NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Calibrating a sextant scale
From: Alexandre Eremenko
Date: 2007 Nov 26, 00:31 -0500
From: Alexandre Eremenko
Date: 2007 Nov 26, 00:31 -0500
Frank, > That doesn't mean a sextant is > "poor". It just means the certificate may be irrelevant. You mentioned that the 0.8' error that you eliminated returned after a highway trip. My statement that this was a poor sextant was related to this, not to the difference between the current arc error and the certificate. And to make it even more precise, I mean not "poorly made" but "in poor condition". > Some people aren't worried about that very fine level of > measurement. Mike was talking about a > plastic Ebbco sextant. For an > instrument like that, star-to-star > angles should provide all the necessary > accuracy. No doubt. In all this discussion of calibrarion I was not talking about 1' arc errors or plastic sextants. > could be done in a relatively short period > of time (a night or two?) maybe Hardly. There are usually not enough good distances to measure in a "night or two" to determine the arc error. Aiming at "ultimate accuracy" I use at least 1/2 of the full cycle of Sun and Jupiter. This takes 2 weeks under extremelly lucky weather. And measure the same distance MANY times, on several days. (Again I am NOT talking of plastic sextants and 1' errors. I am talking of making a table of errors which can be used reliably when doing lunars.) > once every five or ten years > by using special apparatus Russian manual says: every two years. Instead of a certificate they issue a little book with empty forms for the arc corrections. The forms have to be filled up every two years in "Navigation Chambers" Besides the arc correction they measure backlash. > Otherwise, I wouldn't drive with them at all, right? I afraid that at least 95% of this list members have to drive from home to reach their boats:-) (I am in even worse condition, I have to take an airplane, drive to the airports etc.) So from my personal perspective a sextant is useless if it can develop a 0.8' ARC ERROR after a transportation in a car. (I don't mean the index or side error which is relatively easy to measure and adjust). > *eventually* change the adjustment > of a sextant and require > re-certification. Frank, let us make a distinction between re-adjustmewnt and certification. Arc error is not adjustable. It can change if you either deform the frame, or if something serious happens with the arm pivot or with the worm screw assembly. I think only the third named thing can be adjusted without special equipment, and even this hardly. (Any such adjustment is explicitly prohibited by the Russian manual). > One of the things that I really like about the SNO-T is > its very sturdy design. > I do expect that your sextant could survive more > banging around than average sextants. When I bought it, it was sold "as new". And it looked new, never used and in factory packing, rubber parts covered with talc. The factory certificate contained only the original factory arc correction. It's true, this original certificate is total nonsense (which shows that nobody really measured anything and that the person who filled the certificate did not understand what s/he was doing). I suppose that my sextant was kept in some warehouse from 1990 when it was made to 2003 when it was sold. And of course I use utmost care in transportation:-) Alex. --~--~---------~--~----~------------~-------~--~----~ To post to this group, send email to NavList@fer3.com To , send email to NavList-@fer3.com -~----------~----~----~----~------~----~------~--~---