[分享] 《天空和望远镜》杂志对MEADE ETX-90/EC的评测。
S&T Test Report:A "Hot" Telescope Gets Even Hotter
By Dennis di Cicco
Adapted from
Sky and Telescope
May 1999
"Oh joy, rapture!
I've got a brain."
--Scarecrow in the movie
The Wizard of Oz
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Meade sets a new standard by providing user-friendly capabilities never before available from an inexpensive telescope
Above: Outwardly similar to its predecessor, Meade's new ETX-90/EC 3½ inch Maksutov-Cassegrain now has motor drives on both axes and a push-button hand controller (an optional computerized controller is available). The optical tube is identical to that of the original ETX, including the same fine optics and a threaded aluminum lens cap. All Sky & Telescope photographs are by Chuck Baker.
When we reviewed Meade's 90-millimeter ETX Maksutov-Cassegrain in the January 1997 issue, we called it the hottest telescope ever. Well, it just got hotter. A lot hotter!
Meade has introduced its new ETX-90/EC, with the EC designating electronic control. (By the way, the ET in ETX is from Meade's founder, John Diebel, who envisioned Everyone's Telescope.) Retailing for the same $595 price as its predecessor, the ETX-90/EC has built-in motor drives and a push-button hand controller.
As welcome as these features are, the real excitement surrounds the optional Autostar controller. For $149 you can skip a trip to Oz and buy the ETX-90/EC a brain. And what a brain it is! Foremost among Autostar's features is the ability to automatically point the scope to more than 12,000 targets stored in an internal database. But it does much more than that. In fact, the Autostar-equipped ETX-90/EC can do things no commercial telescope has ever done before.
Within days of the ETX-90/EC's unveiling last January, Sky & Telescope obtained the first unit Meade loaned for review. While the scope was a production unit (manufacturing had been under way for several months in anticipation of initial demand), the Autostar was still being tweaked (I tested version 1.0c).
The Optics
The ETX-90/EC has a 90-mm (3½ inch) clear aperture and a focal length of 1,250 mm, yielding f/13.8. It is the same optical tube assembly as the original ETX. Will old ETXs fit on the new EC mount? Yes. Will Meade sell the new mount separately? As of press time the question remained open.
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ABOVE： The straight bands recorded in this video image from the author's double-pass, autocollimation test with an 85-lines-per-inch Ronchi grating shows that the ETX-90/EC has smooth, well-corrected optics. Click on image for larger view..
The optics in our test telescope were excellent and consistent with the scope we reviewed in 1997, which was anonymously purchased. Star images came to a precise focus with a crisp, round Airy disk surrounded by a bright diffraction ring and several fainter ones. Star images inside and outside of focus were indistinguishable from one another. The optics are fully baffled to suppress flare and scattered light, and multicoatings provide very contrasty views of the Moon and terrestrial scenes.
At 156x the scope easily showed the companion of the brilliant double star Rigel in Orion. Although the 91/2-arcsecond separation of this pair isn't challenging for small apertures, the nearly 7-magnitude difference in brightness between the two stars can overwhelm optics that deliver low-contrast images. At the same magnification the ETX-90/EC split 4-arcsecond Castor in Gemini with lots of dark sky between the components. What really impressed me, however, was the clean split of Eta Orionis at 250x. With a current separation of 1.7 arcseconds, this pair is within 0.5 arcsecond of the scope's Dawes limit. The companion was readily apparent within the first diffraction ring of the primary, and a thread of dark sky divided the stars during moments of good seeing.
Familiar with how rarely 8-inch and larger telescopes experience moments of good seeing, I was pleasantly surprised by how often the atmosphere served up diffraction-limited conditions for the small-aperture ETX-90/EC. You don't see more detail with the small scope, but the sharp images are very satisfying and a frequent reminder of the excellent optical quality
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Internal power comes from eight AA batteries located under a snap-on cover at the bottom of the scope's base. The holder clearly shows the correct polarity for each battery. The scope can also be powered by an optional 12-volt external source
The Moon was beautiful at any magnification, with black shadows contrasting sharply with illuminated features along the terminator. Venus appeared dazzlingly white with no color fringes except those caused by atmospheric refraction. Like Venus, Jupiter was low in the January evening sky, but several bands were easily seen on the planet's disk.
