Choosing a Telescope
When choosing a telescope, the most important thing is the aperture, the diameter of the main lens or mirror.
Small, bright objects, such as planets require magnification to be viewed well. Large, dim objects, such as galaxies and nebula, require a way of brightening the image. Both of these are improved with a large aperture.
A telescope with twice the aperture has four times as much light collecting area. With all other things equal, it will produce an image four times brighter. A telescope with a diameter of 100 mm (4 inches) has a light collection area of about 78 cm2. The night adapted human eye has a collection area of about 1/2 cm2. The 100 mm telescope can therefore gather more than 150 times as much light.
Whether or not you get a bright, wide field of view or a magnified, narrow field depends on your choice of eyepiece. Long focal length eyepieces will give you bright images and low magnification, great for big, dim nebulas. Short focal length eyepieces give you dimmer, magnified images, great for compact, bright objects such as planets.
The second most important thing is the stability of the mount. If your telescope jiggles around, the magnified image jiggles around even more. This can make it impossible to see anything. The rotation of the Earth causes objects to drift across the view through any telescope that doesn't have a tracking motor. Mounts with flimsy legs or with lots of backlash in the bearings won't stop jiggling until after the object has drifted out of view. Wind can also blow around a telescope with a flimsy mount. Add this to the effects of scintillation, and a poorly mounted telescope may end up stored in the closet.
Scintillation
is what makes the stars twinkle.
Pockets of air of different density shift the light as though passing
through
thousands of weak lenses. Often, this settles down for only a
fraction
of a second out of several seconds. It is during these moments of
"good seeing" that experienced observers see detail. If you are a
beginner and you can't see what someone is showing you, they probably
can't
see it either, except during flashing instants of good seeing!
copyright David Carpenter 2001
Page under development.
For more specific information from major telescope suppliers, visit: Orion Telescopes & Binoculars, Celestron, or Meade Instrument Corp. Also, check out the advertisements in Sky & Telescope or Astronomy magazines, available at practically any library or bookstore.
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