Buying a Sextant, Starfinder and Books

How to choose a sextant

To help you choose your favourite sextant, let me explain the differences and see which sextant might be the right one for you. The only sextant I would strongly advise against is a cheap plastic sextant, since these are really not suitable for navigation.

From left to right: The Astra IIIB by Celestaire, the Freiberger Sextant and the Horizon Ultra by Cassens&Plath

The basic, but very good: The Astra IIIB by Celestaire

The Astra IIIB by Celestaire and made in China.

The Astra IIIB by US-based Celestaire is the sextant I began my career as celestial navigator for almost 20 years. This sextant has an excellent price-performance value. This highly cost-effective and surprisingly accurate sextant is being sold to thousands to yachties every year. It’s designed in the USA and manufactured in China and since its launch in 1986, over 26,000 Astra IIIB have been sold in the USA alone!

The Astra IIIB is accurate enough to get a good fix (20″ which is 1/3 or a minute), but its main drawback is the optics and obviously it lacks the “bells and whistles”. It’s a basic sextant with a number of fixed shades and you need check your index error and take this into account when you do your sight reduction. Also, you have to look up your Dip error when doing your calculations, something better sextants have a correction built in. You also need to be very careful to judge if your sextant is really vertical when taking the sight and do a pendulum movement to make sure that the lower lim of the sun is exactly touching the horizon while you move your sextant to and fro.

There is nothing wrong with it, these are just some more more things to keep in mind and take into consideration.

The simple standard micrometer where you read off the minutes on the arc. Then you have to do the dip and index error calculation when doing the sight reduction. Two more extra steps of maths!

The accuracy of the Astra IIIB is good enough, even though there is a Astra IIIB “Pro” versions as well, with a bronze arc fused to the aluminum frame. It’s accuracy is slightly better than the standard Astra IIIB: 15″ on the arc.

If you buy the Astra IIIB Reginasailing Friends get a discount from Bookharbour.com Please contact me for more details.

The same sextant under the banner of Cassens&Plath: “CP Sailing”

The Cassens&Plath “CP Sailing” sextant which is very similar to the Astra III DeLux

An alternative to the Astra IIIB De Lux would be identical Cassens&Plath CP Sailing sextant. It is therefore not made by Cassens&Plath themselves but they buy it from Celestaire and test them individually giving a complience certificate from Cassens&Plath.

All Cassens&Plath CP Sailing sextants are individually certified.

As a EU resident you would also avoid the hassle with dealing with Customs and pay a service fee to the courier company along with long lead times due to the act of importing from a non EU-country (Brexit!).

The basic sextant made in Germany: The Freiberger Sextant

The Freiberger sextant

If you wish to go up one step, you could also consider the Freiberger sextant, made in Germany. The Freiberger has a better optics and a larger view than the Astra IIIB. The lens is made by Jena with an aperture of 40mm (4 x 40). It’s easier to see through and better in soft light, i.e. for twilight-shooting of planets and stars.

The Freiberger has a bigger telescope part, which gives a somewhat bigger view, but there was some glare due to the shades coming into the vision field, when I tested it last.

It’s still a classic sextant with Index error, Dip and moving the sextant in a pendulum gesture to make sure you take the correct sight.

The basic sextant by Cassens&Plath: “Bobby Schenk”

The Cassens&Plath Bobby Schenk sextant.

A sextant similar in price range as the Freiberger is the German manufactured Cassens&Plath Bobby Schenk sextant. Bobby Schenk is a German pioneer in ocean cruising and helped Cassens&Plath to develop a special “low-cost” version of their top-line sextant (see below). It’s very similar to the Ultra Horizon but is lacking some key features.

Just like with the Horizon Ultra, it has large mirrors which makes sure that you never lose sight of the stars, even when the seas are rough and the boat experiences heavy heeling.

It comes in an elegant wooden case, alternatively in a Pelicase-style rough case and has all the mechanical accuracy as the bigger brother Horizon Ultra has. For instance, it has an accuracy of no more than 10″ on the arc and has a great optical lens. The optics is important for being able to see weak bodies on the sky, such as starts.