Saturn was very impressive. As the planet snapped into focus in the supplied 26-mm (48x) eyepiece, I could easily see the rings' Cassini Division and several bands on the ball of the planet. Even at this modest magnification the moons Titan, Rhea, and Dione were clearly visible, and Tethys became evident when the power was increased to 156x.
The bottom line is that the ETX-90/EC showed me everything I could expect from an instrument of its size. The mechanics of the optical tube assembly are also first rate, with barely a hint of lateral image shift as the focus knob is turned back and forth.
The Mount
Although outwardly similar to the original ETX, the new mount is functionally very different. The manual slow-motion controls are replaced with small DC motors driving metal worm gears through plastic gear trains. Power comes from eight AA batteries in the base, and there is provision for an external 12-volt source (though none is supplied with the scope). Even discount alkaline batteries provided more than 20 hours of operation with heavy slewing. As the batteries near exhaustion, slewing becomes sluggish. With the optional Autostar attached, erratic operation is a sure sign that the batteries need replacing.
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A flip mirror, operated by the arrowed knob, directs light from the straight-through port for a camera to the 1 ¼-inch eyepiece position. The drive is not designed for long-exposure astro imaging, but snapshots of terrestrial scenes and bright astronomical targets are possible.
The mount has internal stops that limit rotation in altitude (or declination, if used equatorially) to about 125° and in azimuth (right ascension) to slightly less than two full turns. The clamp for the azimuth axis is a large lever, and the altitude clamp is controlled by a fluted knob on one fork arm; a user can operate both easily while wearing heavy gloves. The standard controller with the ETX-90/EC is designed for comfortable one-hand operation. Either this controller or the optional Autostar must be connected to the scope for the motors to operate. The standard controller offers four slewing speeds: 5°, 0.75°, 8 arcminutes, and 2 arcminutes per second.
The ETX-90/EC can be set to power up in either altazimuth or equatorial mode. In the former the motors run only when the slewing keys are pressed; in the latter the right-ascension drive operates at sidereal rate and requires polar alignment. At first I considered the factory default of altazimuth rather strange for an astronomical telescope, but I'll bet most people will start out using the scope this way. In addition to checking out a bird feeder and other daytime sights, altazimuth operation is convenient for viewing celestial objects, since the telescope functions like a Dobsonian with electric controls! This bypasses the need for a tripod and the hassles of polar aligning the scope.
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Legs for the tabletop tripod are now an optional purchase ($29.95). But, as mentioned in the text, people are likely to use the telescope in other mounting configurations
The tabletop tripod legs are no longer standard equipment. While this certainly helps keep the cost of the basic telescope down, it's also indicative of how most people will use this instrument, especially with the Autostar option. Meade sells a field tripod ($199.95) for the ETX-90/EC that is well suited to altazimuth and equatorial use. Another possibility for the altazimuth configuration is a simple pier, such as an attractive one available in garden shops for birdbaths or sundials. Piers offer easy access to the telescope regardless of where it's aimed. Another reason for using the altazimuth mode is that it offers full sky coverage. At any latitude lower than about 45°, a polar-aligned ETX-90/EC cannot view objects near the southern horizon because the optical tube hits the base of the mount.
When the scope is polar aligned the tracking rate is very good, easily holding objects in the 48 x eyepiece for more than an hour. The mount and motors are robust enough to support a 35-mm or small CCD camera for snapshots, but the drive is not designed for long-exposure astro imaging. After slewing eastward, especially at the faster speeds, the scope pauses as backlash works its way out of the gear train before it resumes celestial tracking. Sometimes it took 15 seconds or more for the scope to begin moving, and twice as much time could pass before the full tracking rate resumed. Nevertheless, it's easy to eliminate this lag by tapping the west push button for a second or two to speed up the backlash removal.
