NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Centring Error Detector
From: George Huxtable
Date: 2007 Jan 12, 23:50 -0000
From: George Huxtable
Date: 2007 Jan 12, 23:50 -0000
Phil wrote- I'm looking for information about "Centring Error Detectors", machines to verificate and evaluate centring error of sextant. Here is an exampe of this kind of instrument : http://bib.iota.u-psud.fr/Record.htm?idlist=3&record=894512461279 Here is an other kind : http://gita.grainger.uiuc.edu/IOPText/0950-7671/6/10/408/siv6i10p324.pdf ======================= Frank responded- For those who | didn't read it, I'll give my summary... | | The purpose of the instrument(s) is to measure "arc error". This is a | combination of eccentricity (centering error), division error, and any other errors | which are fixed over time and depend on the angle observed with the sextant. | It's the (presumably) fixed, correctable error tabulated on the certificates | pasted to the inside of the sextant case. | | The idea behind the described system is to insert a prism with a known angle | of deflection into the light path in front of the horizon glass of the | sextant or in front of the telescope such that it occupies the part of the field | of view where the horizon side is normally visible. The simplest case is a 90 | degree prism. You place it in front of the horizon side of the sextant's | telescope, and it lets that side of the telescope see straight up instead of | straight ahead. You then set the sextant arm to 90 degrees and aim it at some | object. For example you could look at the sea horizon by pointing the sextant | straight down. The mirrors let you see the horizon on the right side. The prism | simultaneously shows the horizon on the left side. Then the procedure to | measure the error is exactly the same as an ordinary index error sight. You line | up the two images and read off the error. Assuming the prism is an exact 90 | degree prism, the "index correction" that has just been measured using the | prism is actually the "arc error" at 90 degrees (arc error plus normal IC?). In | the "patented" system as described, the sextant user would be provided with a | set of prisms and holders for angles of 30, 60, 90, and (optionally) 120 | degrees. The prisms do not need to have exactly those angles, but they need to | have their actual angles accurately measured before they may be used. By | making observations with each prism, a rough correction table for the sextant's | arc error could be constructed. It's a very clever idea, but if you had such a | system today, would it be any more useful than ordinary sextant observations | (or maybe lunars) in assessing arc error? ===================== Note from George. There's an extra complication here. None of the prisms referred to, in these two documents, are ordinary prisms. They are all PENTAPRISMS, the 5-sided prism that you need to get a defined angle of deflection, however the light comes in. So when Frank mentions a 90 degree prism, it's not the 90-45-45 three-sided prism that you or I would think of first, but a special (and presumably expensive) 5-sided job. Perhaps you may be able to salvage one such, for 90 degrees, from a suitable camera, but not the whole range of different deflection angles, unfortunately. The Hezzanith system, shown in the second reference, is to be used by an observer at sea, using an object at infinity such as horizon or star, but the French version appears to be designed for an indoor test setup for certification. It has a pair of parallel collimators, spaced according to the offset between the two sightlines of a sextant. Both systems use similar pentaprisms. Although the Hezzanith patent offers the four prisms that Frank describes, the French Fabry version is a bit more clever, in offering four prisms such that combining them in different ways, you can test all angles in 15-degree steps up to 135 degrees. An earlier setup at Kew, for certificating sextants, (not shown in those websites) comprised an immense assembly of such collimator-pairs, all looking at a sextant mounted on its side at the centre of the array. Again, that was intended to tackle all angles at 15 degree spacings, by using different combinations of angles between them. George. contact George Huxtable at george@huxtable.u-net.com or at +44 1865 820222 (from UK, 01865 820222) or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK. --~--~---------~--~----~------------~-------~--~----~ To post to this group, send email to NavList@fer3.com To , send email to NavList-@fer3.com -~----------~----~----~----~------~----~------~--~---