Choosing a Telescope
by Mark Miller
- Night or Day
There are two basic kinds of uses for telescopes: daytime terrestrial (bird watching, scenery, sports) and astronomical (planets, galaxies, stars, the Moon) viewing. All telescopes can be used in some capacity for both purposes, but some telescopes are best suited for only one use. Astronomical telescopes, for example, should be as large in aperture (diameter) as possible, to gather more of the faint light which travels such a long distance from its source. Terrestrial telescopes can be built much smaller, for portability and convenience, since they have lots of light to work with and don't need so much magnification. Most people settle on a compromise between these extremes, depending on just what they want from a telescope.
- Telescope Types
Telescopes have progressed through many changes over the years. Many modern designs look quite unlike the traditional image of a telescope. We recommend choosing telescopes not just by technical design, but by suitability for particular uses. Telescope optical designs are many and varied; some commercial designs are hybrids of several basic types, but most telescopes on the market today fit into one of the following groups:
The Refractor - The familiar long tube, with the lens in front and the eyepiece in back, has in modern times evolved into two distinctly different kinds of telescopes. The spotting scope refractor is one of the best terrestrial telescopes, with its compact size and simple operation. Often seen with a zoom (variable power) eyepiece, it has an erect, normally oriented image. Permanently aligned with a closed tube and a simple focus design, it can be used with any conventional camera-type tripod. The best spotting scopes are prismatic, with built-in image erecting prisms. High quality models are of sufficient optical quality to see craters on the moon and the rings of Saturn with little difficulty. The astronomical refractor is the same basic type commonly seen in department stores, with a long white tube and a metal or wood tripod. Nearly all are imported from the Orient, with Japanese models generally being of higher quality than Korean, Hong Kong, or Taiwanese models. All look generally the same in pictures; quality varies tremendously. Advertising claims for high magnification of 400X, 600X, etc., are very misleading. The practical limit is 60X per inch of aperture, or 144X for a typical 60 mm (2.4") scope. Higher powers are useless, and serve only to fool the unwary into thinking that magnification is somehow related to quality of performance. It is not. If equipped with the proper tripod and a precise mounting , a small refractor is very nice for observing the planets, the Moon, many of the larger star clusters, nebulas, and so forth. With an erecting prism or star diagonal, a small astronomical refractor can make a very nice terrestrial telescope. This is a good telescope for the technically-minded young astronomer.
The Newtonian Reflector - A very popular and economical telescope, the Newtonian reflector (invented by Sir Isaac Newton) is primarily an astronomical instrument. It is quite simple in design, resulting in very large apertures at the lowest cost per unit of aperture of any type of telescope. Although heavier and somewhat more difficult to transport than similar sized compound telescopes, the Newtonian is preferred by many astronomers who want large aperture at moderate cost. Six and eight inch Newtonians are easily carried in an automobile, as the tube detaches from the mounting in seconds. Newtonians are generally not suited to earthly observing, as the image is rotated and usually upside-down or sideways. Newtonian reflectors need occasional cleaning and realignment, but you can easily do this yourself. Optical performance is excellent. The large aperture makes them ideal for deep-space views of galaxies, star clusters, and nebulas. The simple optical design results in sharp, high-contrast planetary and lunar views, too.
Rich-field Reflectors - Large diameter, short-focal-length telescopes which cover an unusually large area of the sky at one time. They give very bright, sharp images, can be hand-held or used with simple bases or mounts, and are less prone to vibration than other telescopes. The wide field of view makes it easier to locate an object. The lack of high magnification is largely compensated by the sharper, brighter images and superior light-gathering power. Unlike larger Newtonian reflectors, many small rich-field telescopes often have permanently aligned and sealed optics, and are small enough to be quite portable.
Compound Telescopes - Compound telescopes have combined the best features of other telescopes into very compact, lightweight instruments. They use both mirrors and lenses, resulting in telescopes only about twice as long as they are wide. Unlike the basic refractor and reflector, these telescopes are distinctly modern 20th Century designs, the products of high-technology manufacturing techniques. The features are many - the closed tube, lightweight, rugged designs are easily portable, and the superb optical performance is better in nearly every respect than any other single telescope. Little if any maintenance or alignment is required. Compound telescopes can be used either astronomically or terrestrially. The lightweight optical assembly allows very strong mounts to be made very light in weight. Camera adapters and many varied accessories are widely available. The only significant disadvantage is just what might be expected - compound telescopes cost more than other telescopes. The
Maksutov-Cassegrain telescope is perhaps best known due to the introduction of the Questar telescope in the 1950's. The "Mak", introduced by D. D. Maksutov in 1944, uses a deeply-curved, thick front corrector lens, with a reflective spot on the corrector acting as the secondary mirror. The most popular Maksutov telescopes today are 90 mm dia. "Spotting Scope" models used for both astronomical and terrestrial observing. Large diameter models are very difficult to manufacture and take a long time to reach thermal stability at night.
