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More power and the telescope

When it comes to telescopes, you might think the more magnification the better. This is not necessarily true. Although higher magnification can make an image look large, the amount of light gathered is diffused over a wide space producing a weaker image. Before you succumb to a 200x marketing claim, make sure you know what you are buying and what you will actually be able to see with the 'scope. Most experts point to a top magnification of 40 to 60 per aperture inch.

The real key to great telescope viewing is the aperture. This is the diameter of the objective lens in a refractor scope or the objective mirror in a reflector scope. The larger the aperture, the more light will be gathered and a better image can be seen. There is a caveat, however. A telescope with a large aperture is by definition a large telescope. Are you going to be traveling to a dark field for your viewing? Having to haul a large telescope can get old quickly. Where are you going to store the telescope and how are you going to transport it are two important questions to consider. The telescope will not help you enjoy the stars if it remains in the garage. Recommendations for amateurs point to 60 to 80 mm reflectors and for the more advanced, about twice that size.

Another factor to consider when purchasing a telescope is the focal length, or the distance from the mirror or lens to the point where the image is in focus. You can find the measurement in the telescope's specifications on the box. Generally, the longer the focal length, the higher the magnification. However, just because the telescope appears long, it does not mean the focal length is long as well. The catadioptric telescope's light path is folded in a short tube. Check the measurements for clarification.

After you find the focal length you will want to take a look at the focal ratio, meaning how bright the image is over how wide a field of view. You get this figure by dividing focal length (the distance from the mirror or lens to where the image is in focus) by aperture size. This combination of the lenses is more important than magnification, with a long focal length lens or mirror with a short focal length eyepiece producing a great image. So a telescope with an aperture of f/10 or above is what to look for when doing the stars and planet thing. F/8 is good for general sky viewing. Hunker down to an f/6 or less for catching deep sky mysteries.

Another variable worth looking at is the wave number (sometimes called the wave error). If this was a perfect world all wave numbers would be 0, indicating there are no aberrations in the glass or mirror. However, aberrations exist and are measured in fractions. Look for a wave number of 1/8th or less. Keep in mind that the combined wave length is the key...so if you have two mirrors or lenses with 1/8th wave error each, then combine them for a wave number.

It is not enough to see a blob in the sky...we want to see the details and that depends on a telescope's resolution, or ability to "resolve" objects that are close to each other. It is a combination of factors including the aperture, the grade of the optics and general atmospheric conditions (dark and dry are the optimum).

There are other pieces of equipment that will help determine if your star gazing adventure is a pleasurable one. Particularly important is the telescope mount or stand that will hold the scope steady as you look through it. Keep in mind that the merest jiggle is magnified and can throw the object you are looking right off the view finder. There are generally two types of mounts. The alt-azimuth is the mount we are most familiar with. It is easy to set up and use and relatively inexpensive. You simply set the horizontal and vertical coordinates for the celestial body and then lock it in. Due to the earth's rotation, the object will move out of your view and you will have to readjust.

The alternative is the equatorial mount. A more complicated setup, it is also more expensive than its little brother. You align the mount with the earth's poles, and set the coordinates of the star or other object. The perk is that you will be able to track your star's journey across the sky, and keep it in view. Any kind of photography of the heavens requires an equatorial mount. Look for an equatorial mount with setting circles to help you set the coordinates. There are even options for a computer interface enabling you to use either a laptop computer or a PDA to control the telescope's action.

Whatever option you choose (know that the equatorial mount is the choice of most serious sky watchers), look for stability, strength to support the scope and adjustability to your viewing size when either standing or sitting. Price tags start from $100 way on up to $2,000.

The next important accessory is the variety of eyepieces you will need to change the magnification of the telescope. It is recommended you have at least three: one low, one medium and one high power. There are a number of eyepiece types including the oldest designs, Huygens and Ramsden which are not considered top of the line anymore. Orthoscopic and Plossl eyepieces are more often recommended. Whichever you choose, look for quality in the optics, field size, sharpness and brightness, size of the barrel and that it is aberration free. Another consideration is eye relief, or the distance between your eye and the lens. This is important particularly if you need to use your glasses to see.

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