Yippee..just bought a sextant!!! any advice?

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And plenty of batteries..............
 
that's a good first sextant !

The corrections table for index error looks a little 'unusual' as far as I can see from the photo but it will be easy to make the necessary corrections in whatever proforma you use.

The scope is a x2.4 so not very strong for stars, but should be fine for sun sights, and horizontal and vertical angles.

There are many adjustments to be made if you want to reduce the instrumental errors as far as poss, but until you are very happy with taking angles, I'd leave adjusting the moveable and stationary mirrors for later.

You will need a book like Norrie's tables for all the Dip, Refraction, Parallax, Semi-diameter corrections.

As for how to use it. Stick "How to use a sextant" into a search engine, and discard all the answers which would make Lakey blush. Loads of info and you will have fun finding a set of pages which suits your learning profile.
 
That looks like a great buy! It's great fun and very satisfying when you get a decent fix. Search the posts here for details on downloading free almanacs and reduction tables from the web.
 
melandnick said:
So

We are off next year and I just treated myself to a Carl Zeiss sextant: £125

I reckon it was a bargain. Anything I need to check? Any advice / comments for a newbee? Thanks
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That certainly seems to be a bargain. You should be well pleased.

Given that astro was the mainstay navigation 'system' of maritime ocean trade for at least a couple of centuries, there is a great deal of know-how to wade through, if you're that way inclined. Only a little of that is likely to be needed.

Every astro practitioner has a few practices and habits of his own. You'll develop/adopt your own. Certainly, the first of these I'd recommend is that you acquire a strong broad camera strap and attach it to your sextant handle - for exactly the same reason you'd have such a strap on an expensive camera.

The second is to provide a very secure, permanent home on board for the sextant box, such that in the event of big wash/lurch/knockdown, your sextant will remain safe and secure inside its fixed box instead of becoming a missile.

There are lots of books on 'how to do it'. I'd recommend you pay a visit to the Kelvin Hughes stand at the SIBS next month, or - if you cannot wait - Warsash Nautical Bookshop and have a look at 'Ocean Sailing' by Tom Cunliffe/Fernhurst and the RYA Astro Navigation Handbook by Tim Bartlett. Each does the job very well, and it's really a choice of what style appeals best to you.

The best book I have on sextant maintenance is 'The Sextant Handbook' by Cdr Bruce Bauer, available also from the booksellers above. And the best 'astro proformae' around ( oh, I am inviting flak! ) for most purposes are those from www.cockpitcards.com mentioned in an earlier response.

What to check?

Firstly, have a look for signs of bumps and scrapes. Sextants should be treated as precision optical instruments - which is exactly what they are. Few yotties give them the TLC they need. Have a look at the screws holding/adjusting the mirrors, for damage to the s'driver slots. Have a look at the mirroring for signs of peeling and corrosion damage.

Set the arc and micrometer drum exactly at zero, then peer through the scope at a straight distant horizon. That seen directly ( LHS ) and that seen reflected via both mirrors ( RHS ) should ideally align. Tweak the micrometer drum until the two line-images DO align, and read off the 'error' at the drum's arrow-mark, or 'Index'.

If the 'Index Error' is 5' of arc or less, then fine. If 10' of arc or less, still OK. More than that indicates an adjustment is warranted. Speak to Kelvin Hughes.

Next, set the arc about 30-35 degrees, then place the sextant down on a table, arc and mirrors up. Peer past the pivot towards the arc so that you can see a section of the arc itself and - right alongside to the left - that same section reflected in the mirror. You may need to adjust your viewpoint or the angle-setting slightly, so you can see the REAL ARC and the REFLECTED ARC simultaneously.

The two lines should coincide. If they're close, a simple adjustment is warranted to make the mirror perfectly perpendicular to the frame ( arc ). If there's a big misalignment, an inspection and adjustment by someone with experience/skill is probably called for. Again, Kelvin Hughes can help - and so can lots of other yotties you know. The books mentioned explain all this, with photos/diagrams, rather better than I. It is, of course, better and quicker demonstrated by someone who knows.

