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Re: Astra 3 proffesional
Celestaire are or were the importers, I'd see what they have first.
A book on sextant alignment and checking it over, to make sure it is in good working order. Sometimes there are alignment tools you might want.
Mary Blewitt' s little book, Mixter's Primer of Navigation, and Wm. Buckley's video on how to use the sextant are all classics.
Each approached things differently. Mixter walks you gently through the math and navigation all the way from ancient times to modern logic with spherical trigonometry--which frighteningly makes great sense for navigation!
Basically you need to know if the sextant is "bent" or aligned properly. How to zero it out before each use. And then, what to measure with it, along with how to take the exact time of the measurement. (I've used a cheap digital stopwatch, hacked to the real time, velcro'd onto the sextant so I could just "squeeze" to mark the exact time of a sight.)
Given the time, and the measurement, then you have the choices of using the tables and books and doing the math the traditional way, or punching the numbers into a calculator, a PC program, even an old Palm Pilot (cheap and reliable with good software available cheap) and when the numbers are crunched, you will still not know where you are.
You will, however, know that you are a certain distance from a certain point, i.e. someplace along a big curved line. Once you have done that a couple of times and can get a couple of lines to cross, THEN you know...you're someplace near there. How close you are, depends on how good everything else was.
Navigators, sextant or not, often refer to a "circle of position". You KNOW that you are somewhere in the circle. Sometimes the circle is a half mile wide, sometimes it is two hundred miles wide. With the best of sextants in the best of circumstances, a circle two miles across would be considered EXCELLENT.
And if you are at the shore, or near it? Twenty miles might be all you'll get, because of atmospheric distortions in coastal areas. So be patient, and try to get some used books or find a local helper. One of them will wind up talking in terms that makes sense to you. Might not be the first, or even the fourth.