The Astra IIIB with Split View Mirror is easier to use if you experienced with celestial navigation and using a sextant.  The Split View Mirror is better when using dim stars or in non total darkness conditions.
Construction Details:
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.
Celestaire 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.
Features:
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.
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 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 3.5x40 monocular telescope as standard equipment.

Choosing between an "All View Mirror" and a "Split View Mirror" - Dag Pike says in his  book "Reeds Sextant Simplified": The wide view mirror has the advantage that the horizon can be seen through the whole of the horizon glass rather than just the normal ,split view, half. It is suggested that the horizon is not seen so clearly through this type of mirror, but in practice there is little difference and this type of mirror does make it easier to bring stars and the sun down to the horizon.  However, it does lack the vertical reference of the division between the two halves which is found with traditional split view mirrors and the obersever has to be more careful to check that the sextant is upright at the moment of taking an observation.

The top of the line Davis plastic sextant, the Mark 25 features our 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 Davis Mk 25 Sextant, is a very accurate, precision piece of kit, the best Davis sextant available.

It has an 'all-view' (Full-field Beam Converger) with quartz hard coat which replaces the conventional half-silvered mirror. Horizon and astrobody images are combined in a single full-field for easy, reliable sights under the most difficult conditions.

It features a traditional half-silvered horizon mirror sighting and a beam converger with a full viewing field.  The special mineral coating is selective for different wavelengths of light which greatly increases accuracy. The horizon is transmitted in one colour region and the star or sun in another.

Improved reinforced plastic stiffens frame for greater measurement accuracy.

High-efficiency LED (Solid-state Light Emitting Diode) illumination system, which floods arc and drum with soft light. The LED will last for the lifetime* of the sextant. Batteries, which are included, will last up to 10 times longer than with regular bulbs. Positive push-on switching action, with vinyl boot water protection. The Davis Mk 25 Sextant, is a very accurate, precision piece of kit, the best Davis sextant available.

It has a  Full-field Beam Converger with quartz hard coat which replaces the conventional half-silvered mirror. Horizon and astrobody images are combined in a single full-field for easy, reliable sights under the most difficult conditions.

It features a traditional half-silvered horizon mirror sighting and a beam converger with a full viewing field.  The special mineral coating is selective for different wavelengths of light which greatly increases accuracy. The horizon is transmitted in one colour region and the star or sun in another. Improved reinforced plastic stiffens frame for greater measurement accuracy.

High-efficiency LED (Solid-state LED) illumination system, which floods arc and drum with soft light. The LED will last for the lifetime* of the sextant. Batteries, which are included, will last up to 10 times longer than with regular bulbs. Positive push-on switching action, with vinyl boot water protection.

Mark 15 Sextant This model has many great features including seven large sunshades, a 3 mm x 27 mm star scope, and easy-to-read micrometer drum vernier scale that reads to 2/10 of a minute. The Mark 15 has a 7″ (18 cm) frame radius graduated from 120 degrees to -5 degrees and comes with a traditional half-silvered mirror. The Davis Mk 15 sextant is proven at the equator and the north pole. This sextant is proof that you can navigate without GPS. It features large, easily adjusted mirrors using instrument-quality glass and a special baked on salt-spray resistant backing. Both mirrors have protective housings and the horizon mirror is half-silvered. No adjustment tools are required. The body is moulded using very tough weather-resistant plastic. All Master Sextants feature the following professional specifications: 7 inch (178 mm) frame radius. Graduation 120° to -5°
Index mirror rectangular, 1.98 x 1.49 inches (50 x 38 mm).
Precision machined slow travel gear and worm mechanism.
Highly visible, easy-to-read micrometer drum and vernier allowing readings to 2/10 minute of arc.
Fully ribbed, three-circle sextant body for platform strength and stability.
Large, contoured handle designed for sure, steady grip and angled back for added control.
Seven large sunshades allow complete, independent adjustment of shading density for horizon and index mirrors.
3 x 27 mm star telescope with anti-reflective coated glass lenses, plus separate hooded sight tube for glare-free sun shots.
Neck lanyard frees hand and protects sextant.
Tough, fully padded, shock-resisting carrying case with compartments for extra batteries and mirror kits.

The Astra IIIB with All View Mirror is easier to use if you are new to celestial navigation. Construction Details 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. Celestaire 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. Features 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 "Selecting A Marine Sextant"). 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 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 3.5x40 monocular telescope as standard equipment.

Choosing between an "All View Mirror" and a "Split View Mirror" - Dag Pike says in his  book "Reeds Sextant Simplified": The wide view mirror has the advantage that the horizon can be seen through the whole of the horizon glass rather than just the normal ,split view, half. It is suggested that the horizon is not seen so clearly through this type of mirror, but in practice there is little difference and this type of mirror does make it easier to bring stars and the sun down to the horizon.  However, it does lack the vertical reference of the division between the two halves which is found with traditional split view mirrors and the obersever has to be more careful to check that the sextant is upright at the moment of taking an observation.