North Hawaii News Articles from CFHT
A Rainbow of Telescopes
There are currently 12 telescopes operating at the summit of Mauna
Kea, soon to be 13 when the Submillimeter Array is completed around
the beginning of next year. People often ask why there are so many
telescopes, and wouldn't it be better to have just one big telescope?
There are really two answers to this. The first answer is that it is
important to have many telescopes because the sky is so large, and one
telescope only sees a small area at a time - depending on the type of
observation, sometimes only one star at a time!
However, a more important reason so many telescopes are needed is
because there are many different kinds of telescopes. The most
important difference between many of the telescopes on Mauna Kea is
the type of 'light' the telescope is designed to observe.
Until the 20th century, astronomers could only use their telescopes to
observe stars and galaxies by the visible light they emit. However,
this is a very severe restriction because visible light is only a
small portion of the type of 'light' emitted by astronomical objects.
To understand different types of 'light', what astronomers and
physicists call 'electromagnetic radiation', it is useful to think of a
rainbow and a piano keyboard. You probably know that there are 88
keys on a piano keyboard, arranged with the lowest notes on the left
and the highest notes on the right. The pitch of a note is what an
astronomer calls the 'frequency' of the note.
Just like sound, light also comes in different frequencies. If you
think of a rainbow, you know that the rainbow has colors running from
red on one edge through orange, yellow, green, blue, to purple. This
is light arranged in order of frequency, from the lowest to the
highest, just like on a piano keyboard.
In one sense, a scale on the piano is a lot like a rainbow. If you
start at the C key and play all the white keys D, E, etc, until the
next C key, you will have played a major scale. You will also have
played the keys in one octave, and it turns out that the frequency of
the last note you played is twice the frequency of the first note. By
accident, it turns out that the frequency of the purple light in the
rainbow is just about twice the frequency of the red light in the
rainbow. We can pretend, then, that the colors of light correspond to
the keys of one octave on the piano keyboard.
One octave is just a small fraction of the piano keyboard, and in the
same way, visible light is just a small part of the full rainbow.
There is light with a frequency lower than red and higher than purple.
The rainbow continues on both sides with frequencies of light
invisible to our eyes. There are somewhat arbitrary names for
different parts of the rainbow, or the electromagnetic spectrum. The
radio waves your FM receiver picks up have a frequency about 5 million
times lower than the red part of the rainbow. A microwave oven uses
radio waves with frequencies about 1000 times higher than the FM radio
stations. Those 'heat-lamps' that some restaurants use to keep food
warm emit infrared radiation, most of which has a frequency of roughly
one tenth of the frequency of the red light. On the other end, the
X-rays used by your dentist have a frequency about 1000 times higher
than the purple light in the rainbow. The ultraviolet radiation that
causes sunburn is just a bit higher frequency than the purple you can
see. All of these are examples of 'electromagnetic radiation', and
modern astronomers use all of them to study their objects.
An observation of a star or galaxy in different parts of the
electromagnetic spectrum can tell an astronomer about different
aspects of that object. The general rule is that, the higher the
energy in the system, or the higher the temperature, the higher the
frequency of the light which is emitted. So, cool clouds of dust and
gas where stars form emit most of their light in the microwave or
infrared region. In the same way, cool stars are red, intermediate
stars like the sun are yellowish, and really hot stars are bluish.
Some of the really extreme environments in space, like the
million-degree surface of a neutron star or the gas involved in a
supernova explosion emit most of their energy in the X-ray range.
On Mauna Kea, many of the telescopes work mostly with visible light, or
light emitted just a little in the infrared. Two telescopes, United
Kingdom Infrared Telescope (UKIRT) and Nasa's Infrared Telescope
Facility (IRTF) were built exclusively for infrared observations and
have certain features to make such observations efficiently. Two
other telescopes, the James-Clerk-Maxwell Telescope and the Caltech
Submillimeter Observatory were designed to observe microwave
radiation, and are used to study very cool dust clouds or the gas in
such clouds. The Smithsonian Millimiter Array (SMA