NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: sextant index error measurement
From: George Huxtable
Date: 2006 Nov 3, 11:19 -0000
From: George Huxtable
Date: 2006 Nov 3, 11:19 -0000
Paul Hirose wrote about his interesting suggestion for placing stripes on a distant board for testing index error | > From the vertical distance | > between the telescope and index mirror, and the distance to the target, | > you can compute the parallax. Add parallax to the sextant reading to | > obtain index error. and Bill responded- | This is where I have failed in the past. How does one know the vertical | distance between a moving point on the index mirror and the central axis of | the scope? It seems quite the reverse. If you know the angle the sextant | is set to, the distance to the target (from what point of the system?) AND | the *exact* IE/IC you could compute the vertical distance. It's clear what angle the sextant should be set to; to zero, or very nearly so. Because it'a a check of the index error that is being made. If you have a peep-tube, to show roughly the line of collimation of the sextant, it's a very simple matter to determine the offset between these two incoming sight directions, which are ideally parallel, or very nearly so. Use whatever specs are needed to give you a clear view at a foot or so distance. Put some sort of boldly-marked ruler at a convenient (short; a few inches perhaps) distance in view of the sextant, so that two views of it can be seen, one through the index mirror, one through the horizon glass, and record the offset between them. The ruler has to be roughly at right angles to the line of view, but that's very uncritical. I used a dressmaking tape that my wife has. It's dead easy to get the answer to a millimetre or so. For my sextant, it was 50 mm, give or take a mm. Or you can do it by geometry. For that, you will need the effective distance d between the parallel reflecting surfaces of the two mirrors. You can measure this between the upper part of the horizon glass and the lower part of the index glass, where there will usually be some overlap between those parallels. If they are front-silvered, that's easy. Otherwise, if you want to be precise, you will have to add two-thirds of the combined thickness of the two glasses; the two-thirds factor allowing for the refractive index of glass. And you need the angle A by which the horizon mirror is tilted from the plane that's at right angles to the collimation line of the telescope. You should be able to estimate this using some sort of protractor (or course plotter). Then the spacing between the sightlines is then d x cos 2A. If the sextant has been designed properly, and most are, it will be the same as the perpendicular distance between the pivot of the index arm and the centre-line of the scope; but in fact, the placing of the pivot has no direct influence on the offset between the sightlines. That sightline offset, having been obtained once for a sextant to a mm or so, won't change, and it might be useful to write it on a slip of paper tacked to the inside of the box. It will vary, between one sextant and another, but not by much. Then a nice white board could be made up, with a few black lines inscribed on it at just the right spacing for your sextant, as Paul suggests. Insulation tape would be a convenient method of marking. Perhaps one black line could be cut a bit shorter than the others, to avoid any miscounting. It needs to be stood up roughly vertical, at a convenient height. Because those spacings closely match the sextant's offsets, there's no longer a requirement for that board to be placed a very long way off. If the offset matches within a millimetre, then a board placed 30 metres away would would allow alignment within 0.1 arc-minute; all that's necessary for normal marine navigation. It's an idea that could do with trying out, but I haven't tried it in practice, yet. 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 -~----------~----~----~----~------~----~------~--~---