Telescopes: Reflecting and Refracting

by Jeanette Cain

In 1609, Galileo was the first person to recognize that the right combination of lenses could be used to visually view the heavens. Although Galileo did not invent the telescope, he did make discoveries of Moon craters, Milky Way stars, and four of the moons around Jupiter. Isaac Newton was the next to investigate the lens and how light reflected and refracted. Newton decided that the lenses would always have colored fringe around the images, and designed a telescope that collected light with mirrors. In 1668, Newton built the reflecting telescope made of a solid metal mirror. The mirror was composed of tin, arsenic, and copper.

For astronomers, light from the sky is a source of information from the planets, stars, and nebulae composing the universe. The more light a telescope can collect, the more information it will give of the universe. The two types of telescopes are the reflectors, which capture light using a mirror, and the refractors, which use a lens. The more modern telescopes have reflectors with mirrors located on a mountaintop, since mountains provide less interference from the atmospheric distortions.

The Gemini Telescope is a reflector. Light is caught by the large curved mirror. The image is then capable of being reflected toward any telescope part with help from secondary mirrors. With this telescope, the data-recording equipment does not need to be a part of a moving telescope. The two key advantages of reflector telescopes are the ability to collect light with a mirror to prevent color fringe, and provide the support for the mirror to be on its back, the telescope size to be unlimited.

A refracting telescope catches the light with a lens, then focuses the image onto an electronic light detector, or photographic plate. Although the image is upside down, it makes no difference in astronomy. Refractors are good for looking at bright objects, but also absorb light from the fainter objects. The refractor lens will focus different light colors at different points, but cause color fringing. With the heavy weight of the lens, any lens over 1 m across will bend under its own weight.

The Schmidt telescope is both a reflector and a refractor with its mirrors reflecting star light onto a curve photographic plate. This curved plate allows for a wide angle view, but the lens will need help to be rid of distortions. It has the ability to capture an image of large areas of the sky, which helps with tracking down objects for the reflectors to center on, and provides a sky survey.

The AAT, stands for the Anglo-Australian Telescope, Reflector is in the Warrambungle Mountains of Australia and has a mirror 3.9 m in diameter. An astronomer can sit in a cage to take photographs when the secondary mirror is replaced. Hawaii and Chile have the twin Gemini 8-M reflector telescopes, and are used by astronomers of seven countries.

Sources:

1. Couper, Heather and Nigel Henbest. Space Encyclopedia DK Publishing, Inc.: NY 1999

2. Editors. Secrets of the Universe. International Master Publishing: US. 1999