Aluminum Sextant - "Astra IIIB"
Deluxe Sextant with Whole Horizon Mirror
The Astra IIIB is dependable and more than accurate enough for the job. It is highly unlikely that any navigator regardless of experience could do more accurate navigation underway in a small vessel with any other sextant, even those costing $2,000 or more.
This lightweight aluminium-alloy instrument is, in fact, easier to use than the traditional 4-pound brass models of the more expensive units. The Astra optics are excellent and the instrument has a good feel to it.
The index mirror is rectangular in shape, and has the silvering applied to the front, rather than the rear surface. This is similar to the way astronomical telescopes are silvered. From an engineering standpoint, the rectangular shape allows for a better reflecting profile during high angle observations, and the front surface silvering provides better reflection brightness for all sights. From a more practical standpoint, the biggest advantage is in the durability of the silvering itself. When applied to the front surface it is easy to keep clean and free of saltwater contamination and it is further protected by a durable oxide coating.
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.