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 Astronomy and Cosmology
 Astronomy and Cosmology
 

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Astronomy and Cosmology
 
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Refracting

Astronomical refractors are renowned for providing superb views of the planets, Moon, and double stars. They are ideal scopes for terrestrial observation too. Refractors have a long, relatively thin tube with a main (objective) lens at the front that focuses the incoming light, which is then redirected to a comfortable viewing angle by a diagonal mirror or prism (optional). The image is then magnified by a small eyepiece lens. Refractors are rugged, require practically no maintenance, and are highly portable in smaller apertures. 

While more expensive refractors are prized for their high-contrast, razor-sharp images, the inexpensive variety commonly sold in department stores should be avoided. They generally suffer from low optical quality, poorly fitting plastic parts, and spindly mounts that wobble at the slightest touch, ruining the view. Claims for high magnification-300x or higher-are deceptive and a sure sign of an inferior product. The practical limit (for any telescope) is about 50x or 60x per inch of aperture, or 144x for a typical 60mm (2.4-inch) scope. Going any higher will provide only dim, fuzzy images. Magnification is not related to quality or performance.

A good-quality refractor will have at least a two-element (achromatic) objective lens and sturdy mechanical construction. Fittings for the eyepiece, diagonal prism, and other accessories should be made of machined metal, not plastic. The tripod can be wood or aluminum, as long as it's stable.

 
 
Reflector

The Newtonian reflector (named after Sir Isaac Newton, its inventor) is a popular and economical astronomical telescope. Its simple high-performance design provides tremendous light grasp at the lowest cost per unit of aperture of any type of telescope. It works by reflecting incoming light off a large, curved "primary" mirror at the base of the optical tube up to a smaller, flat "secondary" or "diagonal" mirror near the front end of the tube. The secondary directs the narrowed light cone out of the tube and into the eyepiece. Small Newtonians are very portable because the tube detaches from the mount in seconds. 

 ReflectorLarger Newtonians can be quite large and unwieldy, which is part of the reason more compact scope designs such as the Schmidt-Cassegrain have gained popularity. But reflectors have much to recommend them. Besides their low cost, they're pretty easy to make for the do-it-yourselfer. Optical quality can be very good. The mirrors are adjustable, so you can keep the optics in perfect alignment (collimation). 

Their large apertures make reflectors ideal for observing galaxies, star clusters, and nebulas. They also provide sharp, high-contrast planetary and lunar views. Dew does not readily form on the mirrors because the open tube acts as a passive dew shield. But because the optical tube is open to the air, it should be capped when not in use to prevent accumulation on the mirrors of dust and dirt. Newtonian tubes can be supported on a variety of different mounts: the standard altazimuth mount on a tripod, an equatorial mount, or on a cabinet-style "Dobsonian" mount. Reflectors are not well suited for terrestrial observing, because the image is often rotated upside-down or sideways.

 
 
Schmidt - Cassegrain

The compact design and versatility of the Schmidt - Cassegrain telescope has made it the most popular telescope type among amateur astronomers. The optical tube is much shorter and, thus, more lightweight than that of a Newtonian reflector of similar aperture and focal length because the light path is "folded" inside the Schmidt - Cassegrain. Light enters through a "corrector" lens at the front of the telescope and is reflected off the primary mirror to an adjustable, magnifying secondary mirror on the inside of the corrector lens. The light beam is then directed out the back of the tube to an eyepiece.

The Schmidt - Cassegrain design was made commercially economical in Schmidt-Cassegrainthe late 60's by the production innovations of California telescope maker Tom Johnson. His techniques for mass-producing the Schmidt corrector lens were the foundation for Celestron's affordable and wildly successful 8" f/10 "C8," introduced in 1970. This complex-curved front lens corrects for optical aberrations, and also seals the telescope tube, which helps to keep the internal optics clean.

For visual observation, the Schmidt - Cassegrain is an all-around strong performer. Optically and mechanically, it is ideally suited for astrophotography-from simple piggyback shots to extended, through-the-scope exposures of deep-sky objects. Its compactness allows it to be mounted on a lightweight fork-style equatorial mount, an option not available to long-tube conventional reflectors. As for price, Schmidt - Cassegrains fall in the middle between reflectors, which cost less, and refractors, which cost more, per inch of aperture.

 
 
Maksutov - Cassegrain

Like the Schmidt - Cassegrain, the Maksutov - Cassegrain is a catadioptric, or compound, telescope, combining features of both reflectors and refractSchmidt- Cassegrainors in a compact body. It uses a combination of mirrors and lenses, most notably a deeply concave front ("meniscus") corrector lens. This lens corrects the optical aberrations of the spherical primary mirror to yield sharp images over a wide field of view. Light passes through the corrector to the primary mirror and is reflected to an aluminized spot on the back side of the corrector, then exits out through an opening in the primary mirror to the eyepiece The Maksutov design was introduced by the Russian optical scientist D.D. Maksutov in 1941. "Maks" have a desirable closed-tube design, are highly portable, and are nicely adaptable for photography.
              

The most popular Maks are 90mm-diameter spotting scope models. They're best suited for terrestrial viewing and photography, but can also be used for casual astronomy. Larger-aperture models are difficult to manufacture (and thus are very expensive), and their thick glass corrector lenses take a long time to reach thermal stability at night.
 
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