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Telescopes for the Amateur Astronomer
Although most people think of telescopes it may be best to consider binoculars first. They are portable, convenient and relatively inexpensive. Cheap binoculars are better than a cheap telescope. Consider something like 7 x 35 binoculars. The 7 is the magnification produced by the eyepiece, the other number is the aperture in millimeters. This size is light and easy to hold.
If you are considering a telescope there are essentially three types available to the amateur astronomer.
Refractors are the most readily recognized type and consist of lenses at both ends of a tube. Whatever you do avoid the cheap models sold by department, nature/science and toy stores. They quote large magnifications, but their small apertures make them useless for astronomy. Remember a telescope's most important job is to collect light, not magnify a image. A normal terrestrial telescope has an extra lens to ensure the image is the right way up. However extra lenses cut down the amount of light reaching the eye. The one thing an astronomer wants to avoid is reducing light and therefore information. So the correcting lens is omitted and a true astronomical telescope gives an upside down (inverted) image. Lenses can create false rainbow tints around very bright objects like planets. This is called chromatic aberration and can be ignored or corrected by a filter. Refractors generally cost more per inch of aperture than other types of telescope, and those of more than 4-inch (100mm) aperture are rather long and cumbersome. However smaller good quality ones can make excellent beginner instruments.
The second type of telescope is the reflector. Light travels down a tube before reflecting off a couple of mirrors and through an eyepiece on the side of the tube. Reflectors need to be larger than a refractor to be equally useful. They do not suffer chromatic aberration, but the main mirror may occasionally need repolishing or realigning (collimating). There are kits available for this. Reflectors are often the most comfortable telescopes to use because of the eyepiece position. You don't have to kneel and possess an elastic neck to look straight upwards as you would with a reflector. A popular type of reflector is the Dobsonian, which has a mount near the ground rather than a tripod.
Finally, there is the Schmidt-Cassegrain telescope. This telescope uses both lenses and mirrors to fold light back onto itself inside a compact tube. These cost less than refractors and are dearer than reflectors, and are overall more portable and easier to handle.
The specifications of a telescope may quote numbers which can affect the exposure needed if you pursue astrophotography. They will not affect the image you see; however, it's best to become familiar with the sky before you start with astrophotography.
You may be able to evaluate the different instruments by joining a local astronomy club or attending one of their star parties. Check at your local library. You can also look in a reputable astronomy magazine for reviews and manufacturers advertising good quality telescopes.
Although you should buy the biggest aperture you can afford, don't get a large telescope if you will have to carry it a long way. A smaller telescope would be easier to set up and therefore more likely to be used. You will also be wasting your money on a large telescope if light pollution and atmospheric turbulence in your area are high, as you won't be getting the best from the instrument.
It's best to spend most of your money on a buying a large aperture instead of telescope accessories. Doubling the aperture number will calculate the maximal practical magnification: i.e., a 70mm aperture will have a maximum magnification of 140x. Do not buy an eyepiece that is stated to stretch magnification past maximum aperture value. Start simple with eyepiece choices, such as a Kellner, which is a good all around eyepiece. If you place a Barlow lens between an eyepiece and focuser, magnification can be tripled.
A telescope also magnifies the movement of objects across the sky and the instrument will have to be moved repeatedly to keep the object in the field of view. To cope with this different mounts and drives have been devised that enable the telescope to follow the moving object. You can get electronic drives to point the telescope, but the most important thing is that the mount is sturdy.
It may be saddening to know that many textbook photos have false colors and are long exposures. The eye is not sensitive enough to see color in dim objects, and stars will always look like tiny light points.
So what can you expect to see with a beginners telescope? A 3-inch (75mm) refractor or 6-inch (150mm) reflector will allow you to see many galaxies and nebulae, Saturn's rings, Jupiter's largest moons and hundreds of craters on the moon. Plenty of wonderful objects to serve as an introduction to the night sky.
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About the Author
David Wildash posts information and resources on his website about Telescopes , and you can read more about buying a telescope
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1. The 3 Main Varieties Of Telescopes2. Telescopes for the Amateur Astronomer