Astronomy Telescopes
| All
astronomical telescopes, large or small, share a common goal — to brighten
and magnify your views of celestial bodies. Refractors,
reflectors, and compound (catadioptric) telescopes do their
jobs in different ways — each with its own benefits and drawbacks.
Yet many fundamentals apply to any telescope. Of primary importance is a telescope's aperture: the diameter of its light-gathering lens or mirror. (That lens or mirror is often referred to as a telescope's objective.) Aperture makes a big difference in the level of detail you can see. A telescope that can only be pushed as high as 50x (50 times magnification) will reveal Jupiter's moons, Saturn's rings, and some degree of detail in the brightest star clusters, nebulae, and galaxies. But to discern Martian surface features or to see both members of a tight double star, you really would like to be able to use at least 150x. Depending on optical quality and observing conditions, you can expect to get anywhere from 20x (mediocre) to 50x (excellent) per inch of telescope aperture. Aperture also enables you to see fainter objects. For example, several dozen galaxies beyond our own Milky Way can be discerned through my 4½-inch (105-mm) reflector. Some are more than 50 million light-years away. Not bad for a telescope I can tuck under my arm and carry on a plane! But with my 12½-inch Dobsonian, hundreds of galaxies are within reach. If a telescope's aperture is arguably its most important "spec," its focal length comes in as a close second. Say you have two telescopes with the same aperture but different focal lengths. The one with the longer focus (and hence, a higher f/ratio) will generally lend itself better to high-magnification viewing. (The f/ratio is the focal length divided by the telescope aperture in the same units.) One reason: you can stick with longer-focus eyepieces, which are easier to use, especially for eyeglass wearers. Another reason: "fast" objectives (those with small f/ratios) tend to make fuzzier images, unless you've paid a premium for high-quality optics. "So it seems clear: I should go after the largest, longest telescope I can afford." Maybe; maybe not! A long focal length is preferable if your primary targets are the Moon, the planets, or double stars. And a large objective is a necessity if you dream of viewing numerous distant galaxies. But if you want to take in large swaths of the Milky Way or sparkling showpieces like the Pleiades, a short, small scope is called for. "Why's that?" Because a long focal length only lets you see a small patch of sky at one time. With standard eyepieces (those that have 1¼-inch-wide barrels), a focal length of 20 inches (500 mm) can provide a 3° field of view — enough to take in all of Orion's Sword. A focal length of 80 inches (2000 mm), by contrast, barely lets you encircle M42, the famous Orion Nebula in the Sword's center. "What if I want to do a bit of everything?" Don't worry. There are plenty of acceptable compromises. Many astronomers take the 6-inch (152-mm) reflector to be an ideal "do-it-all" instrument. But keep in mind that even with that aperture, you still face a tradeoff between a wide field of view (f/5 or thereabouts) and high-power performance (optimal at f/8 and up). The long-focus unit will also be heavier and require a beefier mount. Now that you're up to speed on some of the most important concepts and terms, take the time to peruse the ads and product reviews in recent issues of Sky & Telescope or SkyWatch magazine. Then go ahead and call or write to anyone who manufactures instruments you might be interested in. Their brochures and catalogs should tell you much of what you want to know; if not, call the manufacturers or their dealers and ask away. However, nothing substitutes for firsthand experience. By far the best way to acquaint yourself with the wide world of telescopes is to participate in an astronomy club's nighttime observing session, or "star party." There, you can try out and ask about a wide variety of telescopes. (Find an astronomy club or star party using our Resources section.) You may also be able to buy a used telescope from someone in an astronomy club. Used telescopes carry some risks, including undisclosed damage by the previous owner and a lack of warranty coverage. However, they can also be spectacular bargains. You can also find used telescopes on the Internet. (Be sure to take reasonable precautions if buying from a private party online.) Of course, many buyers will find that a new instrument best suits them. This should be bought from a source specializing in astronomical telescopes. Many camera stores are excellent sources of astronomical products as well. If you're set on buying a new instrument, be prepared to spend at least $200. If this is beyond your means, your astronomical aspirations will probably be best served by buying a decent pair of binoculars and a sturdy lawn chair. At the same time, realize that many excellent beginners' telescopes are available for well under $1,000. Remember that whatever investment you make may serve you well for several decades. |
