How to choose your Sextant

How to choose your Sextant
www.zenithocean.com
Sextant
Davis Mk3
Davis Mk15
Davis Mk25
Astra 3B
Astra 3
Professional
Material
Plastic
Plastic
Plastic
Aluminium
Country
Accuracy
Mirror
Scope
Weight
Scale
Price
USA
+/- 180”
Split
0
0.2kg
Vernier
£45
USA
+/- 20”
Split
3 x 40
0.4kg
Micrometre
£155
USA
+/- 20”
WHM
3 x 40
0.4kg
Micrometre
£240
Chinese
+/- 20”
Either
3.5 x 40
1.2kg
Micrometre
£440
Aluminium
with Bronze
Arc
Chinese
+/ 15”
Either
3.5 x 40
1.4kg
Micrometre
£540
Cassens &
Plath
Horizon
Ultra
Brass Alloy
German
+/- 9”
Either
4 x 40
1.8kg
Micrometre
£1100
Tamaya
Jupiter
Tamaya Spica
Freiburger
Drum
Sextant
Aluminium
with Bronze
Arc
Japanese
+/- 12”
Split
3.4 x 40
1.7kg
Micrometre
£985
Aluminium
with Bronze
Arc
Japanese
+/- 10”
Split
3.4 x 40
1.8kg
Micrometre
£1170
Brass Alloy
German
+/- 20”
Either
4 x 40
1.4kg
Micrometre
£750
Figures based on available data, May 2012
The above list shows Sextant’s which are currently in production, most of which can be purchased in the United Kingdom, otherwise online from the
USA.
Beam Converger / Whole Horizon Mirror
On a traditional sextant, your view is split into two parts, one part being directly ahead of you and the other image being reflected through the mirrors.
With a Beam Converger (Whole Horizon Mirror WHM) both images appear overlaid as the mirror is produced with a quarts based coating which allows
different types of light through, thus allowing you to see your object and the horizon in the same mirror. For Sun sights and bright star sights, this makes
life much easier; however it has the opposite effect for dimmer targets and hazy horizons where it can make life more difficult. It’s definitely a personal
choice, to which type is better, and you do have a choice with most manufacturers, but I personally prefer the split view.
Plastic vs. Metal
Plastic sextants aren’t as bad as they once were known, and with a bit of practice it is possible to achieve accuracy of around 5nm or less for someone
experienced. Although many people will talk about the problems associated with plastic in terms of heat expanding the material and distorting the
readings, I tend to find the problem is more to do with quality and that you have to regularly correct the sextant prior to each use to get the accuracy,
whereas a well looked after metal sextant will remain precise for its lifetime.
Aluminium vs. Brass
Essentially Brass or Aluminium with a Bronze Arc is better because of the way the arc can be manufactured with more teeth giving more accuracy on the
micrometer.
Vernier vs. Micrometre
Aside from the Mk.3, it’s unlikely that you will come across a Venier sextant. With a vernier sextant you read directly from the arc, and accuracy can be
limited.
Scopes
Generally a 4 x 40 scope is more than sufficient; it allows enough light it and is strong enough to focus on a target. Stronger scopes require a much
steadier hand, and are really only useful on larger ships where you might need a better defined horizon due to the distance.
Davis
The Davis ranges are generally quite cheap, but are made of hardened plastic and can be great for practicing. In recent years I’ve heard very favourable
reports of the Davis Mk.15 and Mk.25, and with a bit of practice fixes of less than 5 nm are achievable. The downside is that you often have to realign
the mirrors regularly, and also determine the index error prior to each sight. I am however quite doubtful of the claimed 20 seconds accuracy at build,
particularly when the Astra and Freiberger Metal sextants claim the same. The Mk.3 is quite clearly an emergency or training sextant, and could
probably give you a reasonable idea of your location other than Mid Atlantic possibly as detailed as very mid Atlantic.
Astra
The Astra sextants have become very popular in recent years; they are the cheapest of the metal sextants and are an excellent piece of Chinese
Engineering. The Professional offers marginally more accuracy due its Bronze Arc, but I don’t think the increase in price would be justifiable for the
average navigator.
Cassens & Plath
Without a doubt, the Cassons and Plath range are the best Sextants in production to date. The quality is superb, with an accuracy of just +/- 9 seconds
and with the added bonus of polorised shades. For the professional navigator, experienced in Celestial Navigation, this brass sextant would be the
perfect one for precision sights. The only real downside, other than the cost, is the weight, which at 1.8kg will get a little heavy when obtaining a steady
sight. The sextant is available as the Standard model with split view, or as the Horizon Ultra with the Convergence Mirror.
Tamaya
The Tamaya range are the only sextants that I’m listed today which I have no personal experience with, however I’m assured that they are a very
comfortable and high quality sextant. The Spica, made of Aluminium with a Bronze Arc, has an accuracy of 10 seconds, which is almost as good as the
Cassens & Plath. The prices are again at the higher end of the range.
Freiberger
Currently, a 1986 Freiberger is amongst the sextants that I’m regularly using at the moment, and I suspect the new model is identical, although with the
option of the Convergence Mirror. They are a very comfortable sextant to use, and with a totally enclosed worm gear are less susceptible to the
environmental conditions. For the price though, they only offer an accuracy of 20 seconds, the same as the Astra IIIB yet for a much higher price. The
accuracy issue is generally down to the lack of a subscale for decimals of a minute on the micrometer. That being said, I’ve obtained some excellent
sights with this sextant and wouldn’t hesitate to recommend it.
Available Extras
Zero Magnification Sight Tube: These can be used for terrestrial transits, where magnification isn’t necessary.
Bubble Horizon: Artificial Horizon traditionally used in aircraft sextants but is excellent for practicing on land with.
Prism Telescope: Increased Magnification. Most sextants come as standard with x3.5 or x4.0 magnification, which is more than enough. With any more
magnification, it becomes very difficult to keep steady. Where increased magnification does come in useful is for accurate sights of the sun, particularly
on larger vessels (think 150,000GT cruise ships with a large height of eye), where the horizon is much further away and less defined.