THE ASTRA IIIB SEXTANT

The Astra IIIB has somewhat revolutionized celestial navigation. Never before has such a high quality, accurate, metal sextant been available at such a low price. Over 8000 sextants of this model have been sold in the US alone in the past 9 years; far more than any other serious sextant. Its popularity has spread throughout the rest of the world as well. The frame is made from lightweight aluminum alloy which resists corrosion. Aluminum alloy is not new to sextant construction. It is currently used in Freiberger sextants, and by C. Plath for a lightweight version of their famous Navistar Classic sextant. The sextant is electrically lighted by LED on the arc and drum. Its low price, excellent optics, and choice of horizon mirrors and other accessories makes it the perfect selection for beginners. Yet, we know several professional navigators who prefer the ASTRA IIIB merely because they can leave their expensive models at home without sacrificing noticeable accuracy. The ability to quickly and economically change the type of horizon mirror is a major 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. Maintainability is appreciated by schools and other groups who because of heavy usage need inexpensive replacement parts due to the threat of student mishandling.

The Manufacturing The Astra IIIB is made by the Changzhou Celestaire Instrument Co. in China, in accordance with our specific quality and design guidelines. We are consistently impressed (in fact the first time, surprised) 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.

Two Models to Choose From The Astra IIIB is supplied in two models: standard and deluxe. The only difference in the two models is in the index mirror. The Standard model has a round index mirror (shown with the Practice Bubble Horizon elsewhere in this catalog) with the silvering applied to the rear surface of the glass. This is the way all ASTRA IIIBs were before 1994. At that time the Deluxe model was introduced with a rectangular index mirror having the silvering applied to the front surface (the mirror is interchangeable in size with most other rectangular mirrored sextants). From an engineering standpoint, this arrangement allows for a better reflecting profile during high angle observations, and better reflection brightness for all sights. Such improvements, however, are not easily noticed by the user. From a practical standpoint, the biggest improvement is in durability of the silvering itself. When applied to the front surface. it is easy to keep clean and free of saltwater contamination. It is further protected by a durable oxide coating. In contrast, rear silvered mirrors need resilvering more frequently if saltwater becomes trapped between the mirror and its housing and is not faithfully rinsed out. The horizon mirror for either model may be chosen to be of the whole horizon (WH) type, or the traditional half silvered (Traditional) type (please see the discussion about these mirrors). While the deluxe model is more expensive than the standard model, the selection of horizon mirrors involves no difference in price.

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, vial of oil, adjusting wrench, spare LED, and mirror mounting springs. It includes a 3.5x40 monocular telescope as standard equipment. Optional accessories include a 6x30 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 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.

olav hansen aps. 86 99 15 10