NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Bris Sextant
From: George Huxtable
Date: 2005 Nov 7, 10:00 -0000
From: George Huxtable
Date: 2005 Nov 7, 10:00 -0000
Responding to John Rae's >> I then held the apparatus in front of one side of a 7 x 35 binocular. > Alex replied- > As I said before, this is dangerous! Yes, it is, but so is looking at the Sun with the naked eye. We have discussed this matter within Nav-l recently (within the last year or so.) Let me rehearse some of that discussion- Looking at the Sun with the naked-eye, straight-on, is indeed dangerous. On the retina (which is presumably the part of the eye that's most sensitive to damage) the spot caused by an unmagnified Sun is about 0.175mm across. You get that by taking the angular diameter of the Sun (half a degree) and the focal length of the eye (about 20 mm). On that spot falls all the incoming energy (heat and light) from the Sun that has passed through the pupil, which in the circumstances will be contracted to its minimum diameter of about 2 mm. Now compare that with the situation where a telescope has been placed in front of the eye. Assume its magnification is x5. The telescope can now collect light that falls on a 10mm disc at the middle of its objective lens, and compresses it into a light-pencil of 2mm dia., which will all pass through that contracted pupil. That compression, by a factor of 5, goes inevitably with an angular magnification of 5; it's a law of optics. Light falling on outer parts of the objective will be obstructed by the iris diaphragm. So, from the point of view of the retina, light reaching it from the Sun is increased by 25 times. Even if the objective is bigger than 10mm diameter, that will not further increase the amount of light passing through the pupil, though it will increase the total light falling on the iris. So that telescope has caused an increase of x25 in the amount of light entering the eye. But because it has magnified the Sun's image by a factor of 5, that image now paints a Sun-disc on the retina which is x5 greater in diameter, 0.875 mm dia., and 25x greater in area. So we can deduce that the light energy falling on any point of the retina that's within the Sun's image disc is no greater with the telescope than without it. The only difference is that the image disc is much larger in size. That means that the retina can be burned just as readily and as quickly by the naked-eye Sun as when the Sun is seen through a telescope. The only difference is that with the telescope, such burns will be much bigger. So I would qualify Alex's warning about the dangers of looking at the Sun through a telescope, to say that looking at the Sun without a telescope is almost equally dangerous. The moral is; don't do either! On a slightly different topic, I have been told (but can't vouch for its truth) that damage to the retina is mainly caused by heat energy rather than light energy. For that reason, filters made from highly-exposed colour film are somewhat dangerous, as the dyes they rely on, while obstructing light to give the appearance of blackness, do little to reduce heat transmission. If that is indeed the case, welders' glass would be much safer. However, those that I've looked through seem to be overkill even for looking at the Sun, and there would be no hope of seeing a horizon through that glass. About the geometry of the Bris device (gonioscope?). It's important, for a sextant, that both lines of view (to Sun and to horizon) are in the plane of its frame, which is at right angles to the axis of the pivot. In the Bris device, it's the line of intersection between the glass plates that corresponds to that pivot axis. It seems to me that two aspects become important. First, that the intersection line between middle plate and outer should be carefully parallel to the intersection line between middle and inner, when the device is assembled. Second, that the view lines between eye and Sun. and between eye and horizon, are in the plane at right angles to that intersection line, to avoid "collimation error". How does this work out when a Bris is used? Clearly, you have to rotate it so that that intersection axis appears to be parallel to the horixon. But it has two other axes of freedom, in "yaw" and in "pitch". It ought to be insensitive to pitch, just as a sextant is. But how do you get the adjustment right, in yaw? How sensitive is it to such misalignment.? Unlike a sextant, it gives no clue to exactly where your eye is to go, with respect to the intersection axis of the glasses, and to the direction of the Sun. George.