Re: Sextant Purchase

[charles_reed]

I had a black plastic Ebbco on my first two boats - did sterling service, but little astro-nav, for which it was adequate rather than good.
Mostly used for distance-off by height and for horizontal angle measurements for which it was good.
I don't even know if they're still being made (the black ones, in ABS were the best).

 

[AndrewB]

So those sextant makers have been getting it wrong all these years?

I've taken many sights with both metal and good quality plastic sextants. Typically my accuracy has been twice as good with a metal sextant than a plastic sextant (e.g. 1' error compared to 2' error). It feels like there is a very slight amount of 'give' with a plastic sextant, which means in particular you have to allow for backlash when rotating the micrometer. There is a slightly different result whether your last wind is clockwise or anticlockwise. (Best is to make sure your final wind is always the same way, then the error washes out with the index error correction). As the plastic sextant gets more and more use, wear increases this backlash until ultimately it is no longer sufficiently reliable. After 20 years mine is now close to this point.

 

[duncan99210]

The plastic components are simply not quite as rigid as their metal counterparts. Plastic sextants are bulkier than metal ones for this reason but that cannot compensate for the inherent flexibility of the material. In addition, the plastic components wear at a faster rate than metal, giving rise to inaccuracies, which can be compensated for to an extent by calibration.
Metal sextants are so designed that any dimension changes due to temperature will not affect the accuracy of the instrument.

 

[oldvarnish]

Metal sextants are so designed that any dimension changes due to temperature will not affect the accuracy of the instrument.

Yes, my Plath has the metal frame painted white - apparently this was common in sextants mostly used in the tropics.

 

[Santeh]

Do hope it has in-date certification. It's a risk buying a second-hand sextant this old, too often they've had an accident and are knackered, which won't be obvious without proper testing. If you are expecting to use it for real navigation, get it professionally tested asap, or at least ask someone well experienced in taking sights to check that it's accurate.

I am not planning on using it for real navigation, just teaching/demonstration but I was thinking of getting it looked at (is collimated the right word?) anyways.

By the way, since I did not hit my budget I was thinking of adding a Practice Bubble Horizon (please see: https://www.celestaire.com/marine-sextants/astra-iii-professional.html ) for my own learning and demonstration. I really want my students to have a rewarding and beneficial lesson.

 

[Skylark]

It's not just the frame, plastic sextants tend to have lower grade optics in my experience. I've never successfully shot a star at twilight with a plastic sextant. That said, providing you calibrate the horizon with each use, I've found a plastic version to be ok for sun sights. Metal sextants give more precise, accurate and repeatable results and they are a joy to use. What a great way to spend a couple of hours or so working through the days sight reductions.

 

[Neil]

I am not planning on using it for real navigation, just teaching/demonstration but I was thinking of getting it looked at (is collimated the right word?) anyways.

By the way, since I did not hit my budget I was thinking of adding a Practice Bubble Horizon (please see: https://www.celestaire.com/marine-sextants/astra-iii-professional.html ) for my own learning and demonstration. I really want my students to have a rewarding and beneficial lesson.

I bought a bubble horizon myself shortly after I got my sextant, but found it difficult. Better to use a dish of oil as an artificial horizon, and divide by 2

 

[KellysEye]

I'd recommend Freiburger, they were made in East Germany and have a good reputation for accuracy. You might get one on Ebay at the top end of your budget.

 

[lustyd]

So those sextant makers have been getting it wrong all these years?

Gotta love the forum for bringing out random! You think traditional sextant makers chose metal over plastic on purpose? They had no plastic or mass production when Sextants were the norm. The only reason metal ones are still being made is the huge premium they can charge. Just take a moment to think what happens once you find your position, do you use a metal compass card to steer to? I rather suspect its plastic inside that jar these days!
Metal may be slightly more accurate, but with celestial nav I'd suggest the accuracy of the plastic ones are good enough to get you in a position to start using other nav techniques. I'd also prefer a shiny metal one in a special box, but 10 times the money makes it hard to justify

 

[sarabande]

I do not understand the OP's need to use a top-grade sextant at all, and in particular for having it tuned to the last second of arc.