While the Booby Schenk has an adjustment for Index Error, you still have to do the calculation for your Dip in your sight reduction table. And: you still need to move the sextant like a pendulum to find the correction measurement.

Optionally, in order to save yet more money on this fine precision instrument, the Bobby Schenk sextant can also be bought in an inexpensive plastic box. This is called the Bobby Schenk Astro Classic.

Don’t forget that Reginasailing Family Members receive a discount from Cassens&Plath. Please contact me.

The Regina Sextant by Cassens&Plath: The Ultra Horizon with additions

This the the top line of any yacht sextant is the Cassens&Plath Ultra Horizon.

This can be configured according to your own needs and taste. Here is what I chose and what I can recommend as the “Regina Sextant” as I call it. I have specified it to have the following features:

Polarisation Filter for the Index mirror (looking into the sun)
Polarisation Filter for the Horizon mirror (to avoid glare on the horizon)
Clear-View mirror (instead of standard full-few mirror or classic half-view mirror, see below)
Star-spreading glass (astigmatizing lens)
Double Schüler Prism (to make sure the sextant is vertical)
Dip as well as Index error adjustment directly on the drum (standard for Horizon Ultra)
Neck strap (standard)
Non-slip legs (standard)

Let me explain why this Regina Sextant is the sextant of my choice, which I can warmly recommend.

Polarisation shade lenses

The polarisation filters are very handy to get rid of the glare on the horizon while still getting a good picture. You can turn the filter and allow for continuous adjustments toward the horizon as well as the sky. It’s like two dimmers, one for the sun and one for the horizon.

Especially with the free-view mirror, which has no mirror on the sides, the continuous polarisation filter comes in very handy.

Which mirror:
“Half-View”, “Full View” or the “Clear-View” mirror?

The idea with a sextant is that you can see the celestial body and the horizon at the same time. There are three ways to reach this, whereby my personal favourite is the Clear View mirror also found on the Regina Sextant.

The Half-View mirror

The classic way is to use a mirror on one side of the view and nothing on the other side. This is called half-view.

To the left a full mirror (seeing the light strip in the roof) and to the right completely transparent.

Advantage:

A maximum brightness of the horizon view for twilight observations
The edge of the mirror simplifies the vertical orientation of the sextant

Disadvantage:

Star and horizon are separated
The measurement is only possible if the direction to the star is kept exact
“Swinging the arc” to find the base point on the horizon is difficult.

The Full-View mirror

Thanks to the fact that the sun is very bright, an alternative is to make a semitransparent mirror. You build a mirror where you can look through at the same time. This is not a perfect mirror, nor is it very transparent, but a good compromise in order to see the sun and horizon over the entire field of view. Using the sextant during day-time with the bright sun which is reflected in the mirror in any case and the bright horizon behind, you can see both.

A not completely transparent window and a not perfect mirror, but good enough in good sunlight during daytime. A great sextant when only shooting the sun.

Advantage:

The star/sun can easily be kept in the field of view in turbulent seas.
Star/sun and horizon are always in view at the same time
The observation is simplified

Disadvantage:

The brightness of the horizon image is reduced due to the semi-transparent mirror that can make twilight observations more difficult

The Clear-View mirror

The problem with the Full-View mirror is that the lack of transparency prevents the clear view of the horizon when shooting planets and stars in twilight. Remember: When shooting stars and planets it needs to be dark enough to see the celestial bodies but bright enough to see the horizon. With a Full-View mirror, the horizon disappears far too early during sunset and remains invisible far too long during sunrise.

The Regina Sextant has the best of both worlds: It’s completely transparent on the sides and has a semitransparent mirror in the middle. The fact that it is not a perfect in the mirror in the middle is important so it acts as a Full View mirror during daytime when shooting the sun and during night time the Regina Sextant uses and astigmatizing lens to see the star (see below).

The Regina Sextant is completely transparent on both sides and has a semitransparent mirror in the centre.

The Regina Sextant is thus perfect both to shoot the sun during daytime but also to get an immediate fix shooting several stars or planets.

