TRG – 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

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.

In Part One, I pointed out that binoculars can be a viable alternative to telescopes for astronomical viewing. I touched on magnification and aperture as ways of distinguishing and choosing which binocular is right for you. In part two, I’ll talk about digital binoculars

Digital Binoculars

and go into more detail about how magnification and aperture plays into binocular selection.

Digital binoculars are gaining in populararity – these capture a digital image seen through the binoculars. Zoom binoculars have the ability to quickly and very efficiently zoom in on the object of interest, but the image quality is compromised in cheaper models. The extra workings and glass inside reduce the amount of light available, making them unsuitable for astronomy. Avoid binoculars that claim to be “focus-free”.  Also be beware of advertising jargon like “high-powered”. Increasing the magnification will decrease the brightness and field of view, which makes objects faint and fuzzy. A lower magnification will maximise the amount of light transferred.

The larger the aperture, the brighter the image will be; but the greater the size, the binoculars will weigh and cost more. For general astronomy use, choose binoculars with an aperture of 50mm. An observation binocular with a 60mm objective lens will still be fairly portable while an observation binocular with a 80 to 100mm + objective lens is far more suited for static use. The size of the objective lens and the power of magnification are the two major factors that determine the light transmission of the binocular. For example, 50mm (the diameter of the objective lens) divided by 10 (the power of magnification) gives a figure of 5, which is the diameter (mm) of the exit pupil and indicates the amount of light reaching the eye. In general the larger the exit pupil diameter the brighter the binocular will appear and the better the resolution will be, enhancing colour and contrast perception, especially in low light conditions. However, since the human eye pupil dilates on average from 2.5mm to 7mm depending on light conditions it follows that an exit pupil above 7 is not beneficial as the human eye cannot accommodate it.

Remember also that as we age our eye pupil does not dilate so much, so a large exit pupil of 7mm is not so important for a 50 year old person compared with a 25 year old. So a 4mm exit pupil on a 25×100 observation binocular will be more than satisfactory for most users in most conditions, whereas a 40×100 will only give 2.5mm exit pupil, drastically reducing the amount of light reaching the eye. A standard 7×50 pair will have a 7mm exit pupil, the average human eye pupil size at night.

Most binoculars are not suitable for use with eyeglasses. You have to put your eye close to the eyepiece, but the glasses prevent you from getting close enough. You can of course take your glasses off to use the binoculars, but this can be a nuisance. It is possible to buy special binoculars which can be used with glasses. Standard binoculars have eye relief ranging from only a few millimetres to 15 millimetres. Long eye relief (15 to 25 millimetres or more) is necessary for eyeglass wearers. A poorly designed optical system can force the observer to press his or her eye close to the eyepiece in order to see an unvignetted image, or alternatively may have an exit pupil larger than the observer’s pupil at a comfortable viewing position, resulting in loss of light and a dimmer image.

The eyepieces of binoculars are usually permanently mounted in the binoculars, causing them to have a pre-determined magnification and field of view. Usually binocular eyepieces have 3-4 elements with marginal correction for colour and edge sharpness. The correction on Siebert 6 element eyepieces are comparable to a Japanese made Meade 26mm Super Plossl. The eyepieces do not add colour correction or false colour.
Wide-angle binoculars have a field of view that is wider than average (60 or higher).

The Field of View is the size of an area that can be viewed using the binoculars.

Binoculars are often advertised with their field of view specified in one of two ways: angular field of view, and linear field of view. Angular field of view is typically specified in degrees, while linear field of view is a ratio of lengths. For example, a pair of binoculars with a 5.8 degree (angular) field of view might be advertised as having a (linear) field of view of 305 feet per 1000 yards or 102mm per meter. As long as the field of view (FOV) is less than about 10 degrees or so, the following approximation formulas allow one to convert between linear and angular field of view. Let A be the angular field of view in degrees. Let L be the linear field of view in feet per 1000 yards. Let M be the linear field of view in millimetres per meter. Then:

A = 0.0191 \times L
A = 0.0573 \times M
L = 52.4 \times A
M = 17.5 \times A

Generally, higher powered binoculars give you a smaller field-of-view and the opposite is true for lower powered binoculars. For astronomy, a wide field of view is desirable because if offers a more pleasant viewing experience, and you can see more of the sky at a better edge performance compared to a narrower field.

Come back for part three when I talk about the various prism systems employed by binoculars.

Telescopes aren’t the only viewing tool at the amateur astonomer’s disposal. In part one of a series, I’ll discuss how binoculars can be very useful for several reasons: they are relatively inexpensive, have a large field of view and show images right side up (which makes finding things in the sky easier), are easily portable and require little to no setup.

So, how do you choose a good pair of binoculars for amateur astronomy?

There are different ways of categorising binoculars. Usually, they are distinguished by their magnification and the size of the aperture. A combination of a small magnification and large lens produces a brighter view. A 7×50 pair, for example, gives a brighter image than a 10 x 50, but the image magnification is smaller. A standard 7 x 50 pair is considered one of the best all-round binoculars for practicality, performance and price. For high-powered observation, a magnification of 25x with a 100mm objective lens is recommended.

Observation binocular

Observation binoculars come with either straight or angled eyepieces. The benefit of the angled design, usually 45º, is the ease of use and comfort of viewing. The angled eyepiece model is much more user-friendly, allowing more flexibility for people of different heights to use the binoculars without the need to continually adjust the tripod. It is also of benefit when the binocular is trained on the night sky. The straight eyepiece design model will require a higher adjustment of the tripod for each user and this may make the tripod less stable.

If you are forced to go out to the country where you cannot fix your binoculars to something, then consider image stabilised binoculars. These are far more expensive but have other applications as well.
Stabilisation may be enabled or disabled by the user as required. Stabilisation will allow binoculars up to 20× to be hand-held. Major brands making image stabilised binoculars include are Canon, Nikon, and Bushnell.

When I continue with Part 2 of my Astronomical Binocular Review Guide, I’ll talk about various features to look for in a binocular, including digital binoculars.