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Telescopes 101

A telescope makes a wonderful gift for someone who has an interest in the night sky. Even a majority of the smallest inexpensive telescopes are capable of revealing the rings of Saturn, the cloud belts of Jupiter, hundreds of star clusters and even distant galaxies. With a medium or large telescope, amateur astronomers rarely see everything there is to see with a telescope, even after years of observing. Astronomy is truly an interest that can last a lifetime.

Choosing a Telescope

So how do you choose a telescope for a new astronomer? With so many telescope models designed for the beginner, it is sometimes hard to choose, especially if you are not an astronomer yourself. Here are some frequently asked questions about telescopes for the person buying gifts for an astronomer. We have also included some telescope recommendations for beginning astronomers to make shopping easier. Learn how telescopes work with OpticsPlanet and become an avid astronomer for life. We'll tell you all about the best reflecting telescopes, refracting telescopes, Schmidt-Newtonian telescopes, Schmidt Cassegrain telescopes, Dobsonian telescopes and the Maksutov-Cassegrain telescopes.

Sizes and Measures

Since any telescope, even a small one, can produce any given magnification with the right telescope eyepiece, magnification is not a useful way to rate a telescope. Therefore, the size of the lens or mirror (the heart of the telescope) is used as a general way to measure telescope potential. This is usually part of the telescope's model description.

Size of the Lens or Mirror

The larger the telescope lens or telescope mirror, the fainter the objects a telescope will reveal (the more objects that will be visible), and the greater the telescope magnification it can use and still produce a good image. For example, at 120x, a ten inch telescope will reveal hundreds of objects not visible in a three inch telescope. In addition, at 120x, the 10" inch telescope will produce images that are brighter and sharper than the 3" inch telescope.

Large vs. Small Telescopes

For amateur astronomers, a small telescope generally refers to a telescope of 4 inches or less. A medium telescope generally means a telescope of five to eight inches and a large telescope usually means anything over ten inches in size. However, this is a very general way to look at a telescope. Optical quality and special features may allow a small telescope to outperform a larger telescope for some types of observing.

Is Larger Always Better?

Not always. Large telescopes do translate into more objects seen and better images, but they are also more expensive and also less portable. Many astronomers choose smaller telescopes because they need to transport them to locations where the sky is darker and more stars are visible. Even if observing will be done at home in the backyard, portability is still a factor. When a telescope becomes a chore to move and set up because of its size, it is doomed to collect dust in the closet. If you are buying a telescope for a new astronomer, especially a child, it also makes sense to start with a telescope that is suitable to their size, age and interest.

What to Look For

You need a starfinder or planisphere. It will help you locate the most common things to look at in the sky. A great tool for the beginner is the Meade Astrofinder; it's a software program that will help you find various objects in the cosmos. It also provides interesting facts and information about astronomical phenomena. It's a great telescope accessory to get started in stargazing.

Proper Magnification

Not as much as you think. Few astronomical objects require more than 350x magnification and most observing is done at less than 200x, even with a large telescope. It is essential to understand that as magnification goes up, telescope image quality goes down and that it goes down much quicker in a small telescope.

500x Magnification with Small Telescopes

Yes, you can get 500x or 600x magnification, even in a small telescope, if you use the right telescope eyepieces. However, at such extreme magnification the images produced will be useless. Worse yet, especially for the beginner astronomer, as magnification goes up, ease of astronomical telescope use quickly goes down. At 500x, a small telescope is impossible to use, despite what it might say on the box about magnification.

Minimum Magnification with a Good Image

That depends partly on Mother Nature. On nights when the sky is clear and stable, you can use higher magnifications than on nights when the sky is hazy or unsettled. Assuming good conditions, a basic rule of thumb is 30-50x of magnification for every inch of telescope as a maximum. Thus, the top useable magnification for a 2.4 inch (60mm) will be in the neighborhood of 120x on nights when the sky allows it. This is still more than enough to see the rings of Saturn, cloud belts on Jupiter and many star clusters and nebulae. Magnifications above this in a 2.4 inch telescope, even if you have the telescope eyepieces to obtain it, will produce images of low quality. Above 350-400x, even large telescopes have trouble seeing through the atmosphere above us.

Determining Magnification with the Eyepiece

Telescope eyepieces are marked with their focal length in millimeters, not the magnification. To calculate the magnification you will get with any telescope eyepiece, divide the focal length of the telescope eyepiece into the focal length of the telescope. A telescope eyepiece with a focal length of 25 mm when used in a telescope with 1000 mm focal length therefore produces a magnification of 40x.

How To Choose and Buy A Telescope