There are many objects in the sky you can see with your unaided eyes, but these are only a small fraction of the objects you can see if you use a telescope.
A telescope makes these invisible objects visible in two ways: by making dim objects appear brighter, and by making small objects appear bigger.
First, how does a telescope make dim objects appear brighter?
Every telescope has a primary lens or a primary mirror that gathers light from the object being viewed. The diameter of the lens or mirror is called its aperture. The bigger the lens or mirror — the larger its aperture — the more light it gathers.
An object viewed in a telescope with an eight-inch aperture will appear brighter than when viewed in a six-inch aperture. The aperture determines the brightness of the object, not the amount of magnification.
The lens or mirror, which is also known as the objective, is shaped so that it bends the light to concentrate it and to form an image of the object being viewed.
The distance between the objective and the image is known as the focal length, and the location where the image forms is called the focal plane.
If you place a sheet of paper at the focal plane, you can see this image, but it is small. To be useful, this image needs to be magnified.
The second way a telescope makes objects visible is by making them appear bigger. How does it do this?
In addition to the primary lens or mirror, every telescope also has a second lens, called the eyepiece. The eyepiece works like a magnifying glass.
The objective forms the small image at the focal plane, and the eyepiece is positioned so that it magnifies this image, making the object appear bigger when viewed through the eyepiece.
Different eyepieces magnify the images by different amounts, so the eyepiece that is used determines the magnification of the telescope, and by changing the eyepiece you can change the magnification of the telescope.
The eyepiece’s focal length determines its magnification; a shorter focal length produces higher magnification.
We have seen that the objective of a telescope can be either a lens or a mirror. If a telescope uses a lens to collect the light, it is called a refractor; if it uses a mirror, it is called a reflector.
In a refractor, the lens refracts, or bends, the light to form the image at the focal plane. In the simplest form of refractor, the light travels in a straight line, with the objective lens in the front and the eyepiece at the back.
The refracting telescope was invented in the Netherlands in the early seventeenth century and is the type of telescope used by Galileo to make the first astronomical observations.
But these early refractors had serious problems. Light of different colors is bent a different amount by the objective lens, so the image for each color forms at a different distance from the lens. This creates a fuzzy image.
Another problem is the light has to pass through the lens, so the clarity of the lens also affects the clarity of the image.
Both of these problems are solved if the lens is replaced with a mirror.
In 1672 Isaac Newton designed and built the first reflecting telescope. In a reflector, the light enters the front of the telescope and is reflected by a primary (objective) mirror located at the back of the scope.
The mirror is curved so as to produce an image at the focal plane in front of the mirror.
In Newton’s design, a flat secondary mirror inside the telescope intercepts the reflected light and turns the beam 90 degrees so it exits through the side of the telescope. The eyepiece is mounted at this exit point on the side of the scope.
Today most large optical telescopes use this design.
You can now purchase both refracting and reflecting commercial telescopes to view the faint and small objects in the sky that cannot be seen with your unaided eyes.
The quality of these scopes varies along with the price, but with all the modern improvements, any telescope you buy today will be much better than the ones used by Galileo and Newton.
Marty Scott is the astronomy instructor at Walla Walla University, and also builds telescopes and works with computer simulations. He can be reached at firstname.lastname@example.org.