Buying Guide | Binoculars - Astronoscope

Binoculars Buying Guide

The Sky at Night is a wonderful place, and we know you want to get going as soon as possible.

Many people overlook Binoculars when getting into Astronomy and go straight to the Telescope section. Whilst it is true that Telescopes are more flexible in their set up and give the potential of much higher magnification, binoculars are lighter and much more portable than their more muscular telescopic cousins. They also give you a much wider field of view, and this means that it is easier to find celestial objects and combined with the fact that you use both eyes to view objects through binoculars, this gives the user a much more natural viewing experience..

This is why you will find that most, if not all experienced Astronomer will have a good pair of Astronomical Binoculars in their viewing arsenal.

Binoculars offer a quick and easy 'grab n go' option for the novice astronomer. They require little to no setup prep, they are easily portable and light and you can view your desired target with both eyes to give a better depth of field and they don't require any experience to use them.

However, they also have some drawbacks. Unlike a telescope you cannot change the eyepiece and this means that the magnification power is fixed and because most are hand held, the view can be shaky due to minute hand movements while viewing, and the higher the magnification, the worse this becomes. They are also quite limited in terms of aperture, and this is one of the most important factors to consider when choosing your optical equipment. Whilst they are good for viewing things like the Moon, some planets and a limited number of Nebulae and other deep space objects, they could fall short as your interest and experience develops.

Understanding Binoculars

On the back of every pair of binoculars you will find some numbers like those in the picture above. 

The two numbers separated by 'x' tells you the (i) the magnifying power and (ii) the front lens aperture of the instrument in millimetres. In the case of the picture above it is 20x80, other common combinations are 7x40,10x42 and 10x50.

Below these numbers is the field of view information. It is expressed in feet at 1,000 yards or degrees. 1° at 1,000 yards is 52 feet. Generally the field of view of astronomical binoculars is between 5 and 8 degrees.

Buying Astronomical Binoculars

Because star gazing is done at night, your binoculars need to be of higher quality than those used in daylight.

This means you need a bigger front aperture than you do with binoculars used in daylight. The bigger the front lens aperture the more light can be gathered, exactly the same as with telescopes. Bigger is most definitely better. Higher quality optics will also give you a brighter, sharper image. Price is a good indication of optical quality, so don't waste your money buying cheap binoculars for star gazing, they will disappoint. The best binoculars use BAK-4 coated prisms.

Binocular types

Binoculars come in two styles. The traditional stepped style called Porro Prism an the sleeker 'H' shape of the Roof Prism. Roof Prism are smaller and lighter, but because they are more complicated to manufacture are also generally more expensive.

Porro Prism Binoculars

Roof Prism Binoculars

If you opt for larger binoculars (20x80 and above) you will also need a tripod to keep the view steady as they are too big and heavy to hold in your hands without a shaky view.

We do not recommend zoom binoculars for astronomical observation as they require extremely high optical precision that is normally not met.

Magnification and Exit Pupil

Lower magnification binoculars, 7x or 8x will give you wider field of view and are lighter and easier to handle, as you increase this past 10x the view will be more detailed but will cover a smaller area of the sky.

Higher magnification also means that any shake caused by hand movements is magnified by greater and greater amounts so, as mentioned above, you will need to invest in a tripod to enjoy the improved view.

The exit pupil is often overlooked, but is another important consideration. This is the small disc of light that you can see in the eyepiece as you bring the binoculars up to your eyes. This is important because you want the image to fit inside your eye. To work out the exit pupil, divide the front aperture by the magnification. So a 10x50 pair of binoculars have an exit pupil of 5mm. Everyone's eyes are different but age does play a part here, so the more mature astronomer may want to go for higher power instruments.

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