Finally, and on the water, practice taking, reducing and plotting sights while at anchor/stopped somewhere when you know exactly where you are. Then you can compare your results directly, and give attention to whatever part of the process needs it. Same thing at sea - compare your sight(s) with your GPS position. Given an accurate position reference, you'll steadily build accuracy and confidence for the one or two occasions in a lifetime when you'll need both together.

Enjoi!

:)
 
Astro

If you would like forms to help you through the calculations to obtain a position line, pm me with your email address and I will send them to you. Mk l is for beginners and helps them though the workings and Mk ll assumes you have sufficient knowledge so to do, the forms are a little shorter than the Mk l versions.

Sticky
 
Nearly agree except that the order of checks should be different.

There's 4 main errors, three of which you can correct or nearly so, and the 4th, if bad you fix by getting a different sextant than the one that has the errors.

The 4 are Perpendicularity of frame and index mirror - checked as said by looking at the arc and arc image to see if they line up, then perpendicularity of the frame and horizon glass - also already explained in looking at the horizon, 3rd Index error which is important and can be checked and corrected most easily by looking at star and 4th collimation (if that's spelt right) which means the frame and scope are out of line - this can be roughly checked but not fixed by the average user and generally points to the instrument having been dropped.

You won't hurt it by experimenting with the correction screws as long as you don't force them.

Astro these days is generally useless but a lot of fun and I love it! A secondary benefit of it is being able to confuse most of the others at the bar.
 
No one has told you this but it is very important NEVER to lift or replace or hold the sextant by the index arm as it will only serve to distort it.

Always lift it and replace it by the frame only. When in use only hold it by the handle.

Some sextants fit in the box properly when the index arm is at a given angle.

If this is the case note the angle and make sure to put the arm at this angle when putting it back in the box. Do not force it in any way.

Any salt deposits wash away with fresh water, dry, and then apply sewing machine oil to the screws with a cotton bud. Keep the clamps, micrometer, and worm gear lubricated.

Clean the lenses, mirrors and shades with a soft camel hair brush.

Finally NEVER look at the sun without using the shades, otherwise the sun will burn a dot in the centre of your vision in that eye that will remain with you all your life.

It is perfectly safe to look at the moon, however bright, or any other planet or star without using the shades, however.
 
melandnick said:
So

We are off next year and I just treated myself to a Carl Zeiss sextant:

http://cgi.ebay.co.uk/ws/eBayISAPI.dll?ViewItem&item=280550564029&ssPageName=STRK:MEWNX:IT

£125

I reckon it was a bargain.

Anything I need to check?

Any advice / comments for a newbee?

Thanks
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I have the identical model. I have had it since 1988. It is lovely to use because it is extra light, specially made for yachtsmen.

The only feature I don't like is the light. I feel it could have been set better.

See my comment above about setting the index arm, etc.,

Mine has zero error throughout.
 
TQA said:
Check out Celestial Navigation for Yachtsmen by Mary Blewitt

Old but still the best SIMPLE guide on "how to do it" with a sextant.

Her explanations are " simple, elegant, and easy to grasp." -- WoodenBoat
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Really is my lucky day!!
I just got home and looked through my sailing books....there are many....and found that I have a 1980's copy of Navigation for Yachtsmen by Mary Blewitt.
So as soon as I get my new toy I can start figuring out how to use it.

As always....thanks everyone for the invaluable advice, it is very much appreciated.
 
Many people like Mary Blewitt's book, but I never got on with it very well.

If you find that you are struggling to understand her, then try Tom Cunliffe's book on astro-nav. After I found myself struggling with Blewitt and utterly failing to see why people though that she was so wonderful, Cunliffes style made perfect sense to me and was a refreshing change.

Nothing wrong per se with either book - just different styles.
 
john_morris_uk said:
Many people like Mary Blewitt's book, but I never got on with it very well.

If you find that you are struggling to understand her, then try Tom Cunliffe's book on astro-nav. After I found myself struggling with Blewitt and utterly failing to see why people though that she was so wonderful, Cunliffes style made perfect sense to me and was a refreshing change.

Nothing wrong per se with either book - just different styles.
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I was exactly the same. I bought both, and although I was able to follow Mary Blewitt's book with a bit of hard work, I found Tom Cunliffe's just SO much easier to understand.
 
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