CHAPTER 6 | LIGHT AND TELESCOPES 111
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(^3) 3a Grinding a large mirror may remove
tons of glass and take months, but
new techniques speed the process. Some
large mirrors are cast in a rotating oven
that causes the molten glass to flow to
form a concave upper surface. Grinding
and polishing such a preformed mirror is
much less time consuming.
3b Mirrors made of segments
are economical because the
segments can be made
separately. The resulting mirror
weighs less and cools rapidly.
See image at right.
3d The two Keck telescopes, each 10 meters in diameter, are located atop the volcano Mauna Kea in Hawaii. The two mirrors are
composed of hexagonal mirror segments as shown at right.
3c Both floppy mirrors and segmented mirrors sag under their own weight.
Their optical shape must be controlled by
computer-driven thrusters under the
mirror in what is called active optics.
Unlike traditional thick
mirrors, thin mirrors,
sometimes called floppy mirrors
as shown at right, weigh less and
require less massive support
structures. Also, they cool rapidly
at nightfall and there is less
distortion from uneven expansion
and contraction.
Telescope mountings
must contain a sidereal
drive to move smoothly
westward and counter the
eastward rotation of Earth.
The traditional equatorial
mounting(far left) has a
polar axisparallel to Earth’s
axis, but the modern
alt-azimuth mounting(near
left) moves like a cannon —
up and down and left to right.
Such mountings are simpler
to build but need computer
control to follow the stars.
Westward rotation
about polar axis
follows stars.
Polar axis Eastward
rotation of
Earth
To north
North Pole
celestial pole
To north
celestial pole
Computer control of
motion about both
axes follows stars.
Eastward
rotation of
Earth
To north
celestial pole
Equatorial mounting Alt-azimuth mounting
North Pole
Computer-controlled thrusters Support structure
Computer-controlled thrusters Support structure
Floppy mirror
Segmented mirror
Keck I telescope
mirror segments
W.M. Keck Observatory