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I outlined my experience with my Davis Mark 25 in a posting a week or so
ago. It does need care, constant checking of index errors, deliberate
efforts to equilibrate temperatures and so forth but, given that, I get
altitudes within a minute or so of the calculated value on a regular
basis -- and not all of the error can be attributed to the instrument,
no matter how much I might want to believe that my own contribution is
flawless. Maybe I am just lucky with the particular instrument that I
own but I most certainly don't have errors of several minutes of arc
when using the filters.

Granted I have not tried taking serious observations across the full arc
of the instrument but I have across all altitudes that are commonly used
in navigation (20 to 60 degrees or thereabouts). Maybe I should try some
stars at 100+ degrees apart to see how accurate the instrument is at
extreme angles.

As for leaving the instrument in the sun: If you intend to take
observations with the sun shining on the sextant, you had better leave
it in the sun before hand or you will find yourself adjusting the
horizon mirror nearly as fast as you turn the micrometer knob! To be a
bit more explicit: The frame of the sextant warps as it warms and cools
but it does not, in my experience, suffer any permanent warping. Of
course, my experience is at around 45 degrees of latitude and air
temperatures of perhaps -10 to +25 Celsius. Leaving the instrument out
in the tropical sun might have more lasting effects.

Still, I'd not argue with +/- 5 minutes for a Davis sextant. My own is
better but perhaps I am lucky. Extremes of quality control are expensive
and one of the economies that Davis takes is likely to be that some of
their sextants fall short of the average precision. George, however,
quoted a figure of +/- 5 _degrees_, not minutes, which would make a
sextant utterly useless even for low-precision emergency navigation. I
would be really surprised if any Davis sextant was anywhere near that
bad, unless seriously abused by its users.


Trevor Kenchington


Marvin Sebourn wrote:

> I would be surprised if a plastic Davis sextant would have an accuracy
> of +/- 0.5 minutes throughout the arc. [snip]
>
> I would much rather believe +/- 5 minutes over the arc is much more
> realistic than +/- 0.5', and only then with frequent IC checks. The
> Celestaire catalog says that the accuracy of the Mark 15/25 is
> "Unpredictable". [snip]


--
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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