issues, is there anything else to be aware of up front??

[Littlechay]

Originally Posted by secrabtree

On a small sailboat in the Pacific Ocean I have often had to hold the sextant for a few minutes before the combination of the boat being on the peak and not bouncing too severely to get the sight occurred. I suggest that before one buys a metal sextant that he/she first see how steady their arm is after holding the sextant (or equivalent weight) up in the sighting position for 5 minutes, while just holding the other arm in about the adjusting position at the same time.

This is the same observation that I was going to make. I have previously used plastic Davis and a metal Astra. Several months ago I was given a superb top of the range Plath with all the gadgets by a slightly older chap who was retiring from the sea. I used it crossing from Chile to the Gambiers and can attest to the difficulty of holding it whilst waiting for the perfect moment. A serious work out and several times I had to stop and rest. That just doesn't happen with the Davis.

I still got the sights but in those conditions the Davis would have done as well.

I occasionally use a sextant for other things than astro nav to. For example in Chile around the Chiloe are a recent rural power project has meant that lots of power lines have been draped across channels. I used the sextant a couple of times to measure the clearance before attempting to pass.

[secrabtree]

A number of people talk about slop, about getting different readings when turning clockwise or counterclockwise. I think that's a technique error. I am a civil engineer and for decades a major portion of my business was land surveying. The transits and theodolites that we used to measure angles were much more expensive and accurate than even a good Plath sextant. (A sextant is just an instrument to measure angles, most often used to measure the vertical angle between the apparant horizon and a celestial body.) And, when turning the adjusting screw one way or the other the readings were different. The differences were smaller than the smallest reading on most sextants, but they were still different. We were taught that the final turn of the adjusting screw should always be the same way, either clockwise or counterclockwise. Every reading. Every time. If we went too far we were to turn back to beyond the reading and then make the final adjustment in the same direction as every other reading. No exceptions. This was to minimize the "slop", not totally eliminate it. (to totally eliminate it we'd have to measure the force and speed used to turn the adjusting screw so that it'd be the same each time.) If this technique is necessary when making angular measurements with a $20,000 instrument you can be sure that it is necessary to get good results with an instrument costing a couple thousand dollars or less.
And yes, a sextant can be used to measure horizontal angles for land surveying and a theodolite can be used to measure vertical angles like for celestial navigation. I have done both. And one second land surveying instruments will measure celestial angles with more accuracy, much greater accuracy than any sailor's sextant, even celestial objects. (But a tripod mounted instrument, which land surveying instruments are, can't be used from a boat with acceptable accuracy.)
So to get better results when using a sextant ALWAYS make the final turn of the adjusting screw be in the same direction - whether reading the index error, the vertical angle between the apparent horizon and a celestial body, or whatever. Always in the same direction for the final adjustment before any reading. Always!
And it doesn't matter whether it is clockwise or counter clockwise just as long as it is the same direction for EVERY reading.

[Jim Cate]

Originally Posted by banjoship

This website suggests 1982 Our Story | Boeshield T-9®
It has been slow to penetrate the market (pun intended) and has really only been evident in Aus in the last 10 years or so. Ferociously priced,too.

Well, they should know! So, I guess that my memory of first usage is distorted, and I apologize for the mis information. But at any rate, even 1982 isn't recent!

Cheers,

JIm

[Snowpetrel]

Originally Posted by secrabtree

A number of people talk about slop, about getting different readings when turning clockwise or counterclockwise. (Snip)

Always in the same direction for the final adjustment before any reading. Always!
And it doesn't matter whether it is clockwise or counter clockwise just as long as it is the same direction for EVERY reading.

Thats what I have been trying to do to reduce the backlash error. Unfortunately it all happens pretty quickly, the boats moving, the suns moving, the waves are moving and the damned clouds are also moving!

You have to work real quick at times, and say you are getting a mer pass, you only get one shot at it, unless you are smart enough to take a whole bunch of sights and plot them. But it gets annoying real quick trying to reduce 12' of backlash, and its not needed on a good sextant. Any backlash should be minimal and not enough to make a big difference in a practical way.

I suspect its all only really a big issue now we have GPS and can compare every sight for accuracy.

I'll pull it out sometime and have another play. Now I have the metal sextant I can compare results against fixed objects.

[Tibor]

Really today there is little if any need for a sextant at all. A good GPS is all that is required for most purposes. A Sextant with tables should you have the ability to use it may be a handy backup for ocean crossings but I would prefer a second hand held GPS with adequate batteries as a backup.

[Snowpetrel]

Originally Posted by TeddyDiver

You don't need a sextant. Tables, sunrise and sunset would be enough.
In case in the arctic summer stay between the coast and ice..

BR Teddy

I once got a pole star sight that put me 30 miles south of the real position with a plastic ruler and some string rigged up as a Kamal. Accuracy would be better at lower elevations. From memory we were near 35N.

I have heard 1 cm approximatly equals 1 deg at 57cm from the knot in your teeth depending on a complex correction for the exact goofiness factor. Check against star pairs, eg pollux and castor are around 4.5 deg.