NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: "Vernier acuity" of horizon IC tests
From: Greg Rudzinski
Date: 2009 Jul 7, 20:46 -0700
From: Greg Rudzinski
Date: 2009 Jul 7, 20:46 -0700
Frank, As you have suggested I did a series of 30 index error measurements with no sight tube then 30 with the sight tube. The sextant used is a US Navy mark 3 M.Low with a horizon mirror silvered on the back leaving a clean line between the clear half and silvered half. Holding the sextant two feet from the eye for 30 consecutive observations was quite punishing on the arm but steadiness was maintained. The horizon was good showing only a very slight fuzziness. This time I wore sunglasses, blinked, then squinted to get an improved focus of the horizon (5.1 NM away from a height of eye of 20ft.). The sunglasses removed some glare and most importantly kept the wind out of my eyes. The sight tube observations presented a sharper horizon and an overlapping field (acuity not in effect). The observations without the sight tube did not have an overlapping field (acuity in effect). Results: Without sight tube With sight tube Average 2.05' off arc 1.85' off arc SD .40' .44' Spread 1.6' 1.6' Conclusion: Results are very close so if acuity has an effect then it is subtle in nature. And I had a good vision day ;-) Greg On Jul 6, 8:19�pm,wrote: > Hi Greg. > > You should try this without the sight tube. The sight tube serves two purposes: it maintains the collimation of the line of sight so that you are looking along a line of sight parallel to the frame, and by providing a tiny aperture to look through, it generally improves the focus of the eye. We don't need to worry about collimation for IC tests (the error resulting from collimation error goes to zero at zero degrees). The focus of the observer's eye can be equally well improved by wearing eyeglasses. With the sight tube removed, hold the instrument at arm's length. Then you will see the straight line of the horizon on each side of the horizon glass with no "merging" or "fading" zone across the middle. By the way, this possibility of a "hyper-acuity task" only applies to standard style horizon glasses, mirrored on one side, clear on the other, with no dividing line between the sides (just the edge of the silvering itself). > > -FER --~--~---------~--~----~------------~-------~--~----~ Navigation List archive: www.fer3.com/arc To post, email NavList@fer3.com To , email NavList-@fer3.com -~----------~----~----~----~------~----~------~--~---