Telescopes for Novice Astronomers
David Wildash
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.
For those amateur astronomers, there are three types of telescopes available.
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 an 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.
The third type of telescope is the Schmidt-Cassegrain. This uses lenses and mirrors to fold the light path back on itself within a compact tube. They are generally cheaper than refractors, but dearer than reflectors. They are more portable and easier to handle than the other two.
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.
joining a local astonomy club or checking a library, you can evaluate different types of instruments. Also, astronomy magazines have reviews as well as advertising for good telescopes.
Even though you should purchase the largest aperture you find, keep in mind you may not want a large telescope as you may have to carry it around. Smaller telescopes are easier to set up and use, and high pollution in your area can keep a large telescope from producing results to its full potential.
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's important to know that many textbook and telescope advertisement photos are long exposures and have been given false colors. Since the eye isn't sensitive enough, stars will always look like small points of light and color will not be seen in dim objects.
Now, what will you be able to see with a novice telescope? With a 75mm refractor or 150mm reflector, you may see galaxies, nebulae, Saturn's rings, the large Jupiter moons, and countless craters on the moon. There are plenty celestial objects to help introduce you to the night sky.
David Wildash posts information and tips on his website about (
http://www.buyatelescope.info/) Telescopes, and you can read more about (
http://www.buyatelescope.info/buying-a-telescope/) buying a telescope
Categories
