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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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