Buy a sextant

[Douglas Denny]

Hi

I have a Mark 15 plastic sextant and I want to buy a better one.. 
I've heard about Astra IIIb and Astra Professional... differences? what is the best?

Other tips are welcome!!!

Thank you
Alberto

Any relatively modern sextant with a metal frame and in obviously good condition should give good service and quite adequate accuracy for practical navigation. 

A micrometer drum for measuring minutes is now standard, and is easier to use than a vernier sextant, but a good quality vernier sextant is still accurate and more than adequate for practical use for celestial navigation.
The British sailor Sir Alec Rose sailed single-handed around the globe (which is why he was knighted as was Sir Francis Chichester)  using his metal frame vernier sextant - now on display in Portsmouth museum.

The most expensive is not necessarily the "best".   More money usually only means more bells and whistles and better finish, therefore better 'looking': but not necessarily better accuracy of what its function is - measurement of arc - accurately.

I have various sextants but the 'best' of all was amazingly cheap costing about £200  ($240  exchange rate Jan 2016), bought about four or five years ago;  it is a Russian SNO-T and was new, unused in the original box.  These were ex-Russian Navy released presumably as surplus to requirements.

The box it came in was very basic,  but the sextant itself is superb quality and accuracy - better I think than my Zeiss Freiberger Precision surveyor's Trommel (drum) sextant.

Check the 'feel' of the Index arm movement along the scale, and for any movement in the index arm bearing (at the top of the sextant.  If it is smooth and free (but not too free, i.e. not sloppy) with no obvious dents or marks showing distress or dropped, then it should be a fair bet for being Ok.

Douglas Denny.  Bosham.  England.