110 PART 1^ |^ EXPLORING THE SKY
The traditional telescopes described on this page are limited
by complexity, weight, and Earth’s atmosphere. Modern
solutions are shown on the opposite page.
In larger telescopes the light can be focused to a prime focus
position high in the telescope tube as shown at the right.
Although it is a good place to image faint objects, the
prime focus is inconvenient for large instruments.
Asecondary mirror can reflect the light
through a hole in the primary mirror to
aCassegrain focus. This focal
arrangement may be the most
common form of astronomical
telescope.
Secondary
mirrorThe Cassegrain
focus is convenient
and has room for large
instruments.With the
secondary mirror
removed, the light
converges at the prime
focus. In large telescopes,
astronomers can ride inside the
prime-focus cage, although most
observations are now made by instruments
connected to computers in a separate control
room.Traditional mirrors are thick to prevent the optical surface
from sagging and distorting the image as the telescope is
moved around the sky. Large mirrors can weigh many tons and
are expensive to make and difficult to support. Also, they cool
slowly at nightfall. Expansion and contraction in the
cooling mirror causes distortion in the
images.1
1a1c1bNewtonian
focusSchmidt-Cassegrain
telescopeThin
correcting
lensSecondary
mirror
Primary
mirror
(inside)Prime
focus
cageAURA/NOAO/NSFCassegrain
focus
AstronomerMany small telescopes
such as the one on your
left use a Schmidt-Cassegrain
focus.A thin correcting plate
improves the image but is too
slightly curved to introduce
serious chromatic aberration.Smaller telescopes are often
found with a Newtonian focus,
the arrangement that Isaac Newton
used in his first reflecting telescope.
The Newtonian focus is inconvenient
for large telescopes as shown at right.Shown below, the 4-meter Mayall
Telescope at Kitt Peak National
Observatory in Arizona can be used at
either the prime focus or the Cassegrain
focus. Note the human figure at lower right.