How to Choose An Astronomical Binocular Second in a Series

by telescope review guide

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

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.

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