The Magnification Myth


Dividing the focal length of the eyepiece into the focal length of the telescope gives you magnification

It’s one of those “If I had a dollar for every time I’ve been asked that” questions, and I was asked it again at least twice last week: “How much magnification is that?” And again I had to explain that when it comes to telescopes it wasn’t that much and magnification not what really matters.

The first thing to mention is that we live under a moving blanket of air we know as the atmosphere. The more turbulent the air is above our heads, the worse the seeing is, a term astronomers use to describe how the steady the air is at the time of observing. Good seeing means the air is still and the image through the telescope is steady, not waving and blurring as the air moves about. Most nights of average seeing will let you magnify to around 200x before the image degrades into a blur. This is a case where less can be more – lower magnifications often give you the better, clearer, view because they are not focusing on the turbulence. Those still nights of rock-steady seeing usually only occur a few times a year, and they’re so good that when they do happen you often talk about them for years afterwards. These are the nights you can push past the 200x barrier into “silly power” territory.

The focal length of the telescope also comes into play, because the degree of magnification is worked out by dividing the focal length of the eyepiece into the focal length of the telescope. The eyepiece is the thing you slide into the focuser to look at the image the telescope is collecting and they usually have their focal length marked on them in millimetres e.g. 25mm. They are constructed from a series of lenses spaced a specific distance apart depending on the design and if I was to gripe about one of my bugbears it would be people calling eyepieces “lenses”. There are lenses in the eyepiece, people! They aren’t a lens. But getting back to the subject, with a little bit of simple maths you can see that a telescope with a long focal length will allow you to divide the relatively small eyepiece focal length into it more than a short focal length would.

Aha! So we need to go and buy the telescope with the longest focal length to get a whole lot of magnification, then, right? Not so fast, you need to consider the downside to this extra length – a smaller field of view, which makes the telescope hard to point and harder to track accurately. Nonetheless, if your main aim is to observe the planets, you may choose a longer focal length over a shorter one. The opposite applies to a short focal length – you get a wide, low power field of view, great for large open clusters and deep sky astrophotography. So there is a trade-off between focal length and field of view, depending on what you want to look at.

People who get serious about their telescopes soon learn that aperture, the width of the light-collecting surface, is often the by far more important quality to consider. The reason is two-fold: Firstly, the wider the aperture the fainter you can see, and more often than not how bright an object is determines whether it is seen or not, not it’s distance – which relates to another often asked question, “How far can this telescope see?” Distance is largely irrelevant; You can see the Andromeda galaxy, which lies roughly 2.2 million light years away, with the unaided eye, yet you would struggle to see the closest star to us, the red dwarf Proxima Centauri, a mere 4.2 light years away, even using a telescope, because it’s so faint! The second reason is that a larger aperture will usually result in better resolution of an image. It can be likened to having more pixels in your digital image. Some people dream of getting a really large telescope to see all those faint objects out there; you might hear they’ve caught “aperture fever” if they keep talking of this desire. Big telescopes are nice to look though, but their sheer physical size comes with their own set of problems, so they aren’t for everyone.

To round this discussion out, it might be worth mentioning that larger telescopes don’t necessarily magnify more either, despite the fact they often have longer focal lengths as a by-product of their size. What I’ve noticed over my years of observing is that the image in a big scope is usually brighter and more detailed or with better contrast between features, rather than larger. You also might be surprised to learn how big some objects actually are in the sky and a large telescope at high power will just look straight through them. Again, sometimes smaller is better!

As a rule of thumb, the maximum useful power a telescope will reach is twice the aperture in millimetres. This does vary, depending on the quality of the optics, but it’s a good guide to a telescope’s potential. Which means those cheap 60mm refractors you might pick up in a discount store advertising “576x magnification!” on the sides of the box are leading you astray. Don’t go there! And don’t get hung up about magnification. It’s a lesser factor of your considerations when choosing a telescope.

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