NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: The "big" sextant manufactures
From: W F Jones
Date: 2007 Oct 25, 22:13 -0400
From: W F Jones
Date: 2007 Oct 25, 22:13 -0400
You have referenced both the MK II and MK III sextants in your post. I have never examined or used a MK II but vaguely recall years ago discussions regarding the improvements the USN demanded in the MK III version. One significant difference was the MK III mirrors are much larger making measurements at sea significantly easier. Sometime after WWII, the MK II was widely available in the surplus market at very low prices. The same cannot be said for the MK III since very few MK III legally made it to the market place. There was a commercial version of the MK III but I've not seen one and understand very few were sold. Someone from Scientific Instruments posted earlier this summer that they were selling the old inventory and I assume the collimator used to certify the MK III sextant. It seems unlikely to the me that anyone would choose the MK II if a MK III was available. I legally own a MK III and it has proven itself to be accurate and reliable. I usually use a ring sight and leave the telescope in the case. Frank J. Rochester, NY ============================================================== From: John KarlTo: NavList Subject: [NavList 3592] Re: The "big" sextant manufactures Date sent: Thu, 25 Oct 2007 12:21:48 -0700 Send reply to: NavList@fer3.com [ Double-click this line for list subscription options ] 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 -~----------~----~----~----~------~----~------~--~---