Choosing a Telescope

HISTORY

The first telescope was invented by a man named Hans Lippershey in 1608. He developed the telescope solely for use on his ship, to see other possible invading ships from a distance. Following Lippershey’s invention of the telescope, Gallei Galileo was the first to point the scope at the night sky to view and study astronomical bodies. Today there are two well-known types of telescopes, refracting and reflecting. Both of these telescopes appear physically different but have the same overall purpose, to collect as much light (photons) from outer space as possible to better observe the sky.

REFRACTING TELESCOPES

Eyepieces, binoculars, eyeglasses and refracting telescopes all use lenses to focus incoming light. As light enters the glass, it is suddenly slowed down and focused, also known as refracting. Refracting telescopes can use two different lenses to refract the light, convex and concave. If you do not recall from elementary school, a concave lens is thicker around the edges than the center while a convex lens is thicker in the center than the edges.   Large convex lenses are called objective lenses and are used in telescopes instead of a primary mirror. Also, a smaller lens is used as the eyepiece lens where the image is magnified for viewing. Calculation of a refracting telescopes magnification power is the same as for a reflecting telescope.

Although refracting telescopes are popular with amateur astronomers because of the affordability and easiness to build, these telescopes suffer from a variety of problems that limit their use as research instruments:

1. Chromatic aberration is where different colors of light are refracted by different amounts, resulting in different colors having different focal lengths. This causes the image to appear blurry, or 3d like when you don’t put the glasses on.
2. Air bubbles in the glass will also cause distorted images
3. The lens can only be supported around the edges to avoid blocking any incoming light. If the lens is large, it may cause it to sag, and in turn, distort the image.
4. Glass does not allow all ranges of wavelength through it, it is opaque to some wavelengths.

REFLECTING TELESCOPES

Isaac Newton built the first reflecting telescope in the seventeenth century.  The basic principle of reflection is that the angle between the incoming light and the perpendicular line to the mirror is equal to the angle of the outgoing/refracted light and the perpendicular line to the mirror. Newton discovered, by using this principle, that by using a concave mirror, all of the light rays merge to a focal point within the telescope. The distance between this point and the primary mirror is called the focal length. A secondary mirror must be used to reflect the light rays to one side of the telescope so that the astronomer can view the image through an eyepiece lens.