The Autostar
Advertisements merely hint at the range of Autostar functions; even in this review I don't have room to list them all, let alone describe them in detail. The more time I spent with Autostar, the more I became amazed at what is packed into this relatively small controller. Here are some examples.
While reading Autostar's two-line, liquid-crystal display (LCD) my first evening out with the scope, I decided to explore the Event menu. Here a keypress or two gives the times of sunrise, sunset, moonrise, and moonset (accurate to a few minutes for the Sun and about 10 or so for the Moon) as well as the dates and times of upcoming principal Moon phases. Also available are dates of meteor showers, solstices, equinoxes, solar and lunar eclipses, and the "Min. of Algol."
Assuming that this last item might be a mystery to beginners, I pressed the Enter key to learn more and was informed that, whatever "Min. of Algol" was, it was going to happen in a few hours. I pressed the Go To key and the scope's motors whirred into action (when slewing at high speed they sound like one of today's ubiquitous remote-controlled toy cars) as the scope headed to a point high overhead. It stopped with a nondescript white star in the eyepiece, and I could imagine someone new to astronomy thinking, "So what?" The answer came with additional keypresses. Algol, I learned from reading the LCD, is Beta Persei. Also displayed were its celestial coordinates, magnitude, spectral classification, and information about it being a multiple star located 72.4 light-years from the Sun. A final keypress launched a 300-plus-word description of this eclipsing variable star scrolling across the LCD. Suddenly Algol (and its upcoming minimum light during eclipse) became a lot more interesting!
Some of the information on Algol would be cryptic to a beginner (some abbreviations left even me wondering). But several terms in the description -- "light-year," for example -- appear within brackets, and a keypress shoots you to a definition. This is the first telescope that can give you a course in astronomy -- impressive stuff.
With upward of a megabyte of compressed text stored in Autostar, there are bound to be some mistakes, but they should be fixed quickly. What makes the unit attractive from this standpoint is that it is engineered with software revisions in mind. An optional cable connects it to an IBM-compatible computer so that the software can be updated with files downloaded from Meade's Web site (www.meade.com). It can even be linked to another Autostar to transfer files between units, including catalogs of user-entered targets.
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left:The most exciting aspect of the new scope is the optional Autostar hand controller -- the "brains" of the system. Among its scores of features is the ability to automatically slew the telescope to more than 12,000 celestial targets. Autostar is smaller, lighter, and more ergonomically friendly than the bulky hand controller for Meade's line of LX200 telescopes. All of Autostar's data and functions can be accessed by using just the Mode, Enter, and scrolling keys. All keys are backlit (right). Click on images for larger views.
With Autostar able to do so much, the question arises as to how easy it is to operate. I learned the answer in an unexpected way.
I assumed my experience with Meade's computerized LX200 telescopes would breeze me through Autostar's basics. Wrong! As the Sun set on that first clear evening, I plugged Autostar into the scope and was confronted with a very unfamiliar initializing procedure. There's no need to explain the steps here, since the manual walks you through them with clear instructions. Furthermore, you can do them anytime -- they don't require viewing the sky. I recommend you spend 30 to 45 minutes indoors with the Autostar the first time you turn it on. This will give you plenty of time to enter data (some of which needs doing only once), make and correct mistakes, calibrate the motors, and, most important, learn to navigate Autostar's menus. Skip entering your observing site's precise coordinates (if you know them), since the proper format isn't obvious on the LCD. Instead, select a nearby geographical location from the huge database in Autostar's memory. The format of these data will guide you when you input your own values later. The unit can be set up for multiple sites, each of which can be selected at the touch of a key.
