| Please help me pick an astronomical telescope.?
When considering a telescope, there are two things to consider. On the one hand, the bigger the telescope, the more objects you will be able to see. On the other hand, the bigger the telescope, the more difficult it will be to set up and use on a given night. You need to find a balance between the ups and downs of a big telescope, because although you want to be able to observe as many deep-sky objects as possible (and see as many features on the planets as possible), you don’t necessarily want to have to lug a sixty-pound monster around, and spend half an hour setting it up.Another thing to consider is which type of telescope you will buy. The three main types of telescope are refractors, reflectors, and catadioptrics. They all have their ups and downs.A reflecting telescope, also known as a Newtonian, uses a parabolic mirror to collect light and focus it into your eyepiece. These telescopes offer you the most bang for your buck, so to speak, since mirrors are relatively inexpensive, even large ones. Another positive aspect of reflecting telescopes is that they focus light into an eyepiece mounted on the side of the telescope, meaning that you can comfortably view almost any spot in the sky. The big drawback to reflectors, however, is that they're huge—an eight-inch mirror requires a tube almost as tall as you!Refracting telescopes are the classic telescope type, and probably what you envision when you think of a telescope. They use lenses to focus light into your eyepiece, and accordingly, are significantly more expensive than comparably powered reflectors. However, they're also smaller for a given resolving power. Additionally, if you want to look straight up, you'll probably find yourself lying on the ground rather awkwardly. This is unfortunate, since looking straight up offers the least light-polluted view of the night sky. Some less-expensive refracting telescopes also suffer from various types of optical aberrations, resulting in distorted images very close to their edges; more expensive optics all-but eliminate this issue.Catadioptric telescopes are the “newest” type of telescope, and are essentially just a more compact type of reflecting telescope. They use more and smaller optics than a Newtonian reflector, meaning the light is dimmer when it reaches your eyes, and the scope is more expensive, but it also means the tube length is much, much shorter.Finally, a telescope requires an adequate mount, of which there are two main types.The altazimuthal (sometimes called Dobsonian) mount is the simplest, and allows you to pivot the scope around, from North to East to South to West and back to North again, and also allows you to pan up and down, from the horizon to the zenith and back. These mounts are cheap and practical, but because they do not align to the celestial grid, are useless for astrophotography.The equatorial mount is significantly more complex. Instead of using the altazimuthal coordinate system, it uses the more useful right ascension-declination system. This means you must center it on the North star each time you set it up, but it also means that you can easily follow the motion of heavenly bodies over the course of the night. If you have a powerful telescope, or plan to do absolutely any astrophotography, you need an equatorial mount. These mounts are heavier, more complex, but more useful. Some mounts come with instruments to help you align to the North star; others do not and require that you buy such an instrument, like a red-dot finder.Some mounts are computerized, meaning that they can be connected to a computer with astronomical tracking software. This allows them to stay focused automatically on an object for as long as you want, and also allows the mount to automatically jump to an object if you know its coordinates. Computerized altazimuthal mounts will track objects, but are still not very useful for astrophotography, since the objects will appear to rotate in the field of view. Computerized equatorial mounts do not suffer from this problem.And as if that weren’t enough information, a telescope by itself is useless; you need at least one eyepiece. Different eyepieces have different fields of view, different levels of magnification, and are of differing quality. There are also eyepiece attachments like optical filters (required for almost any kind of nebular viewing, helpful for lunar viewing, and somewhat helpful for planetary viewing), magnifying pieces called Barlow lenses, and more.Finally, you’ll need either a laptop computer with a good planetarium software (I recommend Hallo Northern Sky, which is free), or a good old-fashioned planisphere star chart for your latitude.By now you see that it’s not as simple as just picking a telescope in your price range. Purchasing a telescope is a huge investment, and requires a lot of supplementary purchases so that you enjoy it right out of the box. It also requires a lot of patience, since you’re not going to know what you’re doing for a while. Trying to tackle astrophotography right away, besides the additional expense of a CCD camera, only makes things that much more difficult, so it’s best to save that for later.Perhaps the best first “telescope” you can buy is a pair of good binoculars, if you don’t already have one. A pair of 10 x 50’s will let you observe Mercury, Venus, Mars, Jupiter (and its four Galilean moons), Saturn, Uranus, and on a really clear night, even Neptune. A few of the bigger asteroids, like Ceres, Vesta, and Pallas, are visible too. You’ll be able to see the Andromeda galaxy, and quite a few open and globular clusters. But most importantly, they help you get oriented in the night sky, learn what objects are where, the relative sizes and brightnesses of everything, and they teach you the kind of patience you’ll definitely need with a telescope.If you’re looking to buy a real telescope, consider buying an Orion XT8 Intelliscope (with or without computerized mount). Eight inches is in the “medium” range for a Newtonian reflector, meaning you’ll be able to see a lot of deep-sky objects, but you won’t waste a huge amount of time and effort setting it up. Although it’s meant as a “beginner” telescope, it’s very high quality and is in fact quite powerful. If you’re looking for a smaller scope, consider the six-inch version.
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06-04-2008, 03:03 PM
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