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| Astronomers at Stanford and NASA have discovered two of the smallest moons yet found around Uranus, the International Astronomical Union (IAU) has announced. The new moons, uncovered by NASA's Hubble Space Telescope, are about 8 to 10 miles across (12 to 16 km) -- about the size of San Francisco. The two moons are so faint they eluded detection by the Voyager 2 spacecraft, which discovered 10 small satellites when it flew by the gas giant planet in 1986. The newly detected moons are orbiting even closer to the planet than are the five major Uranian satellites, which are several hundred miles wide. The Hubble telescope observations also helped astronomers confirm the discovery of another tiny moon that had originally been spotted in Voyager pictures. The discoveries will bring Uranus’ total to 24 moons. S/1986 U 10, a rediscovered satellite, may have broken off from another Uranium moon during a collision with a comet; S/2003 U 1 is a newly detected moon. Photo: Courtesy of NASA and Mark Showalter "It's a testament to how much our [ground-based and Earth-orbiting] instruments have improved in 20-plus years that we can now see such faint objects 1.7 billion miles [2.8 billion km] away," says Mark Showalter, a senior research associate at Stanford who works at the NASA Ames Research Center. Showalter and Jack Lissauer, a research scientist at NASA Ames, used Hubble's Advanced Camera for Surveys (ACS) to make the discovery. The images were taken on Aug. 25. The newly discovered moons are temporarily designated as S/2003 U 1 and S/2003 U 2 until the IAU formally approves their discovery. S/2003 U 1 is the larger of the two moons, measuring 10 miles (16 km) across. The Hubble telescope spotted this moon orbiting between the moons Puck, the largest satellite found by Voyager, and Miranda, the innermost of the five largest Uranian satellites. Astronomers previously thought this region was empty space. S/2003 U 1 is 60,600 miles (97,700 km) away from Uranus and whirls around the giant planet in 22 hours and 9 minutes. The smallest Uranian moon yet found, S/2003 U 2, is 8 miles (12 km) wide. Its orbital path is just 200 to 450 miles (300 to 700 km) from the moon Belinda. S/2003 U 2 is 46,400 miles (74,800 km) away from Uranus and circles the planet in 14 hours and 50 minutes. The tiny moon is part of a densely crowded field of 11 other moons, all discovered from pictures taken by the Voyager spacecraft. "The inner swarm of 13 satellites is unlike any other system of planetary moons," says Lissauer. "The larger moons must be gravitationally perturbing the smaller moons. The region is so crowded that these moons could be gravitationally unstable. So, we are trying to understand how the moons can coexist with each other." One idea is that some of the moons are young and formed through collisions with wayward comets. For example, the Hubble telescope spotted two small moons orbiting very close to the moon Belinda. One of them is the newly detected moon S/2003 U 2, which is traveling inside Belinda's orbit. The other, designated S/1986 U 10, was found in 1999 by astronomer Erich Karkoschka of the University of Arizona, who uncovered the satellite in Voyager pictures. But the finding required confirmation by a second telescope. This is the first time this moon has been seen since Voyager snapped a picture of it. S/1986 U 10 is 750 miles (1,200 km) away from Belinda. "Not all of Uranus' satellites formed over 4 billion years ago when the planet formed," Lissauer says. "The two small moons orbiting close to Belinda, for example, probably were once part of Belinda. They broke off when a comet smashed into Belinda." The astronomers hope to refine the orbits of the newly discovered moons with further observations. "The orbits will show how the moons interact with one another, perhaps showing how such a crowded system of satellites can be stabilized," Showalter explains. "This could provide further insight into how the moon system formed. Refining their orbits also could reveal whether these moons have any special role in confining or 'shepherding' Uranus' 10 narrow rings." Astronomers stretched the limit of Hubble's ACS to find the tiny satellites. "These moons are 40 million times fainter than Uranus," Showalter says. "They are blacker than asphalt, if their composition is like the other small, inner moons. So they don't reflect much light. Even with the sensitivity and high resolution of Hubble's ACS, we had to overexpose the images of Uranus to pinpoint the moons." The newly detected moons, when approved by the IAU, will bring the Uranian satellite total to 24. Uranus ranks third in the number of IAU-certified moons behind Jupiter (38) and Saturn (30). Excluding the outer moons that travel in elongated orbits and are probably captured asteroids, Uranus holds the record for the most satellites with 18 in its inner system. All of them have nearly circular orbits. Saturn is second with 17. The Hubble Space Telescope, a photographer dear to more hearts than the late, but very human, Ansel Adams, is nowhere more celebrated than here inside the Space Telescope Science Institute (STScI), which operates the telescope for NASA. Pinned to a hallway bulletin board is a recent New York Times editorial lauding Hubble for revealing the distant universe, and time itself, with "astonishing clarity." The walls of the STScI are of course plastered with gorgeous posters that testify to Hubble's reputation as a time machine. Many of the pictures were made with light that was generated billions of years ago. Less noticed outside the institute are the unexpected contributions Hubble has made in characterizing other worlds and developing solar systems, places where researchers are eager to look for signs of life.
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