Refractors vs Reflectors

Now that you have made the decision to step up to a medium to large aperture telescope from your department store model, you need to consider the various types of optical designs and decide what best suits your needs. The telescope you own now is most likely a refractor. This design, also called a dioptric telescope, is based directly upon the original opera glass telescope designed by Galileo in 1610. That original telescope used a simple convex lens to gather and focus light and a concave lens at the opposite end of the tube to bring that light to a crude focus. The lens at the front of the scope is called the objective. In any telescope the objective is the lens or mirror element that gathers starlight and directs it to a focus point. With this simple design, Galileo discovered that Venus exhibited phases like the Moon and that Jupiter had satellites circling it. These discoveries led Galileo to realize that the geocentric model of the solar system was incorrect. Earth was not at the center of the solar system, but the Sun was! Galileo would pay dearly for his blasphemy. The church would torment him, ruin him, excommunicate him and finally forced him to recant. He was of course correct. The Sun was at the center of the solar system and all the planets circled it. It would be nearly 400 years before the church would come to admit it early in the reign of Pope John Paul II when Galileo was formally brought back into the Roman Catholic Church.

Science has refined the design of the refractor over the years allowing it to create sharper images with better focus. For an amateur shopping for a telescope, the chief advantage of a refractor is that its lenses are rigidly held in place in the telescope tube making the telescope virtually maintenance free. The lenses never need to be adjusted and are in fact often cemented in place. The refractor lenses generally are of long focal length, often producing f ratios7 of f/12 to f/14. These long-focus telescopes provide sharp images with high magnification without using excessively short focal-length eyepieces. The refractor does have some drawbacks that prevent it from being used as a design for large-aperture telescopes. By the time the scope reaches about 4 inches (100 mm) in diameter, the objective lens starts to become too heavy and the tube too long for use in a design that can still be considered portable. Lenses can be designed that provide a shorter focal length,but that results

7 Focal length is the distance from the objective lens to the point at which light rays come to focus. F-ratio is the focal length divided by the diameter of the objective.

in some loss of image sharpness and they don't get any lighter. Refractors also are subject to an error that is an inherent byproduct of the design. As light passes through the glass lenses, the colors are separated in the same way that a prism might and not transmitted evenly by the lens elements. This causes an error known as chromatic aberration. Objects viewed through a refractor will often have a fringe of false color surrounding them caused by the optical separation of the differing wavelengths of light through the telescope objective lens. To try to minimize the effects of this color distortion, the telescopes lenses may be coated with metallic-based compounds that also improve overall light transmission. For those who desire perfect color in their images, the closest you can get is a telescope with a calcium fluorite lens element. Calcium fluorite is not glass, but a mineral that must be ground and polished in a very time-consuming and expensive procedure. Fluorite lenses however are not subject to the chromatic aberrations that plague glass lens refractors. Refractors do have other limitations as well. The long focal lengths produce very narrow fields of view that cannot contain entire deep sky objects. Long focal lengths are also tough for astrophotography work. The longer the focal ratio of the telescope becomes, the longer exposure times are required for imaging a given object. Refractors can be very frustrating telescopes for taking pictures.

The limitations and chromatic aberrations of the refractor began to lead early astronomers to look for other solutions to the problems of building large-aperture telescopes. By 1681, the noted early physicist Sir Issac Newton invented a telescope using a large mirror as a light-collecting surface rather than a lens. This type of telescope is called a "reflector" or catropic telescope. Reflectors use a large concave spherical mirror (primary mirror or objective mirror) to gather light and bring it to focus on a small, optically flat mirror near the front of the telescope tube. This secondary mirror turns the light 90 degrees to an image-forming eyepiece at the side of the tube. This original type of reflector is called a Newtonian reflector in honor of its inventor. A later type of reflector, called a Cassegrain telescope, focuses light through a hole in the center of the primary mirror to an eyepiece in the back of the telescope. Because light does not actually pass through any of the telescopes surfaces, they do not actually need to be made out of pure glass. Telescope mirrors

Figure 2.2. A short-focus Newtonian reflector. Image by author.

Figure 2.2. A short-focus Newtonian reflector. Image by author.

can be highly polished glass, Pyrex or even in some very early instruments, aluminum. Glass or Pyrex is generally used today because it is easiest to polish and figure to exactly the precise shape needed, then once the mirror is correctly figured, an aluminum overcoat is applied and polished until ready for use. The glass or Pyrex only needs to be thick enough to form and maintain the shape of the mirror. Reflector telescope mirrors are therefore much lighter per inch of aperture than are refractors. This removed many of the limitations that existed on telescope size. The world's largest refractor measures only 40 inches in diameter but reflectors now exist that are ten times that size.

