CHOOSING A TELESCOPE
Given the bewildering array of telescopes on the market, how does an enthusiastic but inexperienced consumer choose the right one ? To answer this question, I will explain the differences between specific telescope types, but for that discussion to be meaningful, it is important first to understand some very basic points about astronomical telescopes in general.
Aperture is the Most Important Factor
The single most important specification for any astronomical telescope is is aperture. This term refers to the diameter of the telescope's main optical element, be it a lens or mirror. A telescope's aperture relates directly to the two vital aspects of the scope's performance: its light-gathering power (which determines how bright objects viewed in the scope will appear), and its maximum resolving power (how much fine detail it can reveal). There are other criteria to be considered in selecting a telescope, but if you learn only one thing from this article, let it be this....the larger a telescope's aperture (the fatter it is), the more you will see.
Don't Get Hung-Up on Power
Unfortunately, the first question most beginners ask is not "What is this telescope's aperture"?? but "What is its magnifying power"?? The truth is, any telescope can be made to provide almost any magnification, depending on what eyepiece is used. The factor that limits the highest power that can be used effectively, on a given scope is, you may have guessed...its aperture. As magnification is increased, and the image in the scope grows larger, the light gathered by the telescope is spread over a larger area, so the image is dimmed. There is also an absolute limit, determined by the physical properties of light, to the resolutoin that is possible with any given aperture. As the magnification is pushed beyond that limit, the image fails to reveal any additional detail and gradually breaks down into a dim, fuzzy blob.
The maxium useful magnification for any telescope is about 50 times the aperture in inches, or two times the aperture in millimeters. This equates to about 100X to 120X with the smallest telescopes, which is enough to see such wonders as the rings of Saturn and cloud bands on Jupiter. The 2X per millimeter figure is a rule of thumb, and can vary up or down somewhat depending on the optical quality of the scope in question and the vision of the individual observer. Experienced observers usually use much less power (i.e.) 0.5X to 1X per millimeter is more appropriate for most objects. Any manufacturer claiming that their 60mm scope can provide good views at 450X (7.5 times the aperture in millimeters) is trying either to pull your leg or pick your pocket !!!!
Bigger is Better, But.......
While aperture is the most important specification of any telescope, there are exceptions to the rule, that, "bigger is better". One rule is obvious....the need for portability. The largest amateur telescopes are very big, and demand either housing in a permanent observatory or possession of a strong back, a truck, and bunch of muscular and motivated observing buddies !!! There is a line to be drawn between performance and portability, and where it will be drawn varies with the individual and his/her capacity for storage and portage. Beginners are encouraged to start out with a scope of sufficient aperture to feed their interest, but of a size that they can manage easily. Avoid the overpowering appeal to "aperature fever". Those of you who decide to choose the largest telescope they can afford without regard to portability of their "monster" scopes...soon gather dust in the garage, due to being too heavy and bulky to use.
The Sky is the Limit
The second limitation on very large scopes is less obvious, but comes apparent after the first couple of viewing sessions. The Earth's atmosphere limits how much we can see. Stars and planets viewed through a telescope appear to shimmer or wiggle, as their light passes through the air and is distorted. This effect is known to astronomers as "seeing", and becomes more noticeable and bothersome as telescope aperture increases. It especially affects observations of the Moon and planets, where high power applied to reveal fine details, also magnifies the air turbulence.
The amount of distortion due to seeing varies, depending upon the behavior of air currents in the upper atmosphere, and to a lesser extent upon the altitude and area of the observing site. But on an average site, air turbulence will limit useful magnification to 250x or 300x, and prevent telescopes larger than about 8" or 10" aperture from achieving their full potential for high-power viewing. Telescopes larger than 10" are most often chosen by observers who want to gather as much light as possible for viewing dim galaxies, nebulas, and star clusters. These "deep sky" objects are most often viewed at much lower power than the planets, so seeing is less of a problem.
Different Scopes for Different Folks
Now that we understand the basic points of telescope performance, we can discuss the three basic optical designs of telescopes: the refractor, the reflector, and the compound (or catadioptric) telescope.