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Left:The standard 8 x 21, erect-image finder has good optics but is very difficult to look through in some orientations of the scope. Many people may prefer an optional 8 x 24 right-angle finder ($49.95) available from Meade. Click on image for larger view. Right: Most field tests were done with the telescope in altazimuth mode using the optional field tripod. This arrangement allowed full access to the sky, but it was difficult to reach the focus knob when the scope was pointed to high altitudes.
An optional electric focuser is available.
After this initial setup, I mounted the scope in altazimuth mode on the field tripod resting on my gently sloping driveway. Because Autostar has no internal battery for its clock, you must input the date and time whenever you power up the telescope. The default values are the last date the scope was used and 8:00 p.m., so a few keypresses will typically be all that's necessary. As an indication of how well Autostar is engineered, when the display requests the date and time, a map light automatically turns on so you can read your watch!
I selected the "easy" alignment procedure and followed the instructions that scrolled across the LCD -- no need to fumble with the manual in the dark. First I leveled the telescope's tube and pointed it north by eyeballing the North Star, which was just emerging out of the twilight. This done, I hit the Enter key and was instructed to center Capella in the finder. Before I could glance up to see if this star was visible, the scope automatically began slewing and stopped with Capella well within the field of the finder. I was astounded! Clearly the accuracy of this maneuver hinges on how well the scope was initially leveled and aimed northward, and my estimates were rough. Using Autostar's slewing keys I nudged Capella to the center of the cross hairs and hit the Enter key.
Now I was told to center Diphda. It made no difference that I couldn't recall which star was Diphda, for the ETX-90/EC was already heading toward the southwestern skyline. Only one star was visible in the finder when the scope stopped, so I centered it and hit Enter. Within a few seconds "align successful" flashed on the LCD and the motors began tracking.
Jupiter was obvious in the twilight and also easy to locate within Autostar's Object menu. I hit the Go To key and the scope began slewing in the right direction. Time has dulled the memory of my first telescopic peek at Jupiter nearly four decades ago, but I couldn't have been more excited than I was this evening as I watched the giant planet slide into the eyepiece field. Aiming a telescope at such an easy target isn't much to brag about, but the ETX-90/EC is likely to be a first telescope for many people. To have it find Jupiter automatically is simply amazing!
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The polar-aligned mode is the only arrangement that allows automatic tracking of celestial objects with the standard hand controller. At latitudes below about 60°, the base of the polar-aligned mount blocks the scope from pointing at the southern horizon.
Venus was near the horizon, but the ETX-90/EC effortlessly positioned it within the field of the main eyepiece. A little higher was the waxing crescent Moon. I was a bit surprised when the scope missed this target -- in the finder, but not the main telescope. Calculating the position of the rapidly moving Moon requires extremely complex algorithms. Indeed, Isaac Newton is said to have complained to Edmond Halley that thinking about the Moon's motion "made his head ache and kept him awake so often that he would think of it no more." So it seems fair to cut the ETX-90/EC a little slack. Furthermore, tapping the slewing keys for a second or two is all it took to center the Moon in the eyepiece.
Saturn was my next target, and the scope placed it dead center. Almost as fast as I could hit the keys I jumped to Aldebaran, the Crab Nebula, Rigel, the Orion Nebula, Betelgeuse, Castor, and the double star Mizar in the handle of the Big Dipper, which was just kissing the treetops along my northern horizon. As a test, I asked the scope to point to M17 in Sagittarius, but the LCD correctly showed "below horizon."
As with a polar-aligned mount, the altazimuth tracking easily kept objects in the 48 x eyepiece for more than an hour. It even tracked stars effortlessly within 1° of the zenith. When the drive motors operate at tracking speed they make a "stuttering" sound reminiscent of a fax machine -- louder than most telescopes, but not annoyingly so -- and there appears to be a small amount of vibration. At magnifications of 250 x the image sometimes seemed to jitter in synchronization with the sound of the motors. It didn't always happen, but it also seemed too much of a coincidence to be just the jiggling caused by atmospheric seeing. It wasn't enough to be a problem.
