NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Calibrating a sextant scale
From: Bill Morris
Date: 2007 Nov 22, 00:22 -0800
From: Bill Morris
Date: 2007 Nov 22, 00:22 -0800
The interested amateur can calibrate a sextant(which should of course be in adjustment) without too much expensive gear, now that so much is available on e-bay. He will need two collimators. A collimator is essentially a lens with illuminated crosswires or other graticule at its focal point, so that the image of the wires appears to be at infinity. He will also need to own or have access to a modern universal theodolite, reading directly to single seconds of an arc, e.g Wild T2 or later. It will be helpful to know how to operate the theodolite, but they are robust and not impossibly complicated. In principle,the procedure is as follows. Focus the theodolite on a distant object, level it and then point at one of the collimators, set up in the same horizontal plane a few metres away. Sight on the vertical wire. It should be in focus, If it is not, make it so by adjusting the collimator cross wires, not the thedolite. Note the reading(s) on the horizontal circle of the theodolite. Rotate the theodolite through, say, fifteen degrees, set up the second collimator and adjust it until its vertical wire coincides with that of the theodolite(it helps to have an assistant and /or a rotary table to do this). You now have, as it were, two images at infinity, fifteen degrees apart. With your sextant supported horizontally, see how it compares. Repeat for as many angles as you like. In practice there are many refinements to get the angles as exact as possible, but the amateur can improvise a collimator from any telescope that has cross wires by the simple expedient of substituting a green LED for the eyepiece; and Edmunds Optical once had a short book "Collimators and Collimation", which describes how to build your own. National testing laboratories like the National Physical Laboratory in Britain would have a battery of up to fourteen collimators set up permanently on masonry columns. There is another method using two instruments known as autocollimators, a large surface plate and a plane mirror, that depends on there being always exactly 180 degrees in a straight line. It is described(in French)in :Canadian Journal of Physics 1976. Vol 54, Number 9, pp 917 - 927. The principle is again simple, but the execution I suspect is not. I will be trying it out over the next few weeks and if there is any interest will give an account of how a Pioneer-Bendix Mk II, a Mark IX bubble sextant and an SNO-T sextant perform. Bill Morris On Nov 21, 12:33 am, Isonomiawrote: > Whilst it is easy to calibrate a sextant's zero reading, is there any > way to calibrate any other readings? > > My thought was to lean over a cliff and view my own reflection, and > then align the mirror with the horizon, this should be somewhere close > to 90deg (give or take the variation of the horizon from horizontal). > > Having established the true 90degree, the 45degrees could be > calibrated by finding two objects precisely 90degrees and finding an > object exactly halfway between. > > Then it occurred to me that I could use a bucket of water at the > seaside, to give two readings (direct and false horizon) one being > exactly** twice the other. (** allowing for known errors) > > at which I've just realised that star charts are far easier ..... > > but it might amuse others to think about leaning over a cliff! --~--~---------~--~----~------------~-------~--~----~ To post to this group, send email to NavList@fer3.com To , send email to NavList-@fer3.com -~----------~----~----~----~------~----~------~--~---