For beginners, a plastic sextant to show all the operating parts and the principles thereof is more than adequate. Using a metal instrument which is so vulnerable to unskilled use - even replacing a sextant in the box needs skill ! - is nonsense at the level he is teaching. Accuracy is one thing; super precision in reducing a sight, and using oil baths is ridiculous, and cannot be justified in teaching terms.

I think we are being trolled.

 

[Santeh]

I do not understand the OP's need to use a top-grade sextant at all, an din particlaulr for having it tuned to the last second of arc.

For beginners, a plastic sextant to show all the operating parts and the principles thereof is more than adequate. Using a metal instrument which is so vulnerable to unskilled use - even replacing a sextant in the box needs skill ! - is nonsense at the level he is teaching. Accuracy is one thing; super precision in reducing a sight, and using oil baths is ridiculous, and cannot be justified in teaching terms.

I think we are being trolled.

If you were directing your comments to me as the OP:

A). I said my budget was $200-300US. From my understanding this is in no way asking about a top grade sextant and, from what I am reading via replies, will net me a decent but not great unit.

B). If you read my replies you will see that I, in no way, denigrated plastic sextants and even said I would look into them. The only plastic sextant I did not like was my college's as they are so cheap I would not like even calling them sextants.

C). I have never inquired about a sextant that will give great precision as I have been clear on what my goal is, and interest, in acquiring a decent sextant.

D). I missed where anyone, including myself, mentioned oil baths.

My initial query was, and is, legitimate as I am trying to gather the tool(s) needed to teach this part of a lab. for my students. I am, then, trying to be the best teacher I can be and am, in no way, trolling you or anyone.

 

[Uricanejack]

You can check quite easily your self. getting it looked at by a professional instrument maker would only be worthwhile if you intend to use for navigation and even then I wouldn't bother I would just check myself.

a Tayama S is a very good sextant.

First thing to look for is everything nice and tight. the arm doesn't move if you try and wiggle it.
If its moves the worm gear is loose or worn and it won't be accurate.
next have a look to see if any obvious signs of damage, if its been dropped there will be.
check the mirrors, they should be clean and clear no loss of silvering or signs of corrosion.
the telescope should be firmly attached and focus easily.

Generally speaking you adjust a sextant starting with perpendicularity, then side error then index.

Check your index error first it will give you a good idea.
Its quite easy.
Set sextant at zero both on the main arc and the micrometer.

Hold vertically.
Look through the sextant at a nice straight edge preferably a 50 or 100m away.
The reflected image in the mirror and the real image through the glass should for a straight line. Adjust micrometer until they do.
Read the micrometer.
It’s the difference between zero and this reading you are interested in.
This is your sextants index error.
If its reads less than zero its known as off the arc.
If it reads more than zero its known as on the arc.

A good well adjusted Tayama will have only a very slight index error.
A really good one like a 1970s Spica or Jupiter it might even be zero. (Yes instructors say its not possible) Tayama made the good enough even a pro user would not be able to measure it. If its really small like this don’t mess with it.

If its several minutes don’t worry its probably just a bit out of adjustment.

The first error you can adjust is perpendicular.
Set the arm somewhere about the middle of the arc.
Hold sextant out at arms length and look into the Index mirror. (it’s the one on the arm at the top) you should be able to see the reflected image of the arc and the arc in a perfectly straight line. If its not the Index mirror is not perfectly perpendicular to the plane of the sextant. A slight difference is not a big deal.
There is 2 adjustment screws. One close to the body of the sextant One on the outside edge of the mirror there are little spring steel tabs holding it in place.
The outside one adjusts the mirror for perpendicular. (don’t adjust yet check each error first)

Next Check the side error. Its easiest using a small bright star. But you can check using a small object like top of a pole. Or by holding the sextant horizontally and looking at the same straight edge.

Set the sextant at zero. Look through telescope at star or pole top. Or hold horizontal and look at the straight edge.

The reflected image and the real image in the horizon mirror and glass( it’s the one in front of the telescope, usually half mirror half glass, some Tayamas had a full glass)
You should see a single image. If its out it will be reflected image will be slightly out to one side of the true object.
A small difference is not a big deal.
Side error can be adjusted by turning the single screw near the edge of the horizon mirror
(don’t adjust yet)

Check the index error again.

The index error is due to the two mirrors not being absolutely parallel to each other. Set sextant to zero. Look through the telescope at star or distant small object or distant straight edge.

