Telescopes for Viewing the Night Sky
If you want to look at the craters on the Moon, the rings of Saturn, or Jupiter’s Great Red Spot, you need a telescope. The same advice goes for observing faint variable stars or viewing all but the brightest galaxies or the beautiful small glowing clouds called planetary nebulae, which have nothing to do with planets.
Focusing on telescope classifications
Telescopes come in three main classifications:
- Refractors use lenses to collect and focus light. In most cases, you look straight through a refractor.
A refracting telescope uses lenses to collect and focus light.
- Reflectors use mirrors to collect and focus light. Reflectors come in different types:
- In a Newtonian reflector, you look through an eyepiece at right angles to the telescope tube.
- In a Cassegrain telescope, you look through an eyepiece at the bottom.
- A Dobsonian reflector gives you the most aperture (or light-gathering power) for your money, but you may have to stand on a stool or a ladder to look through it. Dobsonians tend to be larger than other amateur telescopes (because large Dobsonians are more affordable), and the eyepiece is up near the top.
- Schmidt-Cassegrains and Maksutov-Cassegrains use both mirrors and lenses. These models are more expensive than reflectors with comparable apertures, but they’re compact and more easily taken on observing trips.
Many varieties are available within these general telescope types. And every telescope used for amateur purposes is equipped with an eyepiece, which is a special lens (actually, a combination of lenses mounted together as a unit) that magnifies the focused image for viewing. When you take photographs, you usually remove the eyepiece and mount the camera on the telescope in its place.
Just as with a microscope or a camera with interchangeable lenses, you can use interchangeable eyepieces with almost any telescope. Some companies don’t make telescopes at all, specializing instead in making eyepieces that work with many different telescopes.
Beginners usually buy the highest-magnification eyepieces they can, which is a great way to waste your money. Low- and medium-power eyepieces are recommended because the higher the power, the smaller the field of view, making it tougher to track faint (and possibly even bright) targets. For a small telescope, observation is usually best with eyepieces that give magnifications of 25x or 50x, not 200x or more. (The x stands for “times,” as in 25 times bigger than what you see with the naked eye.) If you see a telescope advertised for its “high power,” the advertisers may be trying to sell mediocre goods to unwitting buyers. And if a salesperson touts the high power of a telescope, patronize another store.
What limits your view of fine details with a small telescope isn’t the power of the eyepiece. Air turbulence (the same factor that makes stars seem to twinkle) and any shaking of the telescope in the breeze are the effects that limit the clarity of your view.
Examining telescope mounts
Telescopes are generally mounted on a stand, a tripod, or a pier in one of two ways:
- With an alt-azimuth mount, you can swivel the telescope up and down and side to side — in altitude (the vertical plane) and azimuth (the horizontal plane). You need to adjust the telescope on both axes to compensate for the motion of the sky as Earth rotates. Dobsonian reflectors always use alt-azimuth mounts.
- With the more expensive equatorial mount, you align one axis of the telescope to point directly at the Celestial North Pole or, for Southern Hemisphere viewers, the Celestial South Pole. After you spot an object, simply turning the telescope around the polar axis keeps it in view. Be sure to polar-align the scope for each viewing session.
An alt-azimuth mount is usually steadier and easier for a beginner to use, but an equatorial mount is better for tracking the stars as they rise and set. However, if a telescope has built-in computer control, either style of mount is fine because the computer takes care of tracking.
The objects you see in the telescope are usually upside down, which isn’t the case with binoculars. Of course, it doesn’t really make much difference in the viewing, but just know that top and bottom are reversed when you view through a telescope. Adding a lens to rotate the image so it’s right side up has the disadvantage that it reduces the amount of light coming through the scope and dims the image, so it’s best not done. When viewed through an equatorially mounted telescope, a star field maintains the same orientation as it rises and sets. But when seen through an alt-azimuth mount telescope, the field rotates during the night, so the stars on top wind up on the side.