A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Perpendicularity and other qstns.
From: Alexandre Eremenko
Date: 2004 Oct 12, 19:48 -0500
From: Alexandre Eremenko
Date: 2004 Oct 12, 19:48 -0500
Tamaya manual confirms what I said EXACTLY. And Bruce Stark (Oct 12 2004 - 11:51:12 EDT ) confirms it PARTIALLY, by his own experiment. (Thanks to everyone who actually did this experiment on my request.) And I have further comments, both on perpendicularity and some of a more general nature (if you have patience to read this message to the end:-) Thanks to everyone who participated in this discussion. And let me try to summarize. Tamaya manual (see Doug Royer, 12 Oct 2004 12:57:26 EDT): "You will notice that as the eye is moved above the sextant,the appearance of the reflected arc changes. The ONLY correct observation point is with the eyes on a level with the sextant frame itself". [My emphasis of the word "only"-A.E.] Further it says: "To make adjustments for perpendicularity, move your line of sight to be level with the tops of the cylinders". Bruce Stark (11:51:12 EDT) says: "Without the vanes, just viewing the two parts of the arc, the alignment changes from the bottom of the mirror edge, where it's nearly perfect, to the top". This is EXACTLY what I was saying. I add: 1. This has NOTHING to do with your mirrors silvering, whether it's front or back. (Tamaya manual is WRONG on this). But this IS related to the size of the mirrors. 2. To see the effect the mirrors have to be LARGE (my ones are 42 mm "across", that is in the direction perpendisular to the frame. 3. The effect can be demonstrated mathematically. And I can post this math if there is interest to it on this list. Now I reply some of the messages on the subject. (Sorry for not replying all of them, there were too many). Bruce Stark: "I put the vanes on, taking care that they were aligned perfectly with the inner edge of the flat surface of the arc, and found that their alignment remained the same from top to bottom of the edge of the mirror". This is not so, at least with my SNO-T sextant. What sextant did you use, what is the size of the mirrors, and what was the distance from you eye to the index mirror? Everyone seems to agree that the "vanes", (cylinders, visors, dominoes whatever you call them) test is MORE precise than the "arc test". So how can it be that you see "non-alignment" with the frame test, and do not see it with the "vanes test"? Herbert Prinz: "The test checks the perpendicularity of the mirror to the LINE passing through the upper edges of both visors." This is not so. Simple proof: move the index arm few degrees in any direction. The plane of the index mirror changes. Or move the visors on the arc. The line connecting the visors changes. But the test is still valid. Herbert Prinz: "You are describing a parallactic effect which can only be caused by a different distance of the individual cylinders from the front edge of the index mirror." No, this is not what I am discribing. "Now, how does the size of the mirror come into play?" It permits you to move your eye up and down substantial distance, (42 milimeters approx.) without loosing the view of both visors. To George Huxtable: No, this has nothing to do with front silvered mirrors. I suspect this was a mistake in Tamaya manual. It has to do with LARGE mirrors. The effect should be clearly visible with modern Cassens-Plath, Astra, Tamaya, Freiberger and SNO-T sextants. If the mirror is small (with respect to the frame) the effect should be very small and hardly noticeable. Unfortunately I have no British sextant for comparison. The effect I mentioned is probably not visible with small mirrors (and/or very large frame). Because in this case, the limits of your eye moves up and down are small. The cylinders indeed help (thanks to Jim Thompson), because of their flat horizontal tops. This permits to keep your eye exactly in the plane of the tops of the cylinders. (I don't have the cylinders, only the standard Russian "visors" which have sharp edges, and thus ARE NOT appropriate for this test). Now I pass to some general observations. 1. The test is described with the similar words in all books I read. (Except the Tamaya manual posted today). Apparently this comes from the very first description of the Headley octant by Headley himself in Proc. Roy. Soc. The authoirs are very vague about the placement of your eye. (Most say: "You should look obliquely"; the Russian manual simply says "Look down", whatever this means. This seems EXTREMALLY strange to me. This is a very basic test. Test Number 1 in all books I know. It is hard to believe that the book authors just copy what previous books say, without really testing this. One objection I forsee that the issue is irrelevant, because FEW DEGREES of error in perpendicularity do not matter. Yes, we are talking of few DEGREES here, not minutes and not seconds. I am going to investigate this further and will inform the list if this is interesting. I mean the influence of non-perpendicularity on the altitude reading. 2. The effect is LARGE. Few milimeters if measured on the right edge of the index mirror (as seen from 1 foot distance), so I am extremally surprised that several people who say they actually performed the test, do not see it. I still do not have any comments from professionals doing sextant adjustment. I cannot imagine that they did not notice the effect. Another question is how important this few milimeters non-alignment is. This is a subject for further investigation. Alex.