Index error can be adjusted using the screw on the index mirror closest to the plane of the sextant.

Every time you adjust the index screw you will put a slight perpendicular error into the sextant.

Go through the process again.

The only one of the three adjustable errors you can measure is index error.

After going through a couple of times and as near as you can eliminating perpendicularity and side errors and the index error is small. Leave it. And just measure it again.

If your errors are very small just leave it alone.

If they are quite noticeable. You have a decision to make.

Look very closely at the adjustment screws. If every thing is nice and clean its probably ok for you to adjust.
If you see signs of verdegris or slight whitish powder there is a bit of corrosion in there. The big worry is how good the retaining springs are.

If its corroded you probably want to get it cleaned and have the springs replaced.

On an old sextant if you move them the springs may not be good enough to keep the mirrors tightly in place after.

If you do the above you will have a good idea of the overall condition of your sextant.

One advantage of an old kind of worn sextant. You can adjust and adjust again and again without worrying about it.

The real difference between a good sextant and the plastic jobs is once you set it up it stays set up and all you need to do is check the index error stays constant.

The plastic ones I’ve never seen one stay constant. Other folks might have.

One you know what an error is its no longer an error. it’s a measured quantity.


On older sextant's there was an adjustable error known as collimation. this results from a telescope which is not perfectly parallel to the plane of the sextant.
on more modern sextants like the Tayamas, frieberegers ect. the telescope is fixed and not adjustable even though they can be changed and sometimes moved in and out from he plane of the sextant.
if you have collimation error you have a duff sextant.

 

[TonyBuckley]

The chap in "all is lost" learnt to use his sextant in five minutes.

Can't be that hard and he located his position within 5m every time at any time of day.

Just tell your students to watch the film

 

[Santeh]

You can check quite easily your self. getting it looked at by a professional instrument maker would only be worthwhile if you intend to use for navigation and even then I wouldn't bother I would just check myself.

a Tayama S is a very good sextant.

First thing to look for is everything nice and tight. the arm doesn't move if you try and wiggle it.
If its moves the worm gear is loose or worn and it won't be accurate.
next have a look to see if any obvious signs of damage, if its been dropped there will be.
check the mirrors, they should be clean and clear no loss of silvering or signs of corrosion.
the telescope should be firmly attached and focus easily.

Generally speaking you adjust a sextant starting with perpendicularity, then side error then index.

Check your index error first it will give you a good idea.
Its quite easy.
Set sextant at zero both on the main arc and the micrometer.

Hold vertically.
Look through the sextant at a nice straight edge preferably a 50 or 100m away.
The reflected image in the mirror and the real image through the glass should for a straight line. Adjust micrometer until they do.
Read the micrometer.
It’s the difference between zero and this reading you are interested in.
This is your sextants index error.
If its reads less than zero its known as off the arc.
If it reads more than zero its known as on the arc.

A good well adjusted Tayama will have only a very slight index error.
A really good one like a 1970s Spica or Jupiter it might even be zero. (Yes instructors say its not possible) Tayama made the good enough even a pro user would not be able to measure it. If its really small like this don’t mess with it.

If its several minutes don’t worry its probably just a bit out of adjustment.

The first error you can adjust is perpendicular.
Set the arm somewhere about the middle of the arc.
Hold sextant out at arms length and look into the Index mirror. (it’s the one on the arm at the top) you should be able to see the reflected image of the arc and the arc in a perfectly straight line. If its not the Index mirror is not perfectly perpendicular to the plane of the sextant. A slight difference is not a big deal.
There is 2 adjustment screws. One close to the body of the sextant One on the outside edge of the mirror there are little spring steel tabs holding it in place.
The outside one adjusts the mirror for perpendicular. (don’t adjust yet check each error first)

Next Check the side error. Its easiest using a small bright star. But you can check using a small object like top of a pole. Or by holding the sextant horizontally and looking at the same straight edge.

Set the sextant at zero. Look through telescope at star or pole top. Or hold horizontal and look at the straight edge.

The reflected image and the real image in the horizon mirror and glass( it’s the one in front of the telescope, usually half mirror half glass, some Tayamas had a full glass)
You should see a single image. If its out it will be reflected image will be slightly out to one side of the true object.
A small difference is not a big deal.
Side error can be adjusted by turning the single screw near the edge of the horizon mirror
(don’t adjust yet)

Check the index error again.

