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Re: Sextant Scope Parallelism (was Re : SNO-T Sextant)
From: Robert Gainer
Date: 2004 Aug 13, 14:43 +0000
From: Robert Gainer
Date: 2004 Aug 13, 14:43 +0000
Frank, I have thought about what you said and still don�t under stand it. So I set up this morning and preformed this little test. Please explain to me where I went wrong in my design for the test. First I want to describe the setup for this experiment. I arranged this so that I could take the shot though the open door of my office. The sextant used was my Cassens & Plath. I believe this instrument to be accurate. I set a stand on a tool cart and locked the sextant into it. Then I measured the angle between the top of the spire and the edge of the roof on a building that is about 7,000 feet away. This distance was measured from the town-zoning map. All the measurements were done with my 4-power scope. With the sextant still locked in position I released the scope and shimmed the bottom of the post with a 1/16� thick spacer. This put a noticeable misalignment in the scope. I also did this a second time with the spacer at the top of the post and saw no difference in the measured angle either way. Next I put the sextant at a 45-degree angle and set the index arm so that the roof was an unbroken line on the horizon glass. I repeated the use of shims to misalign the scope both up and down relative to the frame. I still see an unbroken line on the horizon glass. Unfortunately its overcast and I can�t see any other target that is further away. I am sure the effect of a misalignment increases as the angle does, but I expected at least a small jog in the image. All the best, Robert Gainer >From: Frank Reed>Reply-To: Navigation Mailing List >To: NAVIGATION-L@LISTSERV.WEBKAHUNA.COM >Subject: Re: Sextant Scope Parallelism (was Re : SNO-T Sextant) >Date: Thu, 12 Aug 2004 23:31:10 EDT > >Robert G, you wrote: >"That’s where you got me. If the sextant is vertical you are measuring >the >correct angle. If the scope is pointing off center by a small amount then >you see the sun a little displaced from the center of the scope. But >isn’t >the sun still in the center of the horizon mirror and therefore in the >right >place for the measurement.� If you insist on keeping the sun in the center >of the scope I don’t think that you can get the sun to the bottom of the >arc, it will wander off center as you get to the bottom.� Can you force it >both to the center of the scope and the bottom of the arc at the same time >if the scope is out of Parallelism?" > >The purpose of "swinging" the sextant is to ensure that you are holding it >vertically. So let's set that aside. Let's suppose you have ensured that >the >sextant is exactly and precisely vertical by swinging or even by mounting >the >instrument on a pole. With that done, you can still rotate the sextant >about its >vertical axis so that the Sun and the spot on the horizon beneath will pan >from left to right across the field of view of the telescope. If the >telescope is >not parallel to the sextant frame, the contact between horizon and Sun will >change as you pan, typically only by a few tenths of a minute of arc. In >practice, you would almost never notice this effect with a standard Sun >sight, >except at noon, because the Sun is moving so fast. The best way to test for >this >alignment issue is by bringing two star images together. Observed angles >between >stars do change (because of changing refraction as the stars travel across >the sky), but the change occurs very slowly and would not be obervable for >several minutes. So you bring two stars together in the field of view of >your >sextant. Usually you will do this somewhere near the center of the field of >view. >Now rock the sextant back and forth so that the superimposed star images >shift >from left to right. If they do not remain superimposed across the whole >field >of view, the telescope is not parallel to the frame. When shooting lunars, >where a couple of tenths of a minute of arc can be a big deal, this >alignment >error can lead to surprising errors. Speaking from experience, getting the >scope >parallel goes a long way towards making the sights more accurate. > >Frank R >[ ] Mystic, Connecticut >[X] Chicago, Illinois _________________________________________________________________ Don�t just search. Find. Check out the new MSN Search! http://search.msn.click-url.com/go/onm00200636ave/direct/01/