How to Choose An Astronomical Binocular Third in a Series

by telescope review guide

In part two of my series I discussed digital binoculars and binocular optics and how field of view and magnification played into which binocular to buy. I’ll continue to discuss binocular optics by discussing the various prism systems employed by binoculars.

In binoculars prisms are used to provide correctly oriented images and to shorten the optical path. The typical binocular design can be porro-prism or roof prism. The size and design of the prisms will affect image sharpness, with quality optical glass delivering clarity from edge to edge of the field of view. Prisms are located inside binoculars are like mirrors. It is a reflective coating on glass that bends and refracts light to bring the objects you are looking at to your eyes. The BAK-4 prism is made of a high quality glass and produces sharp images and good edge to edge sharpness. Generally, higher quality binoculars will use BAK-4 prisms. Phase coated prisms have a coating process that enhances the resolution and contrast of images coming through the binocular and are generally applied only on more expensive binoculars.

The Full Moon as seen through binoculars

The Full Moon as seen through binoculars

Roof Prism System

In roof prism binoculars the prisms overlap closely, allowing the objective lenses to line up directly with the eyepiece. The result is a slim, streamlined shape in which the lenses and prisms are in a straight line. An Abbe-Koenig prism is a type of reflecting prism used to invert an image (rotate it by 180°).  The prism is made from two glass prisms which are optically cemented together to form a symmetric, shallow V-shaped assembly.
A Schmidt-Pechan prism is a type of optical prism used to rotate an image by 180°. They are commonly used in binoculars as an image erecting system.
The prism consists of two glass prisms separated by an air-gap. Multiple total internal reflections of the light cause a vertical flipping of the image; a “roof” section of the second prism also flips the image laterally, together causing a 180° rotation of the image. The image’s handedness is not changed.
Compared to the double-Porro prism or Abbe-Koenig designs, the Schmidt-Pechan is much more compact. However, the large number of reflections and glass/air transitions create more light loss than the other designs. The multiple internal reflections also cause a polarisation-dependent phase-lag of the transmitted light, in a manner similar to a Fresnel rhomb. This must be suppressed by special phase-correction coatings to avoid unwanted interference effects on the image. Phase-corrected prism coating and dielectric prism coating are effective techniques for reducing reflections. Light reflected from one roof surface is 1/2 of a wavelength shifted from the light hitting the other roof surface, sometimes referred to as “out of phase” or “phase shift”. Although the light waves are subsequently forced back together when they reach the viewer’s eye, this phenomenon results in reduced contrast and image resolution. This effect does not occur in Porro prism designs.

Porro Prism System

In porro prism binoculars the objective or front lens is offset from the eyepiece. Porro prism binoculars provide greater depth perception and generally offer a wider field of view. A Porro prism binocular will inherently produce an intrinsically brighter image than a roof prism binocular of the same magnification, objective size, and optical quality, as less light is absorbed along the optical path. However the optical quality of the best roof-prism binoculars with up-to-date coating processes as used in Schmidt-Pechan models is comparable with the best Porro binoculars.

Of course, you will want to consider the following factors:
Coatings: reduce glare, and protect against water and other potential damage
Quality of construction: the grade of glass, the quality of the prisms and the material used in the barrel are just a few of the factors to be mindful of. BK-7 borosilicate flint glass are of lower quality; for optimal optics, make sure you have BaK-4 barium crown glass prisms
Long eye relief is a worthwhile feature for eyeglass wearers.

I’ll wrap up what you need to know to buy an astronomical binocular in my next installment and then conclude the series with talking about just what can been seen in the night skies using binoculars.

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