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    Re: Davis Instruments Mark 15 Sextant
    From: Brooke Clarke
    Date: 2003 Feb 18, 20:49 -0800

    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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