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Re: Davis Instruments Mark 15 Sextant attachments
From: Nels Tomlinson
Date: 2003 Feb 18, 21:13 -0900
From: Nels Tomlinson
Date: 2003 Feb 18, 21:13 -0900
Hello, The first two (Istok's and McKeel's) seem to have an attachment, image001.gif, which fails to display in my browser. It may have been a virus, but is most likely just a corrupted gif. I can safely try to open them on my machine, since it's not running Windows. Ken is right: if you run Windows, you should (almost) never open attachments. Nels Brooke Clarke wrote: > Hi Ken: > > I have not seen any attachments on this subject. > > Can you name a message on which you see an attachment? > > Brooke Clarke > > Ken Gebhart wrote: > > >>At Celestaire we have sold a lot of Davis plastic sextants over the years. >>The most persistant problem we see is the lack of the whole horizon (beam >>converger) mirror on the Mark 25 to cope with marginal lighting >>conditions. We have many frustrated people buy the split horizon mirror >>kit for the Mark 15 to install on their Mark 25. Then they have paid twice >>as much and only have a light (which is of small value) to show for it. By >>marginal lighting conditions, I refer to the situation where a group of >>Power Squadron students go down to the shore to take evening stars. If it >>is near to a city which throws lots of light into the air (which is usually >>the case) , they find the Mark 25 to be much harder to use than the Mark >>15. It used to be said that the Mark 25 has a more temperature stable >>frame, but even Davis now admits that the plastic is the same (except for >>color) in both m odels. This is why our catalog explicitly recommends the >>Mark 15 over the Mark 25. >> >>BTW I would have jumped in on this thread sooner, except that the messages >>were all attachments, which we never open. I recommend that everyone put >>their comments in the body of the email message instead of in attachments. >>Any comments? >> >>Ken Gebhart >> >>"Trevor J. Kenchington" wrote: >> >> >>>George Istok wrote: >>> >>> >>>>Just soliciting opinions on this sextant. Any thoughts appreciated. >>> >>>Mine is a Mark 25 but I doubt that there is much real difference from a >>>Mark 15. >>> >>>Working on land, with a sea horizon, I am disappointed any time I get an >>>LOP more than a mile from my true position. The one time I fixed my >>>position in the open sea, I came out half a mile from the GPS position >>>-- though I did have the advantage of the stable deck of a big sailing >>>ship, plus GPS readings that allowed me to advance the morning LOP >>>without errors in dead reckoning. In short, the Davis plastic sextants >>>are quite accurate enough for practical use. >>> >>>However, you do have to adjust the mirrors before every set of >>>observations, you have to check index error before and after each >>>individual sight, and you have to take multiple sights and average them >>>by plotting altitudes against time. If temperatures are extreme (as in >>>winter here in Nova Scotia), you have to leave the sextant in the open >>>air for a while before starting so that it can equilibrate to the >>>temperatures that the sights will be made at. Also, the optics are not >>>the best. The field of view is narrow and the light-gathering power is >>>lower than I would like, which makes sights of the fainter stars >>>awkward. (Efficient use of the brief period when stars and horizon are >>>both visible becomes critical.) >>> >>>Gary Harkins has pointed out that, if you are making serious voyages, a >>>Davis sextant would be useful in an abandon-ship bag. It would also be >>>useful on days when spray is flying, accurate sights are impossible and >>>a precious metal sextant could be easily damaged. For those of us taking >>>sights more for fun than navigation, starting with a Davis will teach >>>you to be very careful and precise. After working with one, using a >>>"real" sextant should seem easy and efficient by comparison! >>> >>>Trevor Kenchington >>> >>>-- >>>Trevor J. Kenchington PhD Gadus@iStar.ca >>>Gadus Associates, Office(902) 889-9250 >>>R.R.#1, Musquodoboit Harbour, Fax (902) 889-9251 >>>Nova Scotia B0J 2L0, CANADA Home (902) 889-3555 >>> >>> Science Serving the Fisheries >>> http://home.istar.ca/~gadus > >