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A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Accuracy: main shades vs eyepiece shades?
From: Brad Morris
Date: 2019 Jul 25, 10:43 -0400
From: Brad Morris
Date: 2019 Jul 25, 10:43 -0400
Tony
Shades are fundamentally glass. Glass has parallel surfaces, unless incredibly poorly made. The prismatic effect you speak of changes the position of the object by the double transition thru air/ glass then glass/air(Snell's Law), but does not change the angle.
Repeat, does not change the angle. Thus, IMHO, it does not matter if you have the shades in or out when it comes to the accuracy of the IE. You can, of course, prove this yourself. Measure the IE several times with shades in and with shades out. You will find either 0 or very negligible error.
The eyepiece shade does not change the angular measurement, but neither do the other shades. I will except very poorly made sextants. No telling what you have with a two hundred year old wooden octant. However, for any sextant made in the last 125 years, this is a non-issue*
Brad
*Some may quibble about the number of years. I selected ~ 1900 as a date by which nearly all sextants were made using float glass. Its probably earlier.
On Thu, Jul 25, 2019, 10:33 AM Tony Oz <NoReply_TonyOz@fer3.com> wrote:
Hello!
A spin-off from here: is it methodologically correct to use an eyepiece shade for IE determination - assumed that the main (the index and the horizon) shades are not used in that particular measurement?
My take is - it is not correct to do so, because no matter how good the main shades are, they still have some degree of "prismatic" property. If one does the IE measurement without the main shades - he introduces an unaccounted prismatic error when doing a normal sight (when the main shades are involved). Doing the IE sight through the main shades automagically accounts for "prismaticity" of them.
Please comment.
Regards,
Tony60°N 30°E
