Because when using binoculars to observe the night sky you want the biggest possible amount of light to reach your eyes and not be reflected back into space from the objectives, modern lenses have antireflection optical coatings on at least one of the air-to-glass surfaces. Very good models will have all glass surfaces coated, but tend to be more expensive.
The most used and least expensive coating is a single-layer of magnesium fluoride (MgF), but there are also modern broadband multicoatings. To save money, some optics manufacturers coat only some of the air-to-glass surfaces, and basically we can talk about four levels of coatings used on binoculars.
Coated lenses are the lowest quality, with a single-layer MgF coating on some of the optical surfaces. Fully coated means that all air-to-glass surfaces are coated with a single-layer MgF coating. Multicoated lenses have multi-layer coatings applied on some surfaces, and finally, fully multicoated lenses have multi-layer coatings applied to all of the surfaces.
Recently the market has been invaded with binoculars that have so-called “ruby” coatings intended to reduce glare in bright light and improve the contrast between brown and green objects. Avoid any binocular that uses these coatings, it will perform poorly for astronomical use.
All binoculars are built with prisms that serve as mirrors to reflect the incoming light between the widely spaced objectives and the narrowly spaced eyepieces. They also have the role of inverting the image that the objective lenses project, in a right-side up and not reversed left to right view. Prisms come in two types: roof and porro prisms.
Roof prisms are in line inside the optical tubes, allowing binoculars to be made small and light. This is a great advantage for hikers and birders, but for astronomical use roof-prism binoculars prove to be poor performers.
In roof prism binocular design the light beam is split in two parts, then recombined. This process leads to “phase shifting”, meaning that less light is transmitted in the eyepiece and contrast is decreased. Besides, roof-prism binoculars are generally much more expensive than porro-prism binoculars of equal quality.
Porro-prism binoculars align the objective lenses and eyepieces in an offset arrangement, with the objective lenses farther apart than the eyepieces. They offer a wide field of view and are very affordable. However, porro prisms have a minor drawback: they are easier to knock out of alignment than roof prisms.
Prisms are made of two types of glass, BK-7 borosilicate flint glass and BaK-4 barium crown glass. For most designs, prisms made of BaK-4 are preferred over the standard BK-7 because they have a higher refractive index and give brighter and well defined images.
To check for yourself the type of prisms in your binocular, hold it pointed towards a light source and take a look at the exit pupils. If the prisms are made of BaK-4 glass the exit pupils will be round and evenly illuminated. If the prisms are of BK-7 glass you will notice squarish, gray edges in the exit pupils.