The index error is due to the two mirrors not being absolutely parallel to each other. Set sextant to zero. Look through the telescope at star or distant small object or distant straight edge.

Index error can be adjusted using the screw on the index mirror closest to the plane of the sextant.

Every time you adjust the index screw you will put a slight perpendicular error into the sextant.

Go through the process again.

The only one of the three adjustable errors you can measure is index error.

After going through a couple of times and as near as you can eliminating perpendicularity and side errors and the index error is small. Leave it. And just measure it again.

If your errors are very small just leave it alone.

If they are quite noticeable. You have a decision to make.

Look very closely at the adjustment screws. If every thing is nice and clean its probably ok for you to adjust.
If you see signs of verdegris or slight whitish powder there is a bit of corrosion in there. The big worry is how good the retaining springs are.

If its corroded you probably want to get it cleaned and have the springs replaced.

On an old sextant if you move them the springs may not be good enough to keep the mirrors tightly in place after.

If you do the above you will have a good idea of the overall condition of your sextant.

One advantage of an old kind of worn sextant. You can adjust and adjust again and again without worrying about it.

The real difference between a good sextant and the plastic jobs is once you set it up it stays set up and all you need to do is check the index error stays constant.

The plastic ones I’ve never seen one stay constant. Other folks might have.

One you know what an error is its no longer an error. it’s a measured quantity.


On older sextant's there was an adjustable error known as collimation. this results from a telescope which is not perfectly parallel to the plane of the sextant.
on more modern sextants like the Tayamas, frieberegers ect. the telescope is fixed and not adjustable even though they can be changed and sometimes moved in and out from he plane of the sextant.
if you have collimation error you have a duff sextant.

Wow! Thank you very much! I am, in no way, a qualified technician but I will look into all of these items once my sextant arrives!

Also, it is very good to know that the Tamaya 635-S I bought is a good sextant. I do not have much money, and am paying for this on my own, thus I really could not afford to make a mistake. This is why I both did some readings on different sextants, as well as elected to ask you all.

Thank you again!

 

[Santeh]

The chap in "all is lost" learnt to use his sextant in five minutes.

Can't be that hard and he located his position within 5m every time at any time of day.

Just tell your students to watch the film

I was not aware of the film, thus I looked it up on IMDB. The movie sounds interesting, and I like Robert Redford, thus if I get a chance I might watch it. Ta!

 

[TonyBuckley]

I was not aware of the film, thus I looked it up on IMDB. The movie sounds interesting, and I like Robert Redford, thus if I get a chance I might watch it. Ta!

Please don't. It is quite cringy if you have ever seen a boat, let alone sailed one. RR acts his role well, but sailors need to watch with a VERY open mind and allow their own knowledge to be suspended.

However, the UK DVD comes out 28th April

 

[Skylark]

RR acts his role well,

It couldn't have taken him long to learn his lines (or should that be line).

 

[BruceDanforth]

I used one of those plastic sextants you are describing to get a set of sights pretty much everyday right across the Pacific and it worked OK. The amount a small boat bounces around having a vernier rather than a micrometer drum didn't seem to really make that much difference. I didn't seem to do any worse than another guy on the boat with a much better instrument. The lack of telescope made star sights pretty much impossible though.

Hello all,

I am new to the sextant world, yet in one of the college courses I teach there is a small section on celestial navigation, of which the sextant is described and used. Unfortunately my college has some very, very cheap sextants (hard to even call them that as there is no micrometer drum, clamps, or even true telescope) that I find very hard to try and work with. I was therefore thinking of getting a real sextant so that I can both learn how to effectively use the device, as well as show my students what a true sextant is about.

I was therefor wondering what kind if sextant you all might suggest? I have been eyeing some Tamayas, yet if this might be a good type are there any models that I should focus on (thus also those to avoid)? If a Tamaya is not an appropriate choice, then what might be a good one for my purpose? I was hoping to spend $200-300US or so if possible.

Also, out of curiosity what does MAC mean in regards to sextants?

Thank you all for your suggestions!

 

[Deleted member 36384]

You will also need a marine chronometer in a gimballed frame mounted in a box.

 

[VO5]

You will also need a marine chronometer in a gimballed frame mounted in a box.

And a stopwatch on a lanyard.