NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: sextant precision.
From: George Huxtable
Date: 2005 Jun 22, 10:10 +0100
From: George Huxtable
Date: 2005 Jun 22, 10:10 +0100
Bill wrote, about the possible effect of offsetting incoming light rays to a sextant, with direction unaltered- >I have thought it (and sketched it) and am not clear I am wrong. Let's use >a star, and assume its rays to be parallel for all practical purposes. We >make our horizon and index mirrors parallel. Yes, OK. >This sets equal angles between >the scope line of sight/horizon mirror and horizon mirror/a point on the >index mirror (hopefully at the axis). The reflected and glass images >coincide. If we were to offset the parallel ray striking the index mirror >(or change the geometry of the sextant by raising the index-mirror axis >vertically) the images would no longer coincide. That's where your mind-picture is wrong, Bill. Forget about "a point on the index mirror (hopefully at the axis)". For a plane mirror, one point is as good as another; none has any special significance. If you unshipped the index mirror from its mountings, you could move it about IN ITS PLANE, and as long as it could still reflect some starlight down to the horizon mirror, the star images would still coincide. I think that's probably obvious. Now shift the index mirror away from the horizon mirror in a direction PERPENDICULAR TO ITS PLANE, moving it upwards and backwards so it can still reflect some incoming starlight into the index mirror. As long as the index mirror is kept exactly parallel with the horizon mirror, you could shift it is far as you wanted, many feet away if you wish, and the two star images will remain coincident. It's because the starlight is coming from infinity, and is all exactly parallel. That would not be the case if the light was diverging from a nearby object; indeed, that's how a rangefinder works. >My contention is that a optically perfect shade with faces parallel will >still offset the ray if not perfectly perpendicular to the ray path. Yes, it will; we agree about that offset. >You >are probably correct that the offset is negligible in practice, but >nonetheless does exist. No, I didn't argue that at all. The offset can be as big as you like. However big the parallel offset is, as long as the DIRECTION of the light is unchanged, the star images seen in the telescope will still exactly coincide. If Bill (or anyone else) remains unhappy about that argument, I hope he will express his concerns, and we can thrash it out between us a bit further. Perhaps he isn't the only listmember to be troubled by these concepts, so it may be worthwhile clearing the matter up in a way that makes everyone happy about it. ========================== On a related topic, Renee Mattie wrote- I attempted the following experiment with my Freiberger: * I set my sextant near 0, took it into the closet, and laid it on its back on the carpet. * I shone a laser pointer through the scope and observed the spot on the wall. Actually, I observed two half-spots unless I made a suitable index correction. But I extinguished the horizon half-spot with the entire stack of shades so it would not distract me from the index-mirror half-spot. * When I swung the densest index-mirror shade in front of the mirror, it extinguished the half-spot. * Therefore, I could not observe any deflection of the spot caused by the darkest shade, did not repeat the experiment on a more stable platform, and did not attempt to observe deflection caused by the other 3 shades. The laser pointer is 7 or 10 years old, and there is no way to replace the battery. I wonder if a brand new one would be bright enough to shine through even the darkest shade? ======================= There's an ingenious example of lateral thinking! I haven't come across a laser being employed in sextant alignment before, but it may indeed have its uses. Renee is using what I vaguely remember as being called the "reciprocity principle" for light. This is a fancy way of saying that if a light-ray takes a certain path through an optical system, if you send light back through the system in exactly the opposie direction, it will exactly retrace the same path. Perhaps that's so obvious as to go without saying. To get the light-beams to converge at a spot so close as the facing wall of Renee's closet would require quite a lot of "index correction": and "off-the arc", if I've assessed it right. Unless Renee has a VERY big closet... Renee's conclusion doesn't surprise me; that the darkest shade is so very, very, dark that not enough of the laser light is transmitted as to be visible. Perhaps, if that shade has a blue tint, like mine does, its transmission of the red light from the laser is even less. George. =============================================================== Contact George at george@huxtable.u-net.com ,or by phone +44 1865 820222, or from within UK 01865 820222. Or by post- George Huxtable, 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.