There are a number of factors to take into account when setting out to buy a telescope. The major considerations being space, storage and cost, and who will be the main user of the telescope.
It is handy to know that a telescope size is measured across the tube diameter, or the mirror’s diameter. So a 6-inch Dobsonian telescope has a mirror 6-inches in diameter, in a 6-inch wide tube. It is so tempting to go for the biggest telescope you can afford, such as a 10-inch Dobsonian on an altazimuth mount. Unfortunately, this telescope and stand is over 6 feet tall and more than 2 feet wide. Not only do you have to consider where you can store the telescope, but whether you can easily transport it to your viewing area.
A 6-inch reflector on a German equatorial mount will give you a good view of the planets and stars. It is also a lot more manageable if you have to transport your telescope to a good viewing site. Surprisingly, the cost of the 10-inch Dobsonian, when compared to the 6-inch reflector telescope with an equatorial mount is quite similar. It’s just that the Dobsonian telescope has a simple stand, which is a lot cheaper than the complicated equatorial mount.
A basic 3-inch refractor can be a good compromise for a complete beginner, or if your child is showing an interest in astronomy. It is lightweight, and compact enough to be stored away unobtrusively, but will give you enough access to the objects in the night sky to be worthwhile.
What you really need to look at is the quality of the telescope you are buying. There are a lot of cheap telescopes out there, with very poor quality lenses and mirrors. A poor quality mirror will distort the images, sometimes to the point of making the image impossible to view properly. Poor quality lenses will have a similar effect on the astronomical object you are viewing.
Always go for the best lenses you can afford, even on a basic telescope. What you don’t want to do is fall into the beginner’s trap of over magnifying your lenses. You never need to go higher than a 4 x Barlow magnifying lens. Preferably stick with a 2 x Barlow lens. There is a set limit as to how much you can magnify an image and it is tied into the width and length of your telescope.
Some telescopes are of such poor quality, and the image is so bad that the manufacturers actually stop down the telescope by fitting a smaller ring inside it. This can mean that although you can think you’re buying a 2 or even 3-inch telescope, the stop ring can reduce the viewing field of the telescope by half. Take a look at the end of the telescope, if there seems to be a flat washer just inside the tube, then it may be a stopping down ring.
Before you buy a telescope, insist that you take a look through the lens. You can do these tests in daylight; just don’t look at the sun while doing them. A cheap telescope will give false colours around the edges of high-contrast objects, such as power cables. These telescopes are also prone to astigmatism, that is, an image can show distortions over its shape. Pinpoint lights can also show flaring, which means that once you get the chance to view an astronomical object, you will be perpetually trying to focus the telescope to no avail.
The quality if the mount is also very important. Too light or unstable a mount and stand, will leave you with a telescope you are frightened to adjust. Even a gentle touch on the controls will leave the image bouncing around like a demented thing. The movement of the mount should be smooth, stable, and positive. You should be able to have confidence that when you make an adjustment, It wont leave you waiting thirty seconds for the image to settle down again.
If at all possible, try out as many telescopes as you can before buying. Decide where you want to spend your money, on the mount or on the telescope itself. Don’t buy cheap, you will just end up with an inferior product. Check out the Internet for advice on what’s out there, and compare prices. Buy a telescope which suits your needs, and remember that you will probable have to move your telescope about, from where it’s stored, to where you do your viewing. You should also be confident that you can move the telescope easily and safely by yourself.