How to Choose a Telescope for Beginners
- this is an exciting time to become an amateur astronomer.
- before you buy anything.
- you must determine what’s important to you.
- what do you most want to look at?
- how dark is your sky?
- how experienced an observer are you?
- how much to you want to spend?
- what storage space do you have?
- how much weight do you want to carry?
- Answer these above key questions, familiarise yourself with what’s on the market, and you’ll be well on your way to choosing a telescope that will satisfy you for many years to come.
Before examining the different telescopes available, it’s worth knowing the basics of how they work.
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Aperture – a telescope’s Most Important Specification
- the most important aspect of any telescope is its aperture.
- the diameter of its main optical component, which can be either a lens or a mirror.
- a telescope’s aperture determines both its light-gathering ability (how bright the image appears).
- its resolving power (how sharp the image appears).
- when learning how to choose a telescope, knowing all you can about the aperture is crucial to your ability to see the night sky.
What does this mean?
- the bigger the aperture the better.
- with a 8-inch telescope you can discern craters on the Moon as small as about a mile across – half the size of those visible in a 3-inch scope (under the same conditions using the same magnification).
- the same two instruments turned toward a faint galaxy on a moonless night would tell an even more dramatic story. Because the surface area of a 6-inch mirror is four times that of a 3-inch mirror, it collects four times as much light, meaning the galaxy would appear four times brighter.
Magnification Isn’t Everything
It may surprise you, but a telescope’s aperture is not what determines its magnification (“power”). When seeing a telescope for the first time, a novice will usually ask, “How much does it magnify?” The answer is, “Any amount you want.” Any telescope can provide an almost infinite range of magnifications, depending on the eyepiece you put into its eye end.
But don’t get the idea that super-high powers will do you any good. Two main factors limit the power that shows a decent view with a given instrument: aperture (again) and the atmospheric conditions.
Only so much detail exists in the image created by a telescope’s main mirror or lens, so you must find the optimum magnification to see this detail — without spreading out the target’s precious light too much, making a dim object too dim to see or turning a bright object into just a big blur.
This is why observers generally use low powers for looking at faint things like galaxies and nebulae, and no more than medium-high powers for bright things like the Moon and planets. Just as enlarging a photograph too much will simply show you the grain in the film or the pixels on the chip, so too will excess magnification just make your target blurry.
How much power is too much? There’s a simple rule to find the top useful magnification: 50 times your telescope’s aperture in inches, or twice its aperture in millimeters. And that’s if the scope has perfect optics and the night air happens to be unusually steady.
This means that a high-quality 4-inch (100-mm) scope should not be pushed beyond about 200x. To put this in perspective, even a small instrument that has good optics will show you Saturn’s rings or the principal cloud belts on Jupiter, since these can be seen at a magnification of 75x. On the other hand, if you see a small, 60-mm department-store telescope scope labeled as delivering “300 power!!!”, you’ll know it’s advertising hype and you should wisely look elsewhere.
Calculating Magnification
Now you know the maximum practical power for any given instrument. But how do you get it? What do those little numbers on the eyepieces tell you about the magnification they give?
Every scope has a focal length, which is effectively the distance from the primary lens or mirror to the image it forms. (This is not always the same as the length of the tube, since, as we’ll see later, some telescopes optically “fold” the light path internally.) Focal length is the large number you’ll often see printed or engraved on the front or back of the scope, usually between about 400 and 3,000 millimeters depending on the scope’s aperture and type.
Eyepieces have focal lengths too — 25mm or 10mm, for example. Simply divide the focal length of the scope by that of the eyepiece; that’s the magnification. For instance a 1,000-mm focal length scope, used with a 25-mm eyepiece, delivers 1,000 / 25 = 40 power (or 40x).