Celestron CR-150 HD User Manual

Page 6

6 • Introduction

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.

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.

Figure 1-1

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

The Refractor Optical

The Refractor Optical
System

System

The Mount