Telescopes |
(Think about these carefully before you consult the answers below)
QUESTIONS
1) Spectacles (glasses) were invented in the late 1200's, more than 300 years before the telescope, but are considered by many as the direct ancestor of the telescope itself. If a telescope is really just the combination of lenses, why do you think it took so long for the telescope itself to be invented? <answer>
2) Theoretically, with the right eyepiece even a small telescope can be set up to give 1000 or even 1,000,000 power. Why might this not be practical? <answer>
3) Imagine that you want to buy a telescope for multiple purposes, not just astronomy viewing. Astronomical telescopes display things upside down, unlike the typical sporting telescopes. What could you do? <answer>
4) Can any useful astronomical observations be made with a small telescope? <answer>
5) Tycho Brahe's detailed observations of the heavens, from which Kepler derived his laws of planetary motion, were made without any telescope at all. How could such important work have been done with the telescope. <answer>
ANSWERS
1) According to Louis Bell in his definitive work, The Telescope , at the time spectacles were invented, the dominant eye problem they could be used to correct was presbyopia , the farsightedness that often accompanies aging. The weak convex lenses used to treat this condition did not produce a telescopic effect. ( Convex lenses bulge outward.) Only after myopia , or nearsightedness become more common (or at least more recognized) did concave lenses become common. ( Concave lenses curve inwardly.) This was not until perhaps the late 1500's, and only then was the combination of concave and convex lenses that formed the first telescope even possible.
2) It is true that the right combination of lenses can yield very large magnifications, but there are practical limits. One limit is simply the quality of the lenses. Increased magnification also increases the effect of any defects in the material. Any deviation from a perfect curvature of the lens or mirror, or of the quality of the material that makes them also will be magnified. Resolving power in seconds of arc, and thus the telescope's ability to reveal detail, is limited by the size of the objective to about 13/D where 13 is the size of the objective in centimeters. Thus a 13 cm objective should, in theory, be able to resolve detail as small as one second of arc (about 1800th the apparent diameter of the Moon). The telescope could magnify past this point with the right eyepiece lens, but the image would simply get bigger and more blurred. Perhaps even more important is the simple fact that the size of the objective lens or mirror limits the amount of light it can gather. Each increase in magnitude causes the observed object to appear larger but dimmer because the same about of light is being spread over a larger area. Past a point the image will simply be too dim to see or record. For practical considerations, magnification should be limited to no more than about 20 times the diameter of the objective in centimeters. Thus the maximum usable magnification for a telescope with a 15 cm objective would be about 300X. To make things even more complicated, the effects of our own turbulent atmosphere limit the effective magnification even more, such that perhaps only 75 percent of the maximum usable magnification is practical. So a 15 cm telescope would be limited on most occasions to maybe 225 power, again depending on the choice of eyepiece. (The "75 percent" noted above is a personal guess, not supported by data.)
3) You have a couple of options. First you can get a ordinary spotting or sports telescope and use it to view the heavens. This isn't entirely satisfactory because the mountings, if any, are usually not good for the purpose and the images may not be as bright as you would like for night-time objects. Secondly you could get an astronomical telescope with a special erecting image for terrestrial viewing. Your choice really should be based on what you will use it for most. In many cases, a good pair of binoculars is the best bet!
4) Absolutely they can! In addition to being superb teaching and learning tools, small telescopes have been used in various scientific studies. The large observatory telescopes are used to study very small portions of the sky, and consequently if they are not pointed in the right direction, may miss a new asteroid, comet or even an exploding star. Amateur astronomers using small telescopes and scanning wider areas of the heavens have made many important discoveries along these lines.
5) Tycho did have certain instruments for measuring positions in the sky. In fact the ancients were careful observers of the positions of stars and planets, from the time of Stonehenge and the Pyramids through the Greeks, the Arabs, the Mesoamerican Indians others up to the invention of the telescope. They developed various instruments such as the quadrant, the cross-staff and sextant for measuring precise angles and positions. The really remarkable thing is the accuracy of the trained human eye and brain combination. It is not necessarily true that astronomers in earlier times made excellent non-telescopic observations because their eyes were better than ours today. But they exercised and trained their eyes to see greater detail. Now, most astronomers today don't actually spend much -- or for some even any -- time looking through telescopes. Amateur astronomers, however, can attest to the fact that the ability of the human eye to discern detail increases (up to a point) with experience. For instance, upon first observation through a telescope, Jupiter usually appears as an oval blob with a few darker lines across it. But with each new observation, smaller details become apparent, and eventually fairly small detail in the cloud structure can be see. Like your muscles (and your brain), the eyes become stronger with exercise. Tycho practised, exercised and trained to become an Olympic-class observer.
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