Advantage:

Clear view of the horizon
Star and horizon are simultaneously in view in the centre
Good light conditions at sea by day and night
Combines the advantages of the traditional split-view and full-view mirrors
Observations are simplified

Not sure? Take two!

If you are not sure which mirror you should buy, you might wish to buy both. They are easily interchangable on a Sextant by Cassens & Plath, you just have to re-do the alignments (perpendicularity, side-error, index error) when you change the mirrors.

The mirrors are easily interchangeable and held in place with two simple screws.

Star-spreading glass

The Regina Sextant has an astigmatizing lens that can be moved in front of the star. But this, the dot of a star becomes a horizontal line. This line can then easily be placed on top of the horizon. This is a great help in order to see if the sextant is hold exactly vertical since swinging to and fro can be difficult when observing the fain horizon and a small dot is not easily chased either.

Schüler Double Prism

The classic way to ensure that the sextant is held perfectly vertical is to swing it to and fro so the sun makes a pendulum movement. If not, you measure a too large angle. Only if the sextant is held exactly vertical you are measuring the correct angle. And this is where most sailors have their problems and you get your measurement wrong!

A quicker way is to directly see when the sextant is held correctly by moving the Schüler Double Prisma in front of the horizon mirror. The double prism according to Schüler is also part of the Regina Sextant. It is mounted in front of the shadow glasses in direction of the horizon. The lens is shaped in such a way that it cuts out a vertical area in the middle of the horizon viewing beam.

This provides two advantages for sun observations:

The sun’s reflections on the water surface are faded out and there are no measurement errors caused by glare.
An inclination of the sextant can be clearly recognised as a step in the horizon.

Setting the Index Error and Dip Error directly on the sextant

As part of the celestial theory course you learn to correct for the Index Error of the sextant and thereafter to take your hight of the eye into consideration and correct fort that, which is called the Dip Error. But since the index error seldom changes (unless you change something on the sextant) and the hight of the eye in principle always remains the same on the same boat, why not pre-setting these once for all your sextant? Then you don’t have to bother about these corrections any more (other than checking them from time to time).

The Cassens&Plath sextants corrects for Index Error, while the Cassens&Plath Horizon Ultra and the Regina Sextant corrects for both. It was actually Bobby who invented the adjustment of the Index error (and I honour Bobby for this during my courses!), while the Dip correction is a further development by Cassens&Plath.

Setting the dip and index errors and then “fix” them by the means of an allan key.

Read off the correct angle on the sextant with Index Error and Dip already taken into account!

Check here, how it is done in a step-by-step explanation.

Start Finder

A very nice thing to own is the Weems&Plath Star Finder 2102-D. During your celestial practical sailing course, you will learn to use it and find it really helpful to find stars and planets for twilight shootings.

How it works? Check out my video here.

Necessary books

To work with a sextant you need the following books, which you can buy at Bookharbour as well:

– Nautical Almanac for the corresponding year (e.g. Paperback or Admiralty)

– Sight Reduction Table No 1 for Selected Stars (e.g. Paperback or Admiralty)

– Sight Reduction Table No 2 for Lat 0 – 40 (e.g. Paperback or Admiralty)

– Sight Reduction Table No 3 for Lat 39 – 89 (e.g. Paperback or Admiralty)

You can also buy them with your sextant from Cassens&Plath here.

Please note:
– The Paperback and Admiralty are both the HO249 tables and identical. One is just a bit more fancy and larger than the other one
– Only the Nautical Almanac needs to be purchased annually, the Sight Reduction Tables are good for decades.

Templates

For years, I have been developed my own templates for celestial navigation which I use when I teach. They are also part of the video course here.

For a contribution to my work I can send you these as PDF’s. Please contact me if you are interested in the templates.

10% Discount from Cassens&Plath

For all Reginasailing Family Members, you get a discount on products from Cassens&Plath’s online shop, including sextants, compasses, celestial and tidal templates and much more. Contact me for more information!

Or simply contact me, and I will put you in contact with the sales manager of Cassens&Plath.