6 Things To Know About Telescopes Before you Buy

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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.

Telescopes give skygazers a great way to see magnified views of objects in the sky. Buying that first (or second or fifth) telescope is an exciting moment, so it's a great idea to be fully informed before heading out to the stores.  There's a lot to learn and a telescope is a long-term investment, so it's important to learn the terminology.

Practice setting up the telescope before use.
A telescope with eyepiece (lower end), finderscope,and a good mount are important for long-term enjoyment of stargazing.  Andy Crawford/Getty Images

Power Is Overrated

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, which is important. However, the light gathered by the scope is spread over a larger area which creates a fainter image in the eyepiece. So, it's important to keep that in mind. Also, "high-powered" scopes have specific requirements for eyepieces, so it's a good idea to check out what eyepieces work best with a given instrument. Sometimes, lower power provides a better viewing experience, particularly if observers are checking out objects that are spread out across the sky, such as clusters or nebulae. 

Telescope Eyepieces: Power is Not the Only Object

Any new telescope 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 it's used to look at a nebula, it will only show a part of the object. So, high- and low-power eyepieces each have their place in observing, depending on what interests the stargazer.

It's also important to remember that while a higher magnification eyepiece may provide more details, it may be harder to keep an object in view. To get the steadiest seeing in such cases, it's important to use a motorized mount.

Remember: 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 the observer looks 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 the View

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 an observer will see.

At this point, many beginners just think: "I’ll just buy the biggest telescope I can afford." Unless they can afford to invest in their own observatory as well, they should probably not go "too big" when it comes to an instrument. A small scope that can be transported easily will probably get used a lot more than a larger one that takes some effort to haul 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 f/10.

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 someone has to wrestle with a bit more to get into a vehicle for transport to a viewing area. 

Get a Good Telescope Mount is worth the money. 

The mount for a scope is just as important as the tube and optics. Most people don’t think of that when they decide to buy a scope unless they've done their homework. The mount is 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, it's important to invest in a good, solid 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 the 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 the purchase of 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, and it's better to buy the best one for the budget. However, it's also important to avoid getting suckered in by cheap deals at stores that don't specialize in scopes. 

Being a knowledgeable consumer is key. Read everything about scopes, both in telescope books and in articles online about what is really needed for stargazing. Go to astronomy club meetings and do a little "parasitic viewing" (of course, always ask permission), and ask for advice about telescopes. Most members love to give advice, and there's no better way to experiment with different models of instruments.

Edited and updated by Carolyn Collins Petersen.