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This list has had some interesting comments from people questioning the
accuracy of Davis plastic sextants.

The July/August issue of Ocean Navigator has an interesting note by L.V.
Larson, an experienced USPS celestial instructor, who set out to
methodically determine the accuracies of plastic and metal sextants.

Mr Larson first did a statistical analysis of the sight folders from his
students, half of whom used Davis sextants and half metal ones.  He found
no significant difference in the accuracy achieved.

He then tried his own series of tests using a Davis Mk 20 sextant (he tried
a Mk25 with the full-view mirrors but stated he had problems getting a good
horizon-to-horizon matchup for determining index error, especially when the
horizon was hazy).  He did a series of twilight shots and found only a tiny
shift in IE when the sextant was brought out of an airconditioned car on a
hot day; as soon as the sextant was up to temperature the IE was unchanged.
 He did other tests in which the IE was steady, even though there was up to
20 degrees difference in temperature for different tests.  Finally he
suddenly found one where the IE was dancing around by about 4 min.  This
occured on a sunny, windless day.  Retrospectively, the other tests were
when there was a reasonable breeze.  Mr Larson states that "the shift in IE
was not a result of temperature, but rather of temperature gradient."
Uneven heating of the sextant could cause its IE to shift.

He concludes that the Davis is fine for twilight shots, but caution needs
to be exercised with sun shots (or during daylight in general, eg, doing a
moon-sun fix), with IE checked both before and after the shots, to insure
against these uneven heating effects.  I was taught checking IE before and
after a run of sights is good practice anyway, even with a metal sextant.

I personally wonder if this IE drift would be an issue for a voyager, who
is hopefully surrounded by both decent breezes and the thermal mass of the
ocean.

Lu Abel

   

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