TRG – 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

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.