Telescopes give sky gazers a great way to see magnified views of objects in the sky. But whether you’re buying your first, second, or fifth telescope, it’s important to be fully informed before heading to stores so you can make the best choice. A telescope is a long-term investment, so you’ll need to do your research, learn the terminology, and consider your needs. For instance, do you want a telescope to observe planets, or are you interested in “deep-sky” objects? Those intentions will help you determine which telescope to get.
Power Is Overrated
A good telescope is not just about its power. Three hundred-times magnification sounds great, but there’s a catch: While high magnification makes an object appear larger, the light gathered by the scope is spread over a larger area, which creates a fainter image in the eyepiece. Sometimes, lower magnification power provides a better viewing experience, particularly if observers are looking at objects that are spread out across the sky, such as clusters or nebulae.
Also, “high-powered” scopes have specific requirements for eyepieces, so you’ll need to research which eyepieces work best with a given instrument.
Any new telescope should have at least one eyepiece, and some sets come with two or three. An eyepiece is rated by millimetres, with smaller numbers indicating higher magnification. A 25-millimeter eyepiece is common and appropriate for most beginners.
Just like magnification power, a high-power eyepiece does not necessarily mean better viewing. For example, it may allow you to see details in a small cluster, but if it’s used to look at a nebula, it will only show a part of the object.
It’s also important to remember that while a higher-magnification eyepiece may provide more details, it can be harder to keep an object in view. To get the steadiest seeing in such cases, you may need to use a motorized mount. A lower-power eyepiece makes it easier to find objects and keep them in view. It will also require less light, so viewing dimmer objects is easier.
High- and low-power eyepieces each have their place in observing, so their value depends on the interests of the stargazer.
Refractor Versus Reflector: What’s the Difference?
The two most common types of telescopes available to amateurs are refractors and reflectors. A refractor telescope uses two lenses. The larger of the two, called the “objective,” is at one end; the lens the observer looks through, called the “ocular” or the “eyepiece,” is at the other.
A reflector telescope gathers light at its bottom using a concave mirror called the “primary.” There are many ways the primary can focus the light, and how it is done determines the type of reflecting scope.
The aperture of a telescope refers to the diameter of either the objective lens of a refractor or objective mirror of a reflector. The aperture size is the true key to the “power” of a telescope—its size is directly proportional to the scope’s ability to gather light. And the more light a scope can gather, the better the image an observer will see.
However, that doesn’t mean you should simply buy the telescope with the largest aperture you can find. If your scope is inconveniently large, you are less likely to use it. Typically, 2.4-inch (60-millimeter) and 3.1-inch (80-millimeter) refractors and 4.5-inch (114-millimeter) and 6-inch (152-millimeter) reflectors are popular for amateurs.
The focal ratio of a telescope is calculated by dividing its focal length by its aperture size. The focal length is measured from the main lens (or mirror) to where the light converges to focus. As an example, a scope with an aperture of 4.5 inches and focal length of 45 inches will have a focal ratio of f/10.
A higher focal ratio typically implies higher magnification, whereas a lower focal ratio—f/7, for example—is better for wider views.
A telescope mount is a stand that holds it steady. While it may seem like an add-on accessory, it is just as important as the tube and optics. It is extremely difficult, if not impossible, to view a distant object if the scope wobbles even in the slightest, so a high-quality telescope mount is a good investment.
There are essentially two types of mounts: altazimuth and equatorial. Altazimuth is similar to a camera tripod. It allows the telescope to move up and down (altitude) and back and forth (azimuth). Equatorial mounts are more complex—they are designed to follow the movement of objects in the sky. Higher-end equatorials come with a motor drive to follow the rotation of the earth, keeping an object in the field of view longer. Many equatorial mounts come with small computers that aim the scope automatically.
Just as with any other product, it is true with telescopes that you get what you pay for. A cheap department-store scope will almost certainly be a waste of money.
This is not to say that you should drain your bank account—most people do not need an overly expensive scope. However, it’s important to ignore cheap deals at stores that don’t specialize in scopes and will give you a low-quality viewing experience. Your strategy should be to buy the best one for your budget.
Being a knowledgeable consumer is key. Read about different scopes, both in telescope books and in articles online about the tools you need for stargazing. And don’t be afraid to ask questions once you’re in the store and ready to purchase.