A beginner’s guide to magnification with telescopes

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What magnification is that?

When showing people objects through a telescope for the first time, be it the Moon, a planet or another celestial body, one of the most asked questions is “What magnification is that”. It is important that we as astronomers know not only the answer, but also know how to describe the value of the answer. Like all of my blogs I am going to keep this simple and not try and baffle anyone with long explanations and mathematical calculations. To be honest the mathematical formula is very simple.

Focal length

Let us start with naked eye.  When we look at an object which is an infinite distance away (e.g. the stars), the focal length, or the distance from the cornea to the retina, of a normal relaxed eye is about 1.7 cm (17 mm). When we view an object that is closer, our eyes act as a zoom and change the focal length to about 22mm. If we look at something on the horizon a long way off, we see lots of things around that object i.e. we have a larger field of view. If we then take a pair of binoculars and look at the same object, we see much less of the surrounding area. The object is looking (optically) closer, although it has not moved, we have a smaller field of view. The same thing happens with telescopes. When we look up at the sky at night with the naked eye, our wide field of view has not changed. When we now look through a telescope we are making our field of view small and at the same time bringing the object closer.

Focal length is the distance (given in millimetres) between the telescope’s primary lens or mirror and the point where the light rays come together in focus. All telescopes have a fixed focal length, just like lenses on a camera. So a 20mm focal length will give you a wider field of view than say a 300mm. You will also notice that with a 300mm you get a larger image of the subject, be it a person or animal or even the moon.

If you get a lens or magnifying glass, you will find that you have to move it back and forth from your eye to get the object you are looking at into focus. That point is the focal point. The focus point of a telescope will differ depending on type of telescope and length of tube but it is usually just inside the tube or in in the focuser.  This means that we need to put an aid inside the rear of the tube to enable us to focus on that focal point. For this we use eye pieces which are like a mini telescope, with very short focal length.

Magnification explained

Eye Pieces

Eyepieces, like the telescope, have the focal length written on the outside in (mm), eg, 6mm – 20mm -30mm – 40mm. We now have two bits which both have fixed focal lengths. Now we can determine the magnification we will get with various combinations of eyepieces and the telescope. Let us say that our telescope has a focal length of 1000mm this will make it easier for starting but obviously not all telescopes have the same focal length.I am going to use the eyepieces mentioned earlier for the calculations. The formula is simply the focal length of the telescope divided by the focal length of the eyepiece. So for example 1000mm telescope divided by 10mm eyepiece will give 100 x magnification. 1000 / 10  = 100. This is because 10 goes into 1000, 100 times.


Focal length of telescope Eyepiece X Magnification
1000mm 40mm 25x
1000mm 30mm 33.3x
1000mm 20mm 50x
1000mm 6mm 167x



From the table we can see that with a 6mm eyepiece in a 1000mm telescope we will get 167 x magnification, that is 167 times larger than what we would see with the naked eye. One of the things to remember about eyepieces is exit diameter. If this is too small it can make viewing very difficult. Wide angle eyepieces tend to have a much larger exit opening, making it easier to use.

An important pointer: always work in millimetres. The fact that scope values are sometimes given in inches and sometimes in centimetres and this can make things more complicated than they need to be. Millimetres work out really well, so always try to convert if you can. Multiply inches by 25 and centimetres by 10 to convert to millimetres. You will find that most of the time you work with only a few (two or three) eyepieces and this makes it easier, once you have calculated the magnification, to remember it.

Randy Culp http://www.rocketmime.com/astronomy/Telescope/Magnification.html

Barlow lenses

An extremely useful tool that most amateur astronomers have, in their kit box, is a Barlow Lens. The Barlow lens was invented by Peter Barlow (1776-1862) an English mathematician. A Barlow lens is a concave lens that when placed in a telescopes before the eyepiece, it will increase the focal length of the telescope by 2x, 3x, 4x and so on, depending on the size you use.

The most common Barlow is the 2x Barlow and normally come supplied with most beginner telescopes. A 2x Barlow will double the focal length of the telescope. For example, if you were using a telescope with a 1000mm focal length, you would have 2000mm focal length.

One of the greatest advantages of a Barlow lens is that it not only will double the magnification it will effectively double your eyepiece collection because you can use your telescope with or without the Barlow lens.

It is well worth spending time looking at something big, for example the Moon, and trying different combinations of eyepieces and a Barlow. You will reach a point where you have maximised the magnification of your telescope. This is where the image may well be bigger (closer) but you will struggle to focus and get a sharp image. At the end of the day what you are doing is also magnifying everything between the end of your telescope and the Moon. This includes atmosphere, air movement, light pollution, pollen and dust particles. Bigger is not always better, and you may have to reduce magnification to get a clearer, sharper view.

If you would like to know more about your telescope, eye pieces and magnification then have a look at our Telescope Workshops here at Astrofarm. Bring your telescope along, have help setting it up and using it and start using it to enjoy the fantastic night sky.


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Andrew Davies
Astronomer at Astrofarm
Andrew is the resident astronomer at Astrofarm residential astronomy centre in the South of France. He has been hooked on the night skies since a schoolboy and has never tired of both solar and night time observing as often as he is able. As a teacher of both astronomy and photography classes Andrew has a wealth of knowledge and likes nothing better than to share with others.

He founded both the Mid Cheshire and Runcorn & Widnes Astronomy clubs and organises and presents the annual North West Astronomy Festival. He excels in outreach and inspiring beginners and loves to share his knowledge as well as to learn from others. In addition, Andrew is a keen photographer of both the night sky and the world around - you will see examples of his interest and skill throughout our website www.astrofarmfrance.com