The Basics of Telescopes

A typical type of backyard-type telescope.
This is a typical type of backyard-type telescope. NASA

Sooner or later, every stargazer decides it's time to be buying a telescope. It's an exciting next step to further exploration of the cosmos. However, as with any other major purchase, there's a lot to learn about these "universe exploration" engines, ranging from power to price. The first thing a user wants to do is figure out their observational goals. Are they interested in planetary observing? Deep-sky exploration? Astrophotography? A little of everything? How much money do they want to spend? Knowing the answer to those questions will help narrow down a telescope choice.

Telescopes come in three basic designs: refractor, reflector, and catadioptric, plus some variations on each of the types. Each has its pluses and minuses, and of course, each type can cost a little or a lot depending on the quality of the optics and the accessories needed.  

Refractors and How They Work

A refractor is a telescope that uses two lenses to deliver a view of a celestial object. At one end (the one farther away from the viewer), it has a large lens, called the "objective lens" or "object glass." On the other end is the lens the user looks through. It is called the "ocular" or "eyepiece." They work together to deliver the sky view.

The objective collects light and focuses it as a sharp image. This image gets magnified and is what the stargazer sees through the ocular. This eyepiece is adjusted by sliding it in and out of the telescope body to focus the image.

Reflectors and How They Work

A reflector works a bit differently. Light is gathered at the bottom of the scope by a concave mirror, called the primary. The primary has a parabolic shape. There are several ways the primary can focus the light, and how it is done determines the type of reflecting telescope.

Many observatory telescopes, such as Gemini in Hawai'i or the orbiting Hubble Space Telescope use a photographic plate to focus the image. Called the "prime focus position", the plate is located near the top of the scope. Other such scopes use a secondary mirror, placed in a similar position as the photographic plate, to reflect the image back down the body of the scope, where it is viewed through a hole in the primary mirror. This is known as a Cassegrain focus. 

Newtonians and How They Work

Then, there's the Newtonian, a kind of reflecting telescope. It got its name when Sir Isaac Newton dreamed up the basic design. In a Newtonian telescope, a flat mirror is placed at an angle in the same position as the secondary mirror in a Cassegrain. This secondary mirror focuses the image into an eyepiece located in the side of the tube, near the top of the scope.

Catadioptric Telescopes

Finally, there are catadioptric telescopes, which combine elements of refractors and reflectors in their design. The first such telescope was created by German astronomer Bernhard Schmidt in 1930. It used a primary mirror at the back of the telescope with a glass corrector plate in the front of the telescope, which was designed to remove spherical aberration. In the original telescope, photographic film was placed at the prime focus. There were no secondary mirror or eyepieces. The descendant of that original design, called the Schmidt-Cassegrain design, is the most popular type of telescope. Invented in the 1960s, it has a secondary mirror that bounces light through a hole in the primary mirror to an eyepiece.

The second style of catadioptric telescope was invented by a Russian astronomer, D. Maksutov. (A Dutch astronomer, A. Bouwers, created a similar design in 1941, before Maksutov.) In the Maksutov telescope, a more spherical corrector lens than in the Schmidt is utilized. Otherwise, the designs are quite similar. Today’s models are known as Maksutov –Cassegrain.

Refractor Telescope Advantages and Disadvantages

After initial alignment, which is necessary to have the optics work well together, refractor optics are resistant to misalignment. The glass surfaces are sealed inside the tube and rarely need cleaning. The sealing also minimizes effects from air currents that can muddy the view. This is one way that users can get steady sharp views of the sky. Disadvantages include a number of possible aberrations of the lenses. Also, since lenses need to be edge supported, this limits the size of any refractor.

Reflector Telescope Advantages and Disadvantages

Reflectors do not suffer from chromatic aberration. Their mirrors are easier to build without defects than lenses are since only one side of a mirror is used. Also, because the support for a mirror is from the back, very large mirrors can be built, making larger scopes. The disadvantages include ease of misalignment, the need for frequent cleaning, and possible spherical aberration, which is a defect in the actual lens that can blur the view.

Once a user has a basic understanding of the types of scopes on the market, they can focus on getting the right-sized one to view their favorite targets with. They can learn more about some mid-range-priced telescopes on the market. It never hurts to browse the marketplace and learn more about specific instruments. And, the best way to "sample" different telescopes is to go to a star party and ask other scope owners if they're willing to let someone take a look through their instruments. It's an easy way to compare and contrast the view through different instruments.

Edited and updated by Carolyn Collins Petersen.