North American Skies for September 1997

Telescopes 101, Part I

by Cathie Havens, S&S Optika
and reprinted from The Woman Astronomer

Click here for Part II

Telescopes are available in three different optical designs: refractor, reflector, and catadioptric. All three will perform well for planetary, lunar and at least some deep sky work. Each type has its strong points, and of course, some weak areas.

Refractor - This type of telescope was first used by Galileo in the 17th century and is what most people think of when they hear the word "telescope." Using lenses, the light is refracted or bent so as to be focused to a point behind the "objective," or main lens. This point is then viewed through an eyepiece at the rear of the telescope tube. The objective lens should be at least an "achromatic lens" or a doublet lens system. A single lens breaks up the light into a spectrum (much the way a prism does) resulting in false color and loss of detail.

Refractors are known for their contrast and sharp images. This makes them ideal for planetary and lunar observation. Double star splitting and observation of clusters of stars will be easy through a refractor. Because of their excellent contrast, even the brighter deep sky objects, like nebulae and galaxies, will be very visible through a good refractor.

I would recommend a refractor that is at least 80mm in aperture. A good 80mm refractor with equatorial mounting can be purchased for about $400.

Advantages: Sharp, clear images with excellent contrast. A refractor will also accommodate the use of an erecting diagonal, so if you would also like to use your telescope for looking at the neighboring area, it can be done right-side up and left to right corrected. Refractors will usually be able to focus with a camera. Disadvantages: Smaller diameter (usually 2 to 4 inches) means the scope will not gather as much light. The refractor is also somewhat more expensive, inch per inch, than other designs, especially with larger apertures.

Reflector - The most common reflectors were invented by Sir Isaac Newton in the 17th Century. It is a simple system consisting of two mirrors. The "primary" or "objective" mirror is curved, to bring the light to an exact point of focus, and the secondary mirror is flat. The secondary mirror is used to divert the light to the side of the tube where it is viewed through an eyepiece.

Reflectors usually have an f/ratio between f/5 and f/8. These short f/ratios will yield a wider field of view and brighter image than in the long f/ratio refractors.

Reflectors are generally the preferred telescope if "deep sky" observation is high on your list of objects to view, but will also give very pleasing views of the planets and the moon. A good 4.5-inch Newtonian reflector on an equatorial mount can be purchased for just under $400, a 6-inch on a simple dobsonian mount for around $500.

Advantages: Larger aperture for more light gathering and generally the least costly. Disadvantages: Images are always upside down, which for astronomy is not an issue, but this makes it difficult to use for land observation as correcting the image is impractical. Also, most of the beginning Newtonian reflectors will not focus with a camera.

Catadioptric -This type of telescope is a combination of both mirrors and lenses, with the image viewed from the rear. The most common of this design is the Schmidt Cassegrain. This telescope is very versatile and portable because of its "folded" optical path.

This design will perform well for many types of observation. it does not have as much contrast as a good refractor, but will still yield very nice planetary images. At f/10, it will not be quite as bright, or have quiet as wide a field of view as most Newtonian reflectors, but still gives excellent views of deep sky objects.

Advantages: Portability--an 8-inch objective with an 80-inch focal length will have a tube length of less than 24 inches. Versatility--this design works well for all kinds of viewing. This optical system is also the most easily adapted to photography and will accommodate the erecting diagonal for right side up images. Disadvantages: Usually more costly than the Newtonian telescope. Good 8-inch Schmidt Cassegrains, with clock drives, start at around $1,250.

The Denver Astronomical Society has monthly "star parties" at Chamberlin Observatory, and other organizations do the same. Attending such an event is an excellent way to learn about astronomy and telescopes.

And don't forget binocular observing. Under dark sky conditions, my favorite viewing is still done with binoculars. I really enjoy the wide field of view and overall views that binoculars give.


For more information, contact:

Cathie Havens, S & S Optika
5174 South Broadway
Englewood, CO 80110
(303) 789-1089


The Woman Astronomer
P.O. Box 27731
Denver, Colorado 80227-0731



copyright 1997 by S&S Optika.
(Redistribution, with proper credit including URL, is permitted and encouraged for newspapers, broadcast media, and nonprofit purposes.)

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