NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: The "big" sextant manufactures
From: Greg Rudzinski
Date: 2007 Nov 08, 15:38 -0800
From: Greg Rudzinski
Date: 2007 Nov 08, 15:38 -0800
Extraneous reflected light on the Navy Mark 3 can be knocked out by a 3/8 inch piece of electrical tape placed at the base of the horizon mirror. Field of view issues can be reduced if a sextant is preset to the approximate altitude. Any navigation calculator will make short work of the presetting task. Greg Rudzinski On Oct 25, 11:21 am, John Karlwrote: > The Navy Mark III made by Scientific Instruments in Milwaukee is an a > Plath Navistar. As far as I know there were only two suppliers to the > Navy; the other was M. Low of New York city. Scientific Instruments > made their sextants; Low bought theirs from Plath - so I've been > told. I've made careful measurement comparisons between the Low Mark > III and the Navistar and have found them identical (except for the > Mark III's pistol-grip handle). > > The Plath sextants have been a big disappointment to me. After > hearing about them for almost 50 yrs, I looked through one for the > first time at the "Celestial Celebration Weekend" at the Mystic > Maritime Museum. I was shocked -- I thought I was looking through a > darn toy. The field of view was a mere fraction of the Navy Mark II > I'd been using for years; the shades obstructed the FOV; there was > annoying extraneous internal light; and the horizon only showed on the > left side! I considered it a piece of junk. > > After collecting seven popular sextant makes and models, I've > discovered that the Mark II has a rare design that I was blissfully > unaware of for decades. It has a very wide VOF terrestrial scope with > an internal focal plane. Just like a prism scope, even with a > traditional split-horizon mirror, they show a complete image across > the VOF using only half the objective, essentially just like a whole- > horizon mirror. (You can see this in any binocs by placing a opaque > card over half of the objective.) Most sextants have cheap Galilean > scopes that don't behave this way. > > The Scientific Instruments Mark III has a stated arc accuracy of 9", > effectively the same as the Tamaya Spica 10", and, for common uses, > not practically better than the Astra IIIB's 20". I've not seen any > stated accuracy claims for the Plath sextants. > > I don't consider an arc-error correction table an accuracy claim. > After all, when the correction is made, what's the instrument's > accuracy?? And what's the correction between the 10d tabulated > values? ? Incidentally, the Low Mark III arc-correction table that > I've seen has a max arc correction of 6" between 0 and 105d. > > John Karl --~--~---------~--~----~------------~-------~--~----~ To post to this group, send email to NavList@fer3.com To , send email to NavList-@fer3.com -~----------~----~----~----~------~----~------~--~---