6 Things To Know About Telescopes Before you Buy

Every stargazer discovers what she or he needs to enjoy the sky. Take it easy and all good things will eventually come to you. Halfblue/Wikimedia Commons Share and Share Alike license.

If you're getting interested in stargazing, or have been doing it for a while, chances are you've thought about getting a telescope. It's an exciting moment, so make sure you have all the information you need to make a good selection.  There's a lot to learn if you haven't gotten one before, so do your homework before pulling that credit card out to make a purchase.  What you buy should be with you for a long time, so like any good relationship, you want to invest wisely. First, learn the terminology. Here are some sales terms you will run into as you search out a good piece of optics. 

Power. A good telescope is not JUST about the “power”.

If a telescope ad touts claims about "300X" or other numbers about the “power” the scope has, watch out! High power sounds great, but, there’s a catch. High magnification makes an object appear larger, and that's what you want. However, the light gathered by the scope is spread over a larger area which creates a fainter image in the eyepiece. So, keep that in mind. Also, "high-powered" scopes specific requirements for eyepieces, so be sure to check into that as you consider which scope to buy. Sometimes, lower power provides a better viewing experience, particularly if you are looking at objects that are spread out across the sky, such as clusters or nebulae. 

Telescope eyepieces: power is not the only object.

Your new scope should have at least one eyepiece, and some sets come with two or three. An eyepiece is rated by millimeters (mm), with smaller numbers indicating higher magnification. A 25mm eyepiece is common and appropriate for most beginners.

As mentioned above, a telescope’s power or magnification is not the best indicator of a good scope. As with the whole, so the parts. A higher power eyepiece does not necessarily mean better viewing. It may allow you to see details in a small cluster, for example, but if you use it to look at a nebula, you will find yourself looking at only a portion of the nebula. So, high- and low-power eyepieces each have their place in observing, depending on what interests you.

Also, keep in mind that while a higher magnification eyepiece may provide more details, it may be harder to keep an object in view, unless you are using a motorized mount. They also require the scope to gather more light to provide a clearer image.

A lower power eyepiece makes it easier to find objects and keep them in view. Lower magnification eyepieces require less light, so viewing dimmer objects is easier.

Refractor or reflector telescope: what's the difference?

The two most common types of telescopes available to amateurs are refractors and reflectors. A refractor uses two lenses. The larger of the two is at one end; it's called the "objective". On the other end is the lens you look through, called the "ocular" or the "eyepiece". A reflector gathers light at the bottom of the telescope 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.

Telescope aperture size determines what you'll see.

The aperture of a scope 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 ability to gather light is directly proportional to the size of its aperture and the more light a scope can gather, the better the image you will see.

OK, so you’re thinking, "I’ll just buy the biggest telescope I can afford." Unless you can afford to invest in your own observatory as well, don’t go too big. A small scope you can transport will probably get used a lot more use than a larger one you don’t feel like hauling around.

Typically, 2.4-inch (60-mm) and 3.1-inch (80-mm) refractors and 4.5-inch (114-mm) and 6-inch (152-mm) reflectors are popular for most amateurs.

Telescope Focal Ratio.

The focal ratio of a telescope is calculated by dividing aperture size into its focal length. 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 f10.

While a higher focal ratio does not always mean a higher quality image, it often means as good an image for similar cost. However, a higher focal ratio with the same size aperture means a longer scope, which can translate into a telescope you have to wrestle with a bit more to get into your car or truck. 

A good telescope mount is worth the money. 

It’s likely you never even considered a mount when you thought of buying a telescope. Most people don’t. However, the mount is a very important part of a scope. It's a stand that holds the telescope steady. It is extremely difficult, if not impossible, to view a distant object if the scope is not very steady and wobbles at the slightest touch (or worse, in the wind!). So, invest in a good telescope mount. 

There are basically 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). The equatorial is 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 your field of view longer. Many equatorial mounts come with small computers, which aim the scope automatically.

Caveat Emptor, even for a telescope.

Yes, let the buyer beware. This is as true today as it ever has been in the past. It also applies to purchasing a telescope. Just as with any other product, it is almost always true that "you get what you pay for." A cheap department-store scope will almost certainly be a waste of money. 

The truth is that most people do not need an expensive scope, It's better to buy the best you can for the money, but don't get suckered in by cheap deals at stores that don't specialize in scopes. 

Being a knowledgeable consumer is key, no matter what you are buying. Read everything you can find about scopes, both in telescope books and in articles online about what you really need for stargazing. Ask friends to let you try out their observing equipment. Before you go shopping, learn as much as you can about telescopes.

Happy Viewing!

Edited and updated by Carolyn Collins Petersen.