A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Alexandre Eremenko
Date: 2004 Sep 22, 11:13 -0500
From: Alexandre Eremenko
Date: 2004 Sep 22, 11:13 -0500
I recently bought an SNO-T sextant. This is my first sextant and now I am playing with it day and night:-) Btw, the decision to buy this one came after reading some favorable coverage of Soviet sextants in this list. (For those interested in the Russian sextants, I add few remarks at the end of this message). Now I have many questions, and here is the first one: All sextant books and manuals describe how to check perpendicularity of the index mirror to the plane of the arc, but it seems to me that the books I read do not describe this test properly. "Place the sextant on a horizontal table, and put two visors on the arc at 0 and 120 degrees. (One can use any two objects of equal height, with straight edges, like dominoes; Celestaire sells special cylinders for this, and Russian sextants come with special diopters). Then you look at the right edge of the index mirror, trying to align the direct image of the upper edge of the visor at 0 with the reflected image of the upper edge of the mirror at 120. If they are aligned, the mirror is perpendicular to the arc". No book specifies exactly where your eye should be with respect to the sextant. However, everyone who did this test carefully on a sextant with large mirrors should have noticed that the result strongly depends on your eye position, more precisely on the height of your eye over the table. And this height makes a very substantial difference. To see this, just move your eye a bit up and down, trying to align the images of the visors near the upper-right corner of the index mirror and then near the lower-right corner. You will see that this is impossible. Simple geometry confirms this. The same simple geometry shows WHAT this test really checks. It checks the perpendicularity of the index mirror to the plane passing through the upper edges of both visors AND YOUR EYE. This plane strongly depends on your eye position, and in general, it is not necessarily parallel to the plane of the arc. A simplified version of this test, as described in Chauvenet and several other books does not use the visors. You just look on the arc itself, trying to allign its reflected image with the direct image. But when you do this, it is even harder to keep your eye in the plane of the arc! I suppose this problem becomes insignificant if the sextant is large and the mirrors are small (as I see on the pictures of old sextants). Can anyone give a reference on a more complete discussion of this test? Now on my SNO-T purchase. I confirm the opinion expressed earlier in this list that the Russian (tuchkan) and Ukrainian (mmely.ru, maur) dealers are trustworthy. They replied my e-mails promptly. Moreover, they specialize on marine equipment and they KNOW something about the merchandise they cell, unlike some other dealers on e-bay. The SNO-T I bought was made in 1990 but it was sold as "new" (was never used), had factory wrapping, a spare mirror, all accessories, and probably came from a Navy warehouse. It has two scopes: an inverting one (7x30) and a Galilean one, (3x40). So far I noticed two defects: a) the box is very poor. It is made of pine, and has some iron screws in it! One of the two pine clamps that hold the sextant was broken during the transportation (I suppose some custom oficer broke it when trying to take the sextant out!). But due to the careful packing with a lot of foam, the sextant was apparently not damaged. As it was noticed earlier on this mailing list, the box is very small. This has evident advantages but also a disadvantage: you have to detach the telescopes to store the sextant. b) The inverting telescope (7x30) seems to be of poor quality. (Or maybe I don't understand some of its hidden wondeful features:-) As I understand, a modern 6x scope is nothing but a half of a regular binocular. So I compare this SNO-T scope with my binoculars (a very old Zeiss and an old Russian 8x30) and the binoculars seem infinitely better. The Galilean scope is OK and I use it for all my observations. The results of the various tests for the instrumental error are non-conclusive yet. I can report some Sun, Moon and stars measurements which I made from my balcony under the ideal conditions, with an artificial horizon. These measurements seem to show errors of about 0.4' (for the Sun) but I do not have an Almanach yet; I am using the "Complete on board Celestial Navigator" which lists the Sun position with only 1' precision. I hope that more precise Almanach data will permit me to explore the ultimate sextant precision. There is also a unique feature which I like: the non-electric illumination system. (I was taught that "electricity and salt water are incompatible":-) A magnifying glass with luminiscent casing is used to take the arc and drum reading. It works for about 30 minutes after exposure of the sextant to bright light. I find this very convenient for taking lunar distances from my balcony late at night:-) If there is interest in this group, I can continue with my reviews of the old Soviet navy equipment (I recently bought a fine super-precise 3-armed protractor, and a star globe, the things which apparently were not used in the West in the last 50 years). Alex.