wavelengths of light into crisp, color accurate images. These developments have made the refracting telescope more affordable and more compact. Today, many observers are selecting refracting telescopes as their telescope of choice.
Next to the refractor, a long-focus Newtonian reflecting telescope (Fig. 7.2) produces the best image contrast. To many observers, a 6-in. f/8 Newtonian reflector offers as much image contrast as a 4-in. f/15 refractor. These reflectors use a parabolic shaped mirror, located at the lower end of the tube, to collect and bring light to a focus near the upper end of the optical tube. Because the glass for a mirror does not have to be as optically pure as a refracting lens, mirrors are easier and less expensive to make. Also, mirrors can be more easily ground to shorter focal lengths. As a result, the cost per inch of aperture is usually much lower for a reflecting telescope than for a refractor. And, because mirrors are much easier to grind accurately than refracting lenses, the mirror can be formed into a perfect parabolic shape so that every wavelength of light can be brought to the same focal point. This virtually eliminates chromatic aberration in a reflecting telescope. With shorter focal lengths, reflectors can be made shorter and lighter, the mounts do not have to be as large as refractors, and reflectors can be housed in smaller shelters. Consequently, even modestly large reflectors can be transportable. One drawback of reflecting telescopes is that they require more maintenance than refractors. The mirror for a reflecting telescope is mounted in a mirror cell that is itself mounted to the optical tube. To bring the focused image to an eyepiece, a secondary optical flat mirror set at a 45° angle is used at the upper end of the optical tube to divert the light path out the side of the tube to an eyepiece or other instrument. The two mirrors in this system must be perfectly collimated and aligned to produce a usable image. Periodically, these mirrors will require adjustment to keep the telescope in good working order. Also, the mirror
surfaces will need to be cleaned and recoated with reflecting material from time to time. This takes time and money. Because the contrast of Jupiter's features is so subtle, slight misalignment of the optics will smear out fine detail that would otherwise be seen. In spite of these detractors, large reflectors are much more common than large refractors. Today's large, modern professional telescopes are all reflecting telescopes. Newtonian reflecting telescopes are great telescopes for observing Jupiter.
A variation on the Newtonian reflecting telescope is the classic cassegrain telescope (Fig. 7.3). Cassegrain telescopes also use a parabolic concave primary mirror but use a relatively small convex secondary mirror. Instead of reflecting the light out the side, light is reflected back down the tube through a small opening in the center of the primary mirror, arriving at focus behind the primary mirror. Cassegrain telescopes often have very long effective focal lengths, contributing to high image contrast and allowing high magnification. Many modern professional telescopes use the classic cassegrain configuration.
Today, more amateurs are using Schmidt-cassegrain telescopes (SCTs) than any other design. SCTs use a circular rather than parabolic mirror as the primary mirror. This primary mirror is located at the bottom end of the optical tube, as in a classic cassegrain telescope. Because a circular mirror is prone to chromatic aberration, a very thin correcting lens is placed at the front of the optical tube. Mounted on the backside of the correcting lens is a small circular convex mirror. As with a classic cassegrain telescope, the light gathered by the primary mirror is reflected back down the tube again through an opening in the primary to come to a focus behind it. This combination of mirrors and lens in an SCT allows a relatively long focal system to be housed in a relatively short optical tube. The typical
8-in. //10 SCT (Fig. 7.4) with a focal length of 80-in. can be contained in an optical tube only 22-in. long! This explains the popularity of the SCT; it is compact, relatively light, and easily transported! Several of my professional astronomer friends use these portable instruments on field expeditions. With an effective //10 focal length, the SCT is a very good general-purpose telescope. However, the SCT is the least desirable telescope design for planetary observing. The compound lens design of the SCT requires a large secondary mirror. In an 8-in. //10 SCT, this secondary mirror is 2.75-in. in diameter, causing a 35% obstruction of the primary mirror. This large secondary mirror causes a significant reduction in image contrast compared to refractors and Newtonian reflectors. For this reason, the SCT is not as good for visual observing. However, the SCT seems to perform as well as other telescope designs when used with CCD cameras or webcams for planetary imaging. Another problem with SCTs is image shift. Unlike other telescope designs, most commercially manufactured SCTs achieve focus by moving the primary mirror forward and backward in the optical tube. The mirror actually slides on an inner tube and is adjusted by turning a focus knob with a jack-screw that engages the primary mirror assembly. There must be a certain amount of play in this mounting so that the mirror can move without binding. If the amount of play is excessive the observer will notice a sideways shift of the image as the focus is changed. This image shift, while often slightly present in other telescope designs due to tolerances in the eyepiece rack and pinion focusing mechanism, can be very annoying in an SCT that is poorly assembled. I have experienced such large image shift in some SCTs that the primary mirror, when viewing near the horizon, has shifted enough off center as to no longer be
adequately collimated! It would be very gratifying if commercial manufacturers would take pride in their product and truly eliminate this deficiency. In spite off its shortcomings, the SCT will remain popular. In its defense, I have made a high number of serious observations of Jupiter using an SCT. With care and patience, an observer using a Schmidt-cassegrain can produce fine results.
There are other types of telescopes, most of which are variations on the types already discussed. These other types include Maksutov, off-axis reflectors, Maksutov-Newtonians, Schmidt-Newtonians, binocular telescopes and others.
To summarize, the best telescope is one that produces a sharp image with high image contrast. This would favor refractors and long focal length (f/8 or longer) Newtonian reflectors. Size matters, and high resolution and high magnifications require large aperture. Therefore, a 4-in. refractor or 6-in. reflector is the minimum size for serious work. The optics of any telescope should be accurately aligned and collimated, since subtle features can be missed with a poorly working optical system. Finally, never put off an observing program for lack of the "perfect" telescope. Make the best observation you can with the instrument you have. In reality, the perfect telescope is one the observer uses well.
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