Finding your way around binocular basics

No 2. in a series created and compiled by Dave Armeson

The NEXT step – going further than naked eye observing…

If you’ve had a few nights out under the stars and learnt a few constellations, then you might wonder what is there to see just below naked-eye limit. Well, rather than plunging in the “deep end” and buying a telescope that might/might not be suitable for you, a prudent step is binoculars. Binoculars bring into view star clusters, glorious Milky Way star fields, Jupiter’s four largest, Galilean moons and of course some craters on the Moon and some of the brighter “Deep Sky” objects like the Andromeda Galaxy.


One great idea is to contact and/or get together with people from your local Astronomical Society and talk to them about binoculars – the knowledge they have can save you from making a sometimes costly mistake (the same applies to buying your first telescope – more about this in a future article).

Why binoculars…?

Binoculars offer a number of advantages that make them superior to larger telescopes. In fact, binoculars could be looked as two smaller telescopes that are joined together to give your eyes stereo images. They have three main benefits over telescopes: cost, portability and ease of use.

Binocular types: porro prism and roof prism

The most popular and probably best type of binoculars for astronomy are called porro prism binoculars and look like what you imagine when you think about binoculars. The other type of binoculars is the “straight through” type which are called roof prism binoculars. Porro prism binoculars will give a 12% to 15% brighter image than roof prism binoculars as roof prism binoculars employs silvered surfaces that reduce light transmission by 12% to 15%. Roof-prisms designs also require tighter tolerances for alignment of their optical elements. This adds to their expense since the design requires them to use fixed elements that need to be set to a high degree at the factory.

porro prism binoculars
Porro prism binoculars
roof prism binoculars
Roof prism binoculars

Magnification and field of view

Binocular magnification
Binocular magnification

On the back of any pair of binoculars are two numbers. The first number denotes the magnification of the binoculars themselves and the second number is the size of the front lens in mm. For example in this image they are 7x magnifying with a front lens of 50mm diameter. Other popular sizes are 6x or 7x 35mm binoculars.

Easy binocular targets

The Pleiades (Seven Sisters)

The Pleiades open cluster
The Pleiades open cluster (click image to enlarge)

During the autumn and winter months one of the most stunning visual sights is the Pleiades (The Seven Sisters) star cluster in the constellation of Taurus. They are 2,644 million million miles away. With the naked eye you can see a group of up to 10 to 12 stars under a clear dark sky. In binoculars the view is transformed into something like this image.

The Andromeda Galaxy.

Andromeda Galaxy
Andromeda Galaxy (click image to enlarge)

This is the nearest external galaxy to our own Milky Way and lies at a distance of about 2.5 million light years from Earth. It contains about 250 billion stars/suns.

Finder chart for the Andromeda Galaxy
Finder chart for the Andromeda Galaxy (click image to enlarge)

Remember when you are looking at Andromeda in your binoculars, the starlight from the billions of stars that make up that galaxy has been travelling through space for the last 2.5 million years at the speed of light to land on your retina – so you are looking back in time….!!!

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