So, you've decided to buy a telescope. Congratulations. This is a very good decision. Perhaps I'm biased, but I think everyone should have a telescope. I think in the interests of promoting an enlightened and peaceful society, governments should give all their citizens telescopes - no charge. However, since that is unlikely to happen any time soon, we are on our own.
So let's start with some fundamental concepts. What does a telescope do? A telescope collects light. The more light a telescope can collect, the larger and clearer the image you will see. The amount of light a telescope collects depends entirely on the size of its primary mirror or lens. So, the first rule of telescope buying is to buy the largest telescope you can afford. A very small telescope with tiny, indistinct images will only frustrate you, and will spend more time in the back of your closet, than in your backyard.
When we talk about the size of a telescope we are talking about the size of its primary mirror or lens. Personally, I would not buy a telescope smaller than 6 inches (which means a telescope with a primary mirror or lens 6 inches in diameter). But besides its light gathering ability, there are two other very important considerations concerning the size of a telescope: price, and portability. With a little shopping around, you should be able to find a good quality 6 inch scope for around a thousand dollars. For larger scopes, the price rises dramatically, and so does the weight, and the bulkiness. Unless you live in an area with no light pollution, and nice dark skies, you will probably want to be able to load your scope in your car, and head for the country. This can be very problematic with large, bulky, heavy telescopes.
There are two basic types of telescopes: refractors, that pass (refract) light through a primary lens, and reflectors, that bounce (reflect) light from a primary mirror. Refractors were the first telescopes, and are still used today, but the vast majority of astronomical telescopes are reflectors, sometimes called Newtonians, after their inventer. The diagram below shows the basic differences between the two.
The light from relatively nearby objects, such as the Moon and some planets, is relatively energetic, sometimes too energetic, so that filters must be used to overcome the brightness and make features discernable. With most astronomical objects, however, the reverse is true. When viewing the spectacular Whirlpool Galaxy for instance, which is 35 million light years away, the photons of light entering your telescope have been travelling through space for 35 million years, and they're tired, as you might expect. And every time they have to pass through a lens, or bounce off a mirror, they get a little more tired, so by the time they reach your eye, they don't have much energy left. The higher the quality of the lens or mirror, the less energy they lose, which is why you want to buy the best quality you can afford. This is also one reason why astronomical telescopes deliver an inverted, upside down image, because that uses the least amount of optics. Some scopes do re-invert the image by adding another lens, but that degrades the image and increases the cost, and besides, when you are looking at objects floating around in space, "which way is up" is somewhat of an irrelevant question.
The simplicity, and high tech lenses used in modern day refractors allow them to deliver a slightly better image quality than most reflectors, but this is balanced by a much higher cost, and a much longer tube length, so that anything over six inches is hardly practical. However, some astronomers still use small four or five inch refractors to get the most detail they can out of nearby bright objects. Reflectors are much more practical and inexpensive, and are really the only option in sizes over 6 inches.
Never buy a telescope that is advertised as having high magnification, or high power. This is simply a marketing ploy used on cheap telescopes aimed at people who know nothing about them. Magnification (or power) is determined by the eyepiece of the telescope, and you should have at least two or three different eyepieces of various sizes. But high magnification eyepieces are completely useless unless your telescope has the light gathering ability to support them. All you will see is a large, hazy patch of light. The larger the telescope, the more the image can be magnified, but only up to a point, because atmospheric turbulence and small imperfections in the mirrors or lenses will be magnified as well. On a night when the "seeing" is excellent, you might be able to push a six inch scope all the way to a magnification of 200, but you will see very little detail. The largest scopes in the world rarely go over a magnification of 300, so if you see a three inch scope proudly advertised as 400 power, just walk right on by.
Dobsonian telescopes are very simple reflectors that are usually less than half the price of other reflectors. They can be a good choice for someone on a limited budget who wants the largest scope they can get. They are very popular, and rightly so. Their only downfalls are that they have limited controls, so that it can be tricky to track objects, and the eyepiece can sometimes be at an awkward angle, which can be annoying for those of us with bad backs, and stiff necks. Also, the optics are not always the best quality. But millions of people own Dobsonians and are quite happy with them, so they are definitely something to consider. Below left is a three inch refractor. In the centre is a six inch reflector. Below right is a ten inch Dobsonian reflector. They are all the same price.
So you've gone and spent the family fortune, and you bring your new telescope home and set it up, and you take a look through the eyepiece, and a feeling of disappointment comes over you because the image is not quite as spectacular as you expected. Don't despair, because the best telescope in the world is only as good as the person using it, and there are some tricks you need to know. First there is the concept of good and bad "seeing" mentioned above. This is something completely unpredictable, and completely out of your control. It all depends on what's going on in the atmosphere. The night may look perfectly clear, but the atmosphere could be in turmoil, and images through the telescope will be blurry, and indistinct. The good news is that "seeing" can change from one minute to the next, so sometimes all you have to do is wait a little.
But even if the "seeing" is perfect, you may still have trouble at first, until you learn how to train your eyes, and your mind. The first thing to realize is that the Human eye at rest is focused on infinity, which is what you want. So don't tense up. Loosen those shoulders, take some deep breaths, and try to allow your eye muscles to relax. It is also very important to dark adapt your eyes. You cannot be looking at a T.V. or computer screen, and then go directly to your telescope and expect to see anything. You have to remove yourself from any bright lights, and allow your pupils to dilate. This may take several minutes. If you must use a light to see star charts, or stay away from the cliff on your way to the outhouse, use a dim red light. You can buy special red flashlights, or just put something red over the lens of a normal flashlight.
You must also learn to use averted vision. This means that instead of looking directly at an object, look slightly off to the side, and the object will usually be a little more distinct. And remember that your eye is only half of the picture. The other half comes from your mind. If you are driving down a strange road, it may be hard to read a roadsign a block away. But the next time you drive that road, and you already know what the sign says, you can read it easily, because your mind has the picture already in its files. The same is true for looking at astronomical objects through a telescope. Do some homework. Look at some photos of the object before you try to find it in your scope.
Do all these things, and then be patient. You will be standing there, looking through the eyepiece, playing with the focus knob, relaxing, remembering, the image swimming around in a blurry haze, and then - suddenly - it will snap into focus, your breath will catch in your throat, and the word, "Wow!" will inadvertently come out of your mouth. Welcome to the wonders of the Universe.