What size telescope?

How big a telescope do I need?

Aperture is the most important feature of a telescope

The job of a telescope is to collect light. The largest the telescope’s aperture, the more light it can gather. This makes the image brighter and sharper.

The light gathering power increases rapidly as the aperture gets bigger. If you double the aperture, the amount of light collected increases by a factor of four. For example, a 6 inch scope gathers more than twice as much light as a 4 inch.

In principle, a brighter sharper image can be magnified more before the image quality degrades. Bigger aperture scopes can give you much more detailed and impressive views of planets and the moon’s surface because a larger telescope allows you to use higher magnification.

But in practice, on most nights the detail you can see will be limited by atmospheric conditions, not by the size of the scope. When the air is turbulent (astronomers call this ‘poor seeing conditions’) the most you can expect is to be able to magnify 200x, regardless how big your telescope is. In very good seeing conditions you might be able to magnify 300x or more, but those nights will be infrequent.

Most of the time, therefore, if your main interest is to observe the moon and planets then a good 6 inch scope will perform well. You might even get away with a 4-incher. You will see terrific detail of craters on the moon, cloud bands on Jupiter, details on the surface of Mars, Saturn’s rings and perhaps even subtle cloud belts on Saturn as well.

Bigger scopes will allow stunningly more detailed views of the planets, but only on the best observing nights.

Bigger is better for deep sky objects

For deep sky objects – star clusters, galaxies, nebulae – the situation is different. Bigger is definitely better. You will see plenty of deep sky objects in a 4 inch or 6 inch scope, but they will tend to be faint and have much less detail than in a bigger scope. A galaxy or nebula that might look like a fuzzy blur in a small scope may have swirls of spectacular detail in a large scope. At 12 inches or more, you may also start to see colour in nebulas.

For deep sky objects, the clarity or transparency of the air is more important than the turbulence of the atmosphere. Clear, haze-free air is best. The biggest limitation on viewing ‘the faint fuzzies’, as deep sky objects are often called, is light pollution from cities. Larger apertures can help cut through light pollution to some extent – especially for star clusters – but diffuse fuzzy objects like galaxies and nebulas will be hampered no matter how big the scope. If you live in a city and are keen on deep sky viewing, make sure you get a scope that you can fit into your car and take to dark skies every now and again.

As a general rule of thumb, bigger telescopes tend to have a smaller field of view. There are not many viewing objects where this becomes a problem for telescopes of 12 inches or smaller, but there are a few – such as the Andromeda galaxy and some star clusters such as the Pleiades.

Lastly, remember that there is a trade-off between size and portability. A big telescope won’t always show you more – you will see the most in the telescope that you use most often.

General guidelines

Summing up, there are no hard and fast rules when it comes to aperture. It depends on what you most want to see, where you live, and how you plan to use the scope. But in our experience:

  • You can see a lot with a 4 inch scope, but keen amateurs are likely to outgrow it fairly quickly, especially if they live in a city. That is why Astronz doesn’t sell anything smaller than a 6 inch aperture.

  • An aperture of 12 inches or more becomes bulky and can be a hassle to set up or transport. It either becomes quite unwieldly in size, or is heavy when combined with the mount. Big scopes would either have a permanent home, or are set up only for long viewing sessions. You are unlikely to take them outside for just an hour.

All about magnification

How much does a telescope magnify objects? The answer is -- any amount you want, just by changing the eyepiece.

But you can over-magnify, making the image worse. Higher magnifications will not always show more detail. This is because we view from the bottom of an ocean of air, our atmosphere, which is always turbulent to some degree. Telescopes not only magnify the image of distant astronomical objects, they magnify the atmospheric turbulence as well. Also, spreading out the light by magnifying the image makes the view dimmer.

Here's an example of what happens when you over-magnify an image.

Image courtesy of rocketroberts.com

There is an optimal magnification (or optimal choice of eyepiece) depending on the atmospheric conditions and the object that you are observing. You usually work this out through trial and error. On most nights, when observing planets and the moon you will see the most detail with moderate to high magnifications.

Maximum magnification

A useful rule of thumb is that the maximum magnification your telescope can handle is around 50 times the telescope’s aperture in inches. Any higher and the image gets too dim and blurry. So, a 6 inch scope can magnify up to 300x, while an 8 inch scope can magnify 400x. But this is in the best atmospheric seeing conditions. On a normal night, the optimal magnification will be well less than this – often 200x.

Minimum magnification

Surprisingly, there is also a minimum useful magnification. This is related to the size of the image that comes out of the eyepiece. If it is any bigger than the opening in your eye, then the extra light is wasted. For a typical amateur scope, the minimum magnification is 20x to 40x.

How to calculate magnification

Every telescope has a focal length, which is the distance between the main mirror or lens and the focussed image. For example, our 8 inch Dobsonian has a focal length of 1,200mm. All eyepieces also report their focal length, for example a 20mm Plossl. The magnification is the telescope’s focal length divided by the eyepiece’s focal length. In this example, 1200/20=60x. Changing to a 6mm eyepiece would increase the magnification to 1200/6=200x.