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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

   

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