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 5, 10:00 -0000
From: George Huxtable
Date: 2006 Nov 5, 10:00 -0000
Oh, dear. I AM getting error-prone... I wrote, in NavList 1588, about estimating the offset between the two views from a sextant, when it's set to zero on the arc. | | 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. But it isn't, I'm afraid. It's d x sin 2A, if A is defined in the way I had suggested. Sorry about that. I thought there was something wrong, when comparing the offsets by the two different methods I had proposed. I have two very different sextants, a plastic Ebbco micrometer-type, and a Vernier Heath-clone, and in both the offset angle A is pretty near 20 degrees, by a rough visual check using a Douglas protractor; perhaps a degree or so less. A better method may be to lay the sextant flat down, photograph it from vertically above, and measure off the photo with a protractor, but I haven't done that (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 -~----------~----~----~----~------~----~------~--~---