Bad Astronomy: Misconceptions and Misuses Revealed, from ...

HUBBLE TROUBLE 225
The Hubble Space Telescope floats freely over the Earth, prepared to take
another observation of an astronomical object. Despite its clear views of the
universe, the telescope is never more than a few hundred kilometers above
the surface of Earth. (Image courtesy NASA and the Space Telescope Science
Institute.)
design for a mirrored telescope is still called a Newtonian. Four
hundred years later, it still causes confusion.
Lenses are good for smaller telescopes but become unwieldy
when they are bigger than about a half-meter (20 inches) across.
They have to be supported from the edges, lest you block their
view. Large lenses are extremely heavy, which makes them difficult
to use. They also need to be placed at the aperture of the telescope,
at one end of a long tube. That placement makes the telescope
unwieldy and very temperamental to balance.
Since only the front side of a mirror is needed, it can be supported
all along its backside, making mirrors easier to use. A mirror
reflects light, but a lens has to have light pass through it, which
can dim that light. Even better, when you’re making a mirror, you
only need to grind and polish one side and not two. That’s a pretty
good savings over a lens.