Page 287 - NS-2 Textbook

282 NAUTICAL SCIENCES

are located at the Mauna Kea Observatory (two tele-
~ pJeyePiece lens
scopes with 10.2-meter segmented mirrors) and at the
Canary Islands in the Atlantic (a lOA-meter segmented
objective mirror). For the future, sponsors such as the European
flat mirror ~ mirror Southern Observatory have proposed to build an im-
mense 100-meter reflecting telescope of this design
(called appropriately the Overwhelmingly Large Tele-
Newtonian reflecting telescope
scope [OWL]), at a location yet to be determined.
eyepiece lens
Because of their huge size and great precision, not-
I withstanding some ahnospheric distortion, all these latest
telescopes are able to greatly exceed the light-gathering
convex mirror
objective capability and resolution of smaller space-based telescopes
mirror
like the Hubble. Thus, they are enabling Earth-bound as-
tronomers to make observations of such things as planets
Cassegrain reflecting telescope
orbiting distant stars and galaxies near the edges of the
Reflecting telescopes may be either Newtonian or Cassegrain. A universe that would have been impossible only a few
Newtonian reflector has a small, flat secondary mirror that focuses
the image to the side, while the Cassegrain has a convex secondary years ago.
mirror that focuses the image to the bottom of the telescope.
THE SPECTRUM
In the Cassegraill reflector, the secondary mirror causes the By gathering light from celestial bodies and analyzing the
light to focus behind the objective mirror. Thus the ob- colors it contains, astronomers can determine the amolmt
jective mirror must have a hole in the center to allow of hydrogen, helitun, and other elements in the body.
light to pass through. The eyepiece is then placed at the TIlese light rays, though sometimes very weak, carry with
bottom of the telescopic tube. Cassegrain reflectors are them the complete story of their birth. This story is told
nluch more expensive than Newtonian reflectors and are by their spectru1ll, the rainbow of colors that can be pro-
specially designed for telescopes with large mirrors. duced from white light by a prism. TIle spectrograph,
The world's largest conventional reflecting telescope which is attached directly to a telescope, breaks up in-
with a single one-piece primary mirror in existence today coming light into its component wavelengths or colors to
is the 327-in (S.3-meter) Subaru telescope at the Mauna be photographed. It filters light through a prism, which
Kea Observatory in Hawaii. In 2005 a side-by-side assem- divides light into the separate colors of the spectrum.
bly of two similar but slightly larger 33l-in (SA-meter) di- TIle electromagnetic spectrum, arranged in order of
ameter ntirrors became operational at the Mt. Graham In- increasing wavelength, ranges from gamma rays on the
ternational Observatory near Safford, Arizona. Called the short end of the scale through x-rays, ultraviolet rays, vis-
Large Binocular Telescope! it can achieve the same image ible light, infrared light, and radio waves. Visible light is
sharpness as a 22.S-meter mirror. Each of the cast glass only a small part of the total spectrum. Except for certain
mirrors weighs over 16 tons, and is almost a meter thick. radio \,\Taves, most of the other types of electromagnetic
Another variation of the same idea is the recently com- radiation cannot penetrate Earth's atmospheric shield,
pleted Very Large Telescope (VLT) at a European South- which is what makes space-based telescopes like the
ern Observatory in northern Chile. This can combine the Hubble so valuable.
images of an array of fom separate S.2-meter telescopes in The photograph that records the color bands in the
a technique called optical interferometry to achieve the spech'um is called a spectrogra1ll. When analyzed, it tells
same light-gathering ability as a l6-meter mirror. scientists which elements produced the light. Each chem-
To lower the high cost and difficulty of casting one- ical element gives off its own pattern of colors. Light from
piece mirrors as large as these, several other recently con- vaporized iron gives out one pattern, while hydrogen, he-
structed large reflecting telescopes have mirrors that con- lium, oxygen, and the others each emit different patterns.
sist of a munber of smaller ntirrored hexagons called These color patterns are clearly visible in the spectrum of
segments. Each of these is controlled by a computer to incoming light from celestial bodies. TIlis is one way the
within a millionth of an inch precision, and the computer astronomer can tell that the Sun} planets, and stars are
software can make continuous tiny adjustments to the made of the same elements that we find here on Earth.
overall shape of the mirror to compensate for abnos-
pheric refraction. One such telescope is the Southern THE RADIOTELESCOPE
African Large Telescope (SALT) located at Sutherland,
South Africa. It has a 36-foot (ll-meter) mirror made up In the early 1900s, astronomers discovered that in addition
of ninety-one hexagonal segments. Other such telescopes to radiation in the visible light spectrum, many celestial