Figure 23.7: The South African Large Telescope (SALT) is one of the largest reflecting
telescopes on Earth. SALT’s primary mirror consists of 91 smaller hexagonal mirrors, each
with sides 1 m long. ( 21 )
catadioptric telescopes can be made with large diameters but shorter lengths so they are less
awkward to move around.Figure23.8shows a typical catadioptric telescope.
Radio Telescopes
Noticeitsaysabovethatthelargestopticaltelescopesintheworldarereflectors. Opticaltele-
scopes are designed to collect visible light. There are even larger telescopes that collect light
at longer wavelengths—radio waves. These telescopes are called—can you guess?—radio
telescopes. Radio telescopes look a lot like satellite dishes. In fact, both are designed to
do the same thing—to collect and focus radio waves or microwaves from space.
The largest single telescope in the world is at the Arecibo Observatory in Puerto Rico (see
Figure23.9). This telescope is located in a naturally-occurring sinkhole that formed when
water flowing underground dissolved the limestone rock. If this telescope were not supported
by the ground, it would collapse under its own weight. The downside of this design is that
the telescope cannot be aimed to different parts of the sky—it can only observe the part of
the sky that happens to be overhead at a given time.
A group of radio telescopes, such as those shown inFigure23.10, can be linked together
with a computer so that they are all observing the same object. The computer can combine
the data from each telescope, making the group function like one single telescope.