In order to see the stars better, astronomers use a variety of telescopes. The first kind was a refracting ‘scope that gave Galileo his shocking discovery of moons orbiting Jupiter.
A refracting telescope works with two lenses that are held steady at the correct relative distances. Often a solid tube is used to keep the lenses in place (not to keep the light inside!); however, an open web of trusses is just as effective. The larger of the two lenses, located at the end away from the viewer and pointed toward the sky, is the objective lens; the smaller, next to the viewer, is the eyepiece lens. The two lenses work together to gather and focus the light.
Light radiates out from its source in a straight line until it hits an object that stops or bends it. When that object is translucent (that lets light transit it), the light is bent—or refracted—in transit. If the object is flat like a window, the refracted light is displaced slightly but resumes its path on a new plane parallel to its original path.
The path of the exiting light can be modified with a curved translucent object, called a lens, that will direct and concentrate the light at a point. The distance between that point and the objective is the focal length of the objective.
The smaller eyepiece lens is located beyond the focal point of the objective at the distance where it will concentrate and restraighten the original light. That is its focal point. The eyepiece lens will also magnify the image that was brought to focus by the objective. The amount of magnification is related to the focal length of the eyepiece. That is, magnification equals the objective focal length divided by the eyepiece focal length: If the objective focal length is 100 inches and the eyepiece is one inch, the magnification is 100x.
The bigger the lenses the more light the ‘scope collects. The biggest reflector, the Yerkes telescope objective has a diameter of 40 inches; the whole ‘scope is housed in a 63½ foot tube.
Keeping a 63-foot ‘scope rigid is only one problem for refractors. It’s also hard to make a large piece of glass without any air bubbles. When light hits a bubble, it bounces slightly and therefore is distorted. Another problem is that some wavelengths of light won’t go through even the purest glass. And then, because different colors have different focal lengths, a second lens has to be added to bring the colors to the same focal point.
Largely because of the problem of the blurred colors in his ‘scope, Isaac Newton came up with another solution to the question of how to capture and concentrate the light from a star.