The scope moved rapidly between objects, taking just over a half minute to slew the 52° from Rigel to Castor and 45 seconds to cover 101° between Castor and Deneb. A beep that sounds at the end of slewing is not the end of the centering routine; it marks when the scope reverts to tracking mode, after which a few seconds can still be spent automatically refining the centering.
To check the pointing accuracy I used Autostar's internal catalog and the Go To function to slew to more than a dozen bright stars scattered around the sky. All but one lay well within the 1° field of the 26-mm eyepiece.
Autostar's LCD is remarkably robust in cold weather. Coming from a warm house, it was still easy to read after operating for more than three hours at a bone-chilling 10° Fahrenheit. I did, however, have to slow the scrolling speed for the text to remain legible. It's important to have fresh batteries, since a cold-induced voltage drop exacerbated by slewing can cause the Autostar to become erratic. My only field-test failure occurred under just such conditions. Switching the unit off and on cured the problem but required stepping through the alignment procedure again to synchronize the scope with the sky.
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One "aux" port on the scope's control panel is for the optional electric focuser, while the other is for connection to a computer. Unlike other Meade computer-slewed scopes, the "brains" of the ETX-90/EC are in the Autostar rather than the scope's base, so computer control is possible only when the Autostar is attached
In addition to comets and asteroids, Autostar's database includes several dozen Earth-orbiting satellites (more can be added). This is the first inexpensive telescope that is capable of tracking these swiftly moving targets. Orbital elements for satellites grow stale very quickly, and Meade is expanding its Web site so that Autostar owners can download up-to-date elements (this wasn't ready at the time of my tests). An upcoming favorable pass of the Mir space station prompted me to manually update Autostar's orbital elements, which was easily done with Autostar's Edit function. True to claims, Autostar identified the Mir passage. It took only a few keypresses to automatically slew the telescope to the right position and wait for Mir to appear.
The Autostar offers "tours" that are tailored to the present date. They direct observers to showpiece objects, including the Moon and planets if then visible. Most of the objects are accompanied by LCD descriptions. The Moon has different descriptions based on its phase, and prominent features near the terminator are highlighted.
It's exciting to think how the ETX-90/EC can be coupled to a multimedia computer to introduce people to astronomy. Of course, this can be done with most of today's computer-controlled telescopes, but opportunities will be much greater now that the technology is available in a "beginner's" instrument.
Final Points
Price alone makes it difficult to nitpick the ETX-90/EC. At the heart of any telescope is optical quality. Several generations of amateur astronomers have elevated the 3½ inch Questar to icon status because of its fine optics. The three ETXs that I've used have matched the Questars I've tried.
The ETX-90/EC's mount and drive system are not intended for astro imaging. Rather, they are for slewing to and tracking objects for visual enjoyment. They do this very well, though backlash in the gear train requires a deft touch on the motor controls in the altazimuth mode when viewing at high magnifications.
If this were the first telescope with a Go To function, it would be hailed a miracle. Computer-slewed telescopes, however, have been part of amateur astronomy for more than seven years. But you can't compare the Autostar-equipped ETX-90/EC to them, since the nearest competing telescope costs nearly four times as much and still lacks many of Autostar's features.
One final thought. Someone with an affinity for collecting should consider buying an ETX-90/EC and Autostar and pack them safely away. I believe that one day museumgoers will look at one of these instruments in a glass case and read a placard stating that this telescope changed the way everyone looks at the stars.
Courtesy of Sky & Telescope
http://www.analyticalsci.com/Astronomy/Telescopes/Test%20Reports/ETX90/Sky_Tel_Logo.gif 不仅质量好，而且价格便宜，
咳，国内的厂家也看看自己的镜子。 又是英文……！ Familiar with how rarely 8-inch and larger telescopes experience moments of good seeing, I was pleasantly surprised by how often the atmosphere served up diffraction-limited alexhy 发表于 2002-10-6 21:20
不仅质量好，而且价格便宜，
咳，国内的厂家也看看自己的镜子。
价格多少?
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