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A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Star-star distances for arc error
From: Bill Morris
Date: 2009 Jun 26, 00:53 -0700
From: Bill Morris
Date: 2009 Jun 26, 00:53 -0700
Frank and Douglas ""Also, the measurement can only be done to an accuracy of the divisions of the sextant which in most cases is one minute of arc." Most modern sextants can be read to a TENTH of a minute of arc. Even sextants which lack verniers on the micrometer can be read to at worst 0.2'. Of course, this assumes that the instrument has been properly adjusted (mirrors perpendicular to frame, telescope axis parallel to frame, etc.) and has no pathological problems, like a loose micrometer, for example. and you wrote: "the certificate indicates accuracy of max error of 1 minute 30 seconds at points on the scale." But that is a correctable error." Textbooks of metrology usually advise against interpolation, saying that a value closest to the least graduation interval should be read, so in that respect, Douglas has some support. However, I guess that is advice that is often ignored, probably safely most of the time even when interpolating tenths would take one close to the absolute limits of the instrument's accuracy. It is worth bearing in mind that calibration certificates for micrometer sextants give the errors only for whole numbers of degrees, every fifteen degrees for NPL certificates, and every twenty degrees for Heath and Co certificates. In doing so, they are in effect calibrating only the rack and centring error. It isn't really all that safe to interpolate, as the sources of error are complex and the errors do not usually vary predicatably over the measurement range of the sextant. Ideally, makers should perhaps also calibrate the worm and its centring errors through a whole rotation. Noone would consider that an engineer's micrometer had been properly calibrated if this had not been done, as well as checking at intervals of, say, 2.5 millimetres throughout the measurement range. The SNO-T sextant handbook claims a maximum error for the drum reading of 6 seconds but I haven't come across another sextant that addresses the matter. It is a relatively simple and quick matter to calibrate the worm and centring errors of the drum using an autocollimator reading off the index mirror. I have briefly described the instrument in a previous post on calibration in the workshop. Members might be interested in some results. Yesterday, I calibrated the drums of several common sextants at 5 minute intervals. All are in good adjustment and I took appropriate precautions against backlash. The autocollimator's least graduation is 0.2 seconds. Members may be interested in the results. The SNO-T met the claims made for it very comfortably with a maximum error of only 3 seconds. Next best was a Heath Navigation instrument from 1967. It had a maximum error of 17 seconds and most other errors were under 10 seconds, say 3 to 7 seconds. A well-used Husun sextant from 1938 had errors mostly between 10 and 20 seconds, with a maximum of 29 seconds. A Tamaya 632D sextant from 1957 in apparently excellent condition was no better in one part of the worm with three adjacent readings giving errors rising from 20 to 31 seconds though most were in the 4 to 10 second range (this surprised me so much that I repeated the readings with similar results). One might wonder then that some people achieve superb results, and this is probably because averaging evens out the drum and worm errors, which must sum to zero over a whole rotation. I take it for granted that they must also have great observational skills. Given that they do have the skills, it occurs to me that star-star observations might be at least as good a way of calibrating a sextant as a laboratory method that looks only at whole degrees. One short cut when calibrating an engineer's micrometer is to check it at, say, 2.5, 5.1, 7.7, 10.3, 12.9... and so on, rather than every 2.5 mm as well as a full turn at each end. In effect, this is what star-star distances do in a less structured way. Bill Morris Pukenui New Zealand --~--~---------~--~----~------------~-------~--~----~ Navigation List archive: www.fer3.com/arc To post, email NavList@fer3.com To , email NavList-@fer3.com -~----------~----~----~----~------~----~------~--~---