Davis plastic sextants have emerged as truly viable instruments for celestial navigation. Their accuracy is not as great as with metal sextants, and their errors vary with temperature, making frequent IC checks advisable. They make acceptable “first” sextants if finances are a constraint, and are always useful as back-up sextants. Otherwise a metal sextant will be easier to use and will give better feedback to the beginner as to how well they are doing. But once learned you can always get home with a plastic sextant.
The top of the line Davis plastic sextant, the Mark 25 features a Beam Converger™ mirror, sometimes referred to as a “full horizon mirror”. A special coating is applied to high quality optical glass to allow the user to see through this mirror, yet still pick up images as faint as low-light stars.
The Mark 25 is illuminated by a long-life LED and made of an upgraded plastic that is very stable.
The MK 25, and MK 15 have the same basic construction. The former is gray in color, and is said to be of a more stable plastic which is resistant to temperature changes. The latter is black in color. The MK 15 has a traditional split mirror, and is non-illuminated while the MK 25 has illumination and a whole horizon mirror.
Both sextants come with an instruction booklet, plastic carrying case, neck lanyard, 3x scope and a sight tube.