✅ Choosing a Telescope for Astronomy
Given the bewildering array of telescopes on the market, how does an enthusiastic but inexperienced consumer choose the right one? To answer this question we 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 its aperture. This term refers to the diameter of the telescope's main optical ele-ment, be it a lens or a 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 (i.e., 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 resolution 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 maximum 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; 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 is obvious: the need for portability. The largest amateur telescopes are very big indeed, and demand either housing in a permanent observatory or possession of a strong back, a truck, and a gang 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 or 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 succumbing to "aperture fever." Those infected with this psychological malady choose the largest telescope they can afford without regard to portability. Their mon-ster scopes soon gather dust in the garage, exiled for the crime of being too heavy and bulky, while the once enthusiastic would-be stargazers wind up frustrated or in traction.
The Sky is the Limit
The second limitation on very large telescopes is less obvious, but becomes 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 topography of the observing site. But on an average night, at 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-powered viewing.
Telescope Mounts
The last important topic to cover before delving into optical designs is that of mounts. Telescopes are offered on either altitude-azimuth (or altaz) mounts, which move up-down (altitude), left-right (azimuth), or equatorial mounts, which are tilted to align with the rotational (polar) axis of the Earth.
Altaz mounts are generally lighter and simpler to use, and are preferred if the telescope is to be used both for astronomy and daytime observing (or for daytime observing only). The better ones offer slow-motion controls to aid in moving the scope by small increments, and are useful for powers up to about 150x. The Dobsonian mount is a variation on the altaz mount. It employs unconventional (for telescopes) materials like plywood and Teflon in a compact mounting that moves easily, is extremely stable, and can adequately support large telescopes at a very low cost. Though there are no mechanical slow-motions or electric drives on a Dobsonian, a well-made example glides so smoothly on the Teflon bearings that with a little practice it is quite easy to track objects manually at 200x or more!
Equatorial mounts are designed specially for astronomy, and are not recommended for terrestrial viewing. Their advantage is that they allow easier tracking of the stars across the sky. This motion can be achieved with either a single manual slow-motion control or an electric motor drive (or clock drive). The easier viewing they provide at high power makes equatorials preferred by observers who are most interested in the Moon and planets. Also, you'll need an equatorial mount if you want to do astrophotography.
Different Scopes for Different Folks
Now that we understand these basic points of telescope performance and mounting, we can discuss the three basic optical designs of telescopes: the refractor, the reflector, and the compound (or catadioptric) telescope.
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