The Schmidt-Cassegrain design was made commercially economical due to the optical production innovations of Tom Johnson at Celestron International in the late 1960's. His techniques for producing the complex-curved Schmidt corrector plate were the foundation for every major manufacturer in the business today.
- Telescope Mounts
The telescope mount usually accounts for about half the cost of a telescope system. The best optics in the world are useless without a good, sturdy mount. An astronomical telescope requires a much different mount than a small terrestrial scope. The two basic types of mounts are the altazimuth and the equatorial. The altazimuth mount has two motions, altitude (up and down) and azimuth (left and right). A camera tripod is the most common type of altazimuth mount. Many astronomical refractor telescopes come with altazimuth mounts and, usually, wooden tripods. The "alt-az" is the simplest type of mount to use, requires no set-up alignment, and is low in cost. It is ideal for terrestrial use, although a good altazimuth mount with slow motion controls will work very well for astronomy. The equatorial mount is designed specifically for astronomical use. As the Earth rotates once each day, the stars and planets appear to move across the sky. To follow a celestial object, the telescope must track a curved path at exactly the correct rate. An equatorial mount has one axis tilted so that it is parallel to the Earth's axis of rotation. By then simply rotating the telescope in one axis only, objects will appear to sit still when viewed through the scope. An electric motor drive can be built into an equatorial mount, but not into an altazimuth.
Fork Mounts are the most popular equatorial mount design today, being well suited to the short tubes found on compound telescopes. The companion equatorial wedge tilts the 'polar axis' of the telescope to line up with the celestial sphere for astronomical use. Usually the base of the fork mount can be attached to the tripod without the wedge for terrestrial altazimuth operation.
German-Equatorial Mounts are easily recognized by their counterweight extending opposite the optical tube. Equatorial refractors and Newtonian reflectors are almost always found on this kind of mount, and recently some Schmidt-Cassegrain designs have become popular. These mounts function very well, with the inconvenient and sometimes awkward counterweight being the major drawback to this design.
- Pointing and Tracking
Slow motion controls are a real convenience on any telescope. You'll be adjusting the aiming of your telescope quite often, to follow objects and to find new ones. On a simple telescope, you just push the tube to adjust the aim. Slow motion controls let you simply turn a knob to make precise adjustments, greatly smoothing the movement. Setting circles tell you the celestial coordinates at which the telescope is aimed. They make it much simpler to locate stars, galaxies, and other celestial objects, much as an earthly navigator might locate a position on the earth using the familiar latitude and longitude coordinates. Once your telescope is properly in alignment with the celestial sphere, you can use the setting circle dials on your telescope to locate the coordinates found in a star chart or star atlas for the object you want to see. This way, you can even find objects too faint to be seen in the finder scope. Electric motor drives (clock drives) work along with an equatorial mount to drive the telescope across the sky at just the right speed to cancel the apparent motion of the heavens. It's a real surprise to most people to see how quickly even a large object like the Moon will drift right out of view, unless a motor drive is in use. Motor drives come in many types, some plug into a regular power outlet, some are battery operated. A power converter can usually switch one type to another.
No other characteristic of telescopes is so widely known and yet so misunderstood by most people. When we hear a customer ask, "What's the power of this telescope?" we explain that the question is like asking a car salesman, "What's the speed of this automobile?". The car might, if pushed, do 90 or 120, but does this really matter? The magnification of most telescopes is variable, depending on the focal length of the eyepiece being used. The eyepiece used is the 'gas pedal', and if you want to see any of the scenery, you won't often 'floor it'. As power increases, image brightness and sharpness decreases rapidly. Half the power, four times the brightness. Twice the power, one-fourth the brightness. This is a basic law of optics. With lower powers, you see brighter, sharper images and cover a larger area. It's easier to find your target, vibration is less troublesome, and things usually look more detailed. Experienced telescope users know this, and usually use the lowest power available most of the time. Higher powers are quite useful for viewing small and distant objects like planets and double stars. Most observers have a selection of eyepieces to match the objects they are viewing. The absolute magnification limit for any telescope under ideal conditions is 50 to 60 power per inch of aperture (about 2 power per mm). Powers over 100X are rarely used during daytime hours, because of problems with air turbulence.
