It's easy to see why astronomy is a popular pastime. With the right telescope designs to choose from, so let's start by taking a look at the major kinds of telescopes that are available.
A good type of telescope to start with is a refracting telescope, which is typically 60 x 70 mm. The "60" is the objective lens aperture, with the "70" representing the focal length. Both measurements are in millimetres. Refracting telescopes perform beautifully with simple viewing tasks. One of the keys to viewing objects at long distances is to have a larger aperture when seeking higher power.
A Newtonian telescope has dual mirrors, which have different functions but work together to improve viewing. The larger mirror reflects incoming light rays to a smaller, flat, diagonal secondary mirror, which diverts the light to an eyepiece where the image is brought into focus.
There are two designs in this category -- the Maksutov-Cassegrain and the Schmidt-Cassegrain. These telescopes are portable and are based on a mirror lens design with larger manufacturing costs. An adjustable knob brings the image into focus. They are both popular and convenient options.
Making a telescope selection
When looking for a telescope, there are a few criteria to consider. Probably the two most fundamental aspects of any telescope are the optics and the mounting, which will determine the quality and stability of the images, as well as the ability to maintain focus. There are other features, as well, such as the types of lenses that come with the telescope, or electronic tracking and control units -- but without the right basics in place, the other features won't be much help.
Perhaps the most important thing to look at is the aperture of the telescope. Put simply, the aperture means the diameter of the main lens or mirror, depending on the type of telescope. The size of the aperture affects everything from brightness to sharpness to the ability to focus the telescope. Aperture sizes vary by telescope, but as a general rule of thumb, look for a minimum aperture of at least 70 mm. In order to see some more distant objects, look for something more like 150 mm. The largest amateur telescopes typically have an aperture size of around 250 mm.
Generally speaking, in the case of telescope apertures, size does matter, but it's not the only consideration. Part of the equation in choosing a new telescope is determining where it will be used. Those living in an urban area with a lot of light (which is a bad thing for astronomy), and don't plan on moving the telescope around, should buy as big a telescope as they can afford, because the big aperture will help users see distant objects through all the light and haze of an urban area. But if users plan to take their telescope to different locations, or they live in the Outback, they can get away with a smaller aperture. And remember -- a big aperture means a big telescope, which means more weight to carry around.
Power or Magnification
Closely related to aperture is the power or magnification of the telescope. Magnification simply means how large the telescope can make the object it's focused on appear. It's not nearly as important as aperture, however, because magnification is controlled by the eyepiece that is used. There are many kinds of eyepieces, all with different lenses and filters and magnification abilities. Eyepieces are typically interchangeable, whereas the main lens or mirror is not.
There are two main types of mountings, the equatorial and the altazimuth. Equatorial mounts are designed to track the movement of the night sky as the earth turns, and are a great help in finding and tracking celestial objects. Altazimuth mountings are much simpler, and need to be re-adjusted more frequently to keep up with the movement of the sky. Altazimuth mountings are cheaper, but they also typically mean more hassle to work with, and they provide much less stability.
When buying the right telescope, it all depends on where you'll be using it and if you plan on carrying it with you. While there are many factors that go into selecting the right telescope, focus on aperture above all else and then think about the magnification and mounting.