A refractor is what most non-astronomers think of when they hear the word "telescope". Its tube is most often long and skinny, mounted on a tripod, with a lens at one end and the eyepiece at the other. Refractors were the first type of telescope invented, and the finest refractors still provide the best images of any design for a given aperture. They are often chosen by observers with a dominant interest in the planets and Moon, because they can provide sharp, high contrast views at high magnification and are less bothered by atmospheric "seeing" than the other designs. They also require less maintenance than reflectors or compound scopes, and are therefore popular with beginners. The refractor's good performance at high power and relative insensitivity to light pollution makes it a good choice for a city based observer, as the design performs best on the objects that are most easily seen from urban or suburban locations.
These advantages do not come without a price....literally: refractors are the most expensive telescopes per inch of aperture. Big refractors can cost several thousand dollars, and still are considered too small in aperature for serious deep-sky observing. The long focal length of most refractors restricts the field of view, making it difficult to take in large extended objects like some clusters of stars. And the long tube, with the eyepiece located at the back end, requires a tall tripod, which , if poorly made, can allow the scope to shake and shimmy in the breeze, rendering high power observing difficult.
The reflector uses a mirror, rather than a lens, to gather and focus light. By far the most common design is the Newtonian reflector, which places a concave (dish-shaped) primary mirror at the bottom end of the telescope tube. A small secondary mirror at the other end, directs the focused light out the side of the tube and into the eyepiece. Newtonians offer the largest aperture available at a given price, and when well made, they can provide sharp, contrasty views that rival all but the finest refractors. A Newtonian's low center of gravity and eyepiece location at the top of the tube, allow for comfortable viewing with a more compact mounting, which can be made stable with much less bulk and cost than the tall mounting required by a refractor of similar aperture.
Big reflectors of 10" aperture and larger on Dobsonian mountings are the most popular telescopes for astronomers who seek to gather "buckets of light" for deep sky observing. These giant scopes perform best at remote dark sky site, away from the glare of city lights. The value and versatility of the smaller 4.5" to 8" Newtonians, mounted either equatorially or as Dobsonians, makes them a fine choice for the beginner with general interests.
Newtonian reflectors require occasional maintenance. Unlike the lenses in a refractor, the mirrors in a reflector need periodic alignment, or collimation, for best performance. While many beginners seem intimadated by collimation, it's really not difficult, and takes only a few minutes once you get the hang of it.
A reflector's tube is also more open to air and humidity than that of a refractor, and if left uncovered, the mirrors can accumulate dust and grime, which necessitates occasional cleaning. While these maintenance concerns are often overstated, a Newtonian may not be the right choice for someone who finds the prospect of occasional tinkering with the scope unappealing.
The most modern of the three common designs for amateur telescopes is the compound, or catadioptric type, which uses a combination of lenses and mirrors to gather and focus light. The greatest advantage of this design is its compactness: the lenses and mirrors "fold up" the light path inside the telescope, reducing large-aperture scopes to a manageable size. If an equatorial mounting is desired, the smaller tube can be carried on lighter and more economical mounts than that required by a Newtonian of the same size. Compound telescopes are most popular with observers who desire both generous aperture and an equatorial mounting in a transportable package.
The names Schmidt-Cassegrain and Maksutov-Cassegrain refer to specific designs of compound telescopes, which use differently shaped lenses and mirrors to achieve a similar result. The Maksutov is often cited as offering better image quality, though there is little in the way of optical theory to support this opinion. Most probably the Maksutov has developed its reputation as the superior catadioptric design because its spherical optical surfaces are easier to make to very high precision than the more complex shapes demanded by the Schmidt. As a result, if a telescope maker practices anything less than the strictest quality control, their "average" Maksutov will outperform their "average" Schmidt. In top quality telescopes from careful manufactures, both designs can yeild excellent images.
There are a few drawbacks to all compound designs. As in any telescope that employs mirrors, occasional alignment is required for peak performance. The cost of a compound is higher than that of a Newtonian of the same aperture, though still lower than the cost of a comparably sized refractor. Most significantly for the planetary observer, the secondary mirror in a compound is much larger than that in a Newtonian, and its presence in the light path of the scope reduces contrast somewhat for high powered viewing. In general, astronomers who desire a highly capable, easily transportable telescope, find these worthwhile compromises, and have made the compound scopes very popular.