Everything Science Grade 11

CHAPTER 13. GEOMETRICAL OPTICS 13.4

Primary Lens Eyepiece

Figure 13.19: Layout oflenses in a refracting telescope

Reflecting Telescopes ESBGA

Some telescopes use mirrors as well as lensesand are called reflecting telescopes. Specifically, a
reflectingtelescope uses a convex lens and two mirrors to make an object appear larger. (Figure 13.20.)

Light is collected by theprimary mirror, which is large and concave. Parallel rays travelling toward
this mirror are reflectedand focused to a point.The secondary plane mirror is placed within thefocal
length of the primary mirror. This changes the direction of the light. A final eyepiece lens diverges the
rays so that they are parallel when they reach your eye.

F 1

F 2

Primary Mirror

Secondary Mirror

Eyepiece

Figure 13.20: Lenses and mirrors in a reflectingtelescope.

Southern African Large Telescope ESBGB

The Southern African Large Telescope (SALT) is the largest single opticaltelescope in the southern
hemisphere, with a hexagonal mirror array 11 metres across. SALT is located in Sutherland in the
Northern Cape. SALT isable to record images ofdistant stars, galaxies and quasars a billion timestoo
faint to be seen with theunaided eye. This is equivalent to a person beingable to see a candle flame
on the moon.