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Re: Sextant precision
From: Robert Gainer
Date: 2004 Oct 2, 14:23 +0000
From: Robert Gainer
Date: 2004 Oct 2, 14:23 +0000
George said, �Mostly, quintants were made for marine surveying, for measuring horizontal sextant angles. Not much advantage over a sextant for lunar distances, which as far as I know were only ever tabulated out to 120 degrees�. George, My instrument is indeed intended to be used for surveying and navigation. It looks like a standard issue Navy sextant except with a longer engraved arc. George went on to say, �But I wonder whether Robert's "Brandis" was actually useable over its full range to 162 degrees. Two things happened at large angles: 1. The available slot in the vertical field-of-view would shrink until at some angle, at which the light was tangential to the index mirror, the letter-box shape of the field of view would shrink to zero vertical aperture�. George, This is true and it happens at about 140 degrees. I think the reason that they engraved the arc past this point is twofold. First the vernier needs something past the point of the reading on the arc to be able to select the minuets and parts from the vernier. This adds 13 degrees and 20 minuets to the engraved marks on the limb for this vernier. So if you wanted to read up to 140 degrees you need the arc engraved to at least 160 degrees and 20 minuets. Second, if you add a penta prism to the system you can now read the arc to about 162 degrees and still have the extra 13 degrees and 20 minuets of engraving on the arc for the vernier readings. This will measure an angle of greater then 200 degrees in use. The Freiberger sextant that I have can also be used for surveying (horizontal sights) with a detachable penta prism to extend the measuring range to 215 degrees. This is almost double the 120 degree arc of the sextant. This sextant is said by the manufacturer to have less then 20 seconds of error in the instrument. My Husun is graduated to 125 degrees just like the Freiberger but is supposed to have no error on any measurement until you get to 120 degrees. For day-to-day use that makes it as good as my Cassens & Plath which has �no error for practical use� according to the paperwork with the instrument. They clam the error is less then 9 seconds of arc. I have used a Freiberger in Africa with the penta prism option that was sold by the manufacturer. It was a pain to use and it was better to just turn around and shoot facing the other way with out the prism attached. I never understood why you would need to measure more then 180 degrees under any circumstances. All the best, Robert Gainer _________________________________________________________________ Get ready for school! Find articles, homework help and more in the Back to School Guide! http://special.msn.com/network/04backtoschool.armx