If any one thing affects the overall performance of a telescope the most, it is aperture: the diameter of the main mirror or lens of the telescope. You know how professional observatories are always after bigger telescopes. That's because a telescope with a larger diameter can gather much more light, give a brighter image, and be useful at a higher magnification. For ordinary daytime terrestrial viewing, where lots of light is available, you really don't need a lot of aperture. 60 mm to 90 mm telescopes are capable of providing plenty of detail at the powers usually used. In general, however, you should buy the largest aperture scope which fits your portability and budget requirements.
Ordinary telescopes range in weight from only a few pounds to several hundred pounds, and overall bulk varies tremendously. Some types are quite rugged, while others can require realignment after a little rough handling. It's very important for you to consider portability carefully; your telescope is no good unless you use it, and you may not want to hassle with a bulky, awkward telescope. On the other hand, there is no reason to pay for portability if you don't need it. Compare carefully, considering your individual needs.
Most telescopes can be used for high-powered terrestrial or astronomical photography, as long as you have a removable lens type 35 mm SLR camera. The compound telescope is the most useful for general use, as it's ideal as a telephoto lens. Camera adapters are also available for any telescope with standard 1.25" (31.7 mm) or .965" (24.5 mm) dia. eyepieces. Astrophotography is an interesting and rewarding hobby. It can be either quite simple (photographing the Moon, for example) or very complex (long exposure shots of nebulas, galaxies, etc.). Focal Ratio (f/number) The mysterious "f" number (such as f/6, f/11, etc.) is really quite simple. It's the same number used by photographers, and refers simply to the focal length divided by the aperture of the telescope. For example, a telescope with a focal length of 600 mm and a 60 mm aperture is f/10. Smaller f/numbers result in wider fields of view and lower magnification, best suited for nebulas, galaxies, and many other deep-space objects. Medium focal ratios (f/8 to f/11) can be used for low or high power applications by switching eyepieces. High focal ratios in the f/16 neighborhood are excellent for planetary observations and splitting double stars, but suffer from lower image brightness and limited wide-field range. They also require very long exposure times for photography.
- Mirror Images
Don't be surprised to find an upside-down or backwards view in many telescopes. Refractors and compound telescopes usually give a left-to-right reversed image when using a right-angle "star diagonal" prism. Newtonians usually are upside down or sideways, depending on the viewing position. Finder scopes will usually have an inverted or reversed image as well. Optional image erecting prisms are available for most telescopes. They fit between the eyepiece and eyepiece holder, correcting the view and allowing straight-through viewing. The spotting scope refractor will always give you a normally oriented image, without need of an erecting prism.
- Finder Scopes
Many telescopes have a miniature telescope attached to the side. A finder scope is aligned perfectly with the main instrument, and has a very low power and wide field of view, so you can easily find an object and center it where it can be seen in the main eyepiece.
- Eyepiece & Accessory Sizes
Small terrestrial telescopes usually have either a built-in zoom eyepiece, a fixed-power design, or a special screw-in interchangeable type eyepiece. Standardized eyepieces usually cannot be used on these scopes. Telescopes using standard 1.25" (31.7 mm) or .965" (24.5 mm) dia. slide-in eyepieces allow a lot more flexibility. The larger size is the most popular, although some scopes use the smaller size for reasons of compactness. Adapters are available which let you intermix these sizes. Some very low power, wide-field eyepieces are supplied in extra large 2.0" (50.8 mm) barrels, which have breathtaking 'porthole in space' performance-if your telescope has a suitably-sized diagonal or eyepiece holder which fits them.
It's not very difficult to make a poor quality telescope. Thousands of practically useless telescopes with poorly designed, toy-like tripods and cheap, almost unusable eyepieces are sold in department stores every year. Be sure you are buying a telescope with quality construction and design.
Above all, choose a telescope that seems to fit your lifestyle and personality. Do not worry too much about specifications and other technical details. If you choose a reputable brand, and use common sense in picking a telescope that appeals to you, you won't go wrong.