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Telescopes 101

Fifteen things you need to know before buying a telescope.
RELATED TOPICS: TELESCOPES

6. I've heard "bigger is better" is true when it comes to telescopes. What are the real advantages of big scopes?

The larger the telescope’s diameter, the more light it will collect. A 4-inch refractor, for example, is a great scope for planets, the Moon, and double stars. I know because I own one, and I wouldn’t part with it for love or money. This size scope, however, is a bit small for deep-sky objects such as nebulae, star clusters, and galaxies.

An 8-inch telescope (it doesn’t matter what type) will move you into a new dimension of viewing. The objects you see with an 8-inch scope will reveal more detail. Keep in mind, however, that if you stay interested in observing, you’ll crave even larger scopes. This malady among amateur astronomers is known as “aperture fever.” And, yes, I’m infected. I started many years ago with a 2.4-inch telescope of good quality. I now own a 4-inch refractor and a 12-inch Schmidt-Cassegrain. Am I happy? Yes. Am I satisfied? Not even close. I now dream of a 20-inch telescope. I think you get the idea.

7. Is "achromat" different from "apochromat"? What do the words mean?

Both terms refer to the lens systems used in refracting telescopes. An achromat is a two-lens system. Apochromats also may employ two lenses, but they’re more likely to have three or four. The main difference between achromats and apochromats is the amount of what’s called “excess” color you’ll see on bright objects. Excess color is not a color that’s been added to what you’re viewing. Rather, it usually appears as a purple fringe on one edge of the object. Apochromats deliver an image essentially free of excess color.

Through an achromat, you’ll see excess color on bright objects like Jupiter, Venus, and the Moon. Many observers can ignore the excess color. Apochromats always are more expensive than comparable achromats, and usually by a sizeable amount.

8. I’ve just purchased a new telescope, and am really excited to use it. What’s the first thing I should do?

Read the operating manual several times, and set up your telescope indoors before you try to set it up outdoors at night. This will familiarize you with the location and operation of any switches, buttons, and levers. Knowing how to use your telescope before you head outdoors will enable you to use your observing time efficiently. This will be especially helpful for sessions such as the Messier marathon (locating all Messier objects in one evening).

Let's talk optics

The f/4, f/10, or f/15 represent a telescope’s focal ratio (f/ratio). Focal ratio is determined by dividing the focal length (the distance from the objective to where the light comes to a focus) by the aperture. Here’s an example: A 6-inch telescope has a focal length of 36 inches. To find the f/ratio, divide 36 by 6. This produces f/6. Often, the aperture is in inches, but the focal length is given in millimeters. If so, just multiply the inches by 25.4.

To find magnification, simply divide the telescope’s focal length (fo) by the eyepiece’s focal length (fe). For example, if your telescope has a focal length of 1,000mm and you choose a 25mm eyepiece, the magnification will be 40x. Remember, magnification alone is not a good indicator of telescope quality. Magnification is determined by which eyepiece you use with any telescope. So, a low-quality scope can have a poorly made eyepiece, and the combination can give high power. But any images viewed through it will look distorted — although they will be magnified as claimed.

Optics
The image is “upside-down,” although there is no up or down in space.
Astronomy: Roen Kelly

9. Why are objects I view through my telescope upside-down?

Just before the light collected by your telescope’s main lens or mirror enters the eyepiece, it’s flipped (see “Let’s talk optics” above), so you see an inverted image. A prism assembly (usually called an “image erector”) will re-flip the image, but adding this accessory will cause some light to be lost. And regarding observing, the telescope’s function is to deliver the maximum amount of an object’s light to your eye, so you don’t want to lose light from flipping the image. Besides, keep in mind there’s no up or down in space, and with most objects, you won't even know they're upside-down.

10. Can I use my telescope for views of earthly objects?

Absolutely! Many nighttime observers (usually those with small refractors because of size and portability issues) also use their telescopes for bird-watching or other nature-watching activities. Plus, think about pictures you’ve seen of sailors in centuries past — they almost always are at the bow looking through a telescope.

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