Celestron CR-150 HD instruction manual The Refractor Optical System, The Mount, Introduction

The Refractor Optical System

Developed in the early 1600s, the refractor is the oldest telescope design. It derives its name from the method it uses to focus incoming light rays. The refractor uses a lens to bend or refract incoming light rays, hence the name (see Figure 1-1). Early designs used single element lenses. However, the single lens acts like a prism and breaks light down into the colors of the rainbow, a phenom- enon known as chromatic aberration. To get around this problem, a two-element lens, known as an achromat, was introduced. Each element has a different index of refraction allowing two different wavelengths of light to be focused at the same point. Most two-element lenses, usually made of crown and flint glasses, are corrected for red and green light. Blue light may still be focused at a slightly different point.

Figure 1-1

This cross sectional diagram shows the light path of a classical two-element refractor.

The Mount

In the world of astronomy, there are basically two different types of mounts; Altazimuth and equatorial. Both derive their names from the axial motion of the mount. The Altazimuth moves vertically (altitude) and horizontally (azimuth) relative to the horizon. The equatorial mount has its axis of rotation tilted so that it can be aligned with the Earth’s axis of rotation. When this is done, the telescope (when fitted with an optional clock drive) can track the stars. The directional movements on this mount are North/South and East/West. The Celestron CR- 150 HD, 6 inch refractor uses a German equatorial mount called the CG-5 mount.

6 • Introduction