NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Calibrating a sextant scale
From: Frank Reed
Date: 2007 Nov 24, 23:56 -0500
From: Frank Reed
Date: 2007 Nov 24, 23:56 -0500
George H, you wrote: "Has anyone on this list, by measuring star-star distances or by any other method, ever discovered reproducible errors, outside the terms of a calibration certificate or maker's warranty, in a sextant? Has anyone made calibration measurements of his own, in which he has more confidence than in the manufacturer's scale readings, corrected as necessary by the box certificate? And if the answer is yes, what's the magnitude of those errors?" Sure, I've done this with a few different sextants. Two interesting cases: I had a Tamaya-alike, assembled by a company whose name I can't remember at the moment. Using lunars, I made an arc error table that had a maximum value of 1.8 minutes of arc. The certificate said that the error was insignificant at all angles. This arc error was reproducible. I eventually sold that sextant though not because of the arc error. Another, different Talmaya-alike assmbled by 'International Nautical' has been excellent most of the time with no measurable arc error, but it has a recurring micrometer eccentricity problem amounting to as much as 0.8 minutes of arc. By tinkering with it, I managed to eliminate the micrometer eccentricity at one point. I don't know exactly how I accomplished this. It was one of those cases where I took the micrometer apart, put it back together, and it was fixed. Unfortunately, after a highway trip, the micrometer eccentricity problem came back (this was a sextant that I let Alex use for a while --sadly its perfect performance apparently came to an end just before he had a chance to try it out). And this latter case, I think, tells us something important about most modern sextants: they've been banged around enough in normal shipping that one cannot trust the certificates in the cases. If you're interested in high-accuracy sights like lunars, you may want to find a way to calibrate the instrument again. -FER --~--~---------~--~----~------------~-------~--~----~ To post to this group, send email to NavList@fer3.com To , send email to NavList-@fer3.com -~----------~----~----~----~------~----~------~--~---