Like refractors, reflectors also have an inherent flaw. Because a spherical mirror is used to reflect light to a flat one, some points of the secondary mirror are farther from the primary mirror's point of focus than others. Usually the objective is designed to focus light to the center of the secondary mirror. Portions of the image that fall on the outer parts of the secondary mirror tend to be distorted slightly. This error inherent in the reflector design is called spherical aberration. Stars at the center of the field focus to sharp points but stars near the edge may appear slightly streaked from the center of the field towards the edge. As telescope sales began to grow among the amateur public,two different types of fixes became available. One is to use a primary mirror with a different type of shape called a parabolic mirror. This mirror has a much deeper curve than a spherical mirror. A spherical mirror if continued in shape would eventually form a perfect sphere. A parabolic mirror has a much sharper curve to it and would close into a much more oblate form. This shape causes light to reach the secondary mirror in a more uniform manner creating sharper images. It can also be a very expensive mirror to produce because its curve is so complex. The second type of correcting mechanism that has evolved over the years is called a corrector plate. The corrector plate is a glass lens introduced at the opening of the telescope. The corrector introduces an error into the light path that is exactly opposite of that introduced by the primary mirror. Thus in spite of what your mother told you, in this case two wrongs do make a right. There are two types of correctors commonly in use. Smaller telescopes may use a Maksutov corrector. This is a thick lens element with a heavy concave shape. Maksutov correctors are highly efficient and provide super-sharp images, but like refractor lenses, they become impractical to use once larger than about four inches in diameter. Schmidt correctors are not quite as effective as Mak-sutovs but are thin and very light. They are also relatively inexpensive to produce. Schmidt correctors are preferred for use in any reflector telescope design larger than four inches. Reflectors do have some other disadvantages. The classic Newtonian or Cassegrain design has an optical tube that is open to the elements and therefore must be carefully cared for. Mirror surfaces must be kept meticulously clean on a regular basis. The secondary mirror is in the path of light to the primary mirror and is held in place by a spider support that also blocks some light from reaching the primary mirror. This so-called secondary obstruction created by the mirror and its support can block as much as 15% of the light entering the telescope. The mirrors must be carefully kept in line with each other through a process called collimation. This is particularly true of the secondary mirror, which must be in perfect alignment to properly redirect light to the eyepiece. An improperly collimated reflector can rapidly become a source of great frustration. Telescopes that employ Schmidt or Maksutov correctors have the advantage of being sealed at the front end, protecting the telescope mirrors. These telescopes are also less prone to (but not immune from) collimation problems. The secondary mirror housing of a Schmidt corrector telescope is fixed in the center of the corrector plate and is easily adjustable with an Allen-head wrench. The Maksutov is even simpler. The secondary "mirror" is actually an aluminized spot in the center of the corrector and never needs to (and cannot) be adjusted.

A telescope is only as good as the mounting it sits on. Mounts come in two basic types. The simpler type is called the alt-azimuth mounting. The mount rotates up-down and left-right allowing the telescope to be adjusted in both altitude and azimuth. To follow an object across the sky, one must follow the object in both axes. My 4.5-inch Bushnell sits in a very simple type of alt-azimuth mount,which is simply a bowl in which the bulbous base of the telescope sits and rotates. At the other end of the complexity scale, most GO TO scopes are alt-azimuth mounted with a computer issuing corrections as they track across the sky. Equatorial mounted scopes also move left-right and up-down, but the left-right axis can be pointed directly at the north pole, allowing the telescope to track an object with only a single motion. A drive motor that turns the scope at the same rate as Earth rotates in the opposite direction will enable that scope to keep an object centered in the field so long as the scope is both properly aligned and perfectly level. The most important thing though about a mounting is that is must always be perfectly rigid and not move. If the mount vibrates, no matter how well the scope performs at high power, all you will see is a highly magnified vibration if the mount is not stable.

Reflectors and refractors have other characteristics that cannot fairly be described as either strengths or weaknesses, but will play an important role in determining what type of telescope you will eventually settle on. Refractors tend to have fairly high f-ratios compared with reflectors. A typical refractor will have an f-ratio of anywhere from f/12 to as high as f/15. These telescopes will produce crisp images and high magnification with long focal-length eyepieces. But because the f-ratios are so large, they will be difficult to use for long-exposure astropho-tography. Exposures for deep sky objects will be impractically long. Reflectors, particularly Newtonian types, have very short f-ratios that mean that high powers are not easily usable. Newtonian reflectors will have f-ratios around f/6. They will produce very wide field images that are bright. Reflectors are also very good for long-exposure astrophotography, often requiring less than half the time for an exposure that a refractor of the same size. Refractors will always have the eyepiece at the rear end of the optical tube. With the use of a right-angle prism, the refractor user will always be able to find a comfortable observing position. Newtonian reflectors have the eyepiece near the other end of the telescope on the side of the tube. This can cause an observer to torque his body into many unusual positions while trying to see through his telescope. In the largest Dobsonian designs, a ladder may become necessary when viewing objects near the zenith.

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