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    Re: Davis Instruments Mark 15 Sextant attachments
    From: Nels Tomlinson
    Date: 2003 Feb 18, 21:13 -0900

    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
    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{at}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

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