The Astra IIIB has been made for Celestaire, Inc. in China since 1986 under very specific quality guidelines. It has somewhat revolutionized celestial navigation because never before has such a high quality, accurate metal sextant been available at such a low price. Over 26,000 sextants of this model have been sold in the US alone, far more than any other serious sextant. Its popularity has spread throughout the rest of the world as well, making it the most recognized (and supported) sextant worldwide.
Its low price, excellent optics, and choice of horizon mirrors and other accessories makes it the perfect selection for beginners. Yet, we know many professional navigators who prefer the ASTRA IIIB merely because they can leave their expensive models at home without sacrificing noticeable accuracy.
The frame is made from lightweight aluminum alloy which resists corrosion. Aluminum alloy is not new to sextant construction. It has long been used by many manufacturers for lightweight versions of their heavier brass or bronze sextants in order to reduce arm fatigue and unsteadiness among infrequent users. The sextant is electrically lighted by an LED on the arc and drum.
The Astra IIIB is considered by many to be the easiest sextant to use. Its light weight and excellent balance makes it a pleasure to hold and makes sights easier to take under dynamic weather and sea conditions. Even reading the sextant is easier under low light conditions by having white degree marks against a black background as opposed to scribe marks on a brass arc.
The index mirror is rectangular in shape and is the same size as used on more expensive sextants. This shape allows more aspect of the mirror to be available for high angle sights. This means the sextant does not have to be held as steady for them. An index mirror upgrade kit is available for older models having smaller, round, index mirrors. It comes mounted in a rectangular frame and has instructions for easy installation.
We are impressed with the degree to which each component of the sextant is tested. For example, each mirror and shade glass is tested for perfect flatness by the Fresnel diffraction pattern method. The sextants are assembled and tested in a clean, professional, environment. Random units have been tested by us and several professional navigators, with results that rival the most expensive German and Japanese models.
The ability to quickly and economically change the type of horizon mirror is an attraction of this sextant to many navigators. This is because whole horizon mirrors are generally preferred by beginners who later may wish to change to a traditional mirror as proficiency is attained (see “Marine Sextants Overview“).
Maintainability is appreciated by schools and other groups who because of heavy usage need inexpensive replacement parts due to the threat of student mishandling.
Included and Optional Equipment
The sextant has LED lighting of the arc and drum powered by two standard size AA batteries (not included) which install in the handle. It comes in a varnished wooden case, with instruction book, certificate of accuracy, and adjusting wrench. Case external dimensions are 6.5 x 13 x 11 inches. It includes a 3.5×40 monocular telescope as standard equipment.
Optional accessories include a 7×35 prism telescope, a slip-on rubber eyecup for use while wearing glasses, and a zero-magnification sight tube. A practice bubble horizon is available which facilitates effective back-yard practice without the need for a sea horizon. Although the sextant comes with a choice of either whole horizon mirror or traditional split horizon mirror, an extra mirror of either kind is available at nominal cost, and comes already mounted in its frame. Changing mirrors is done quickly by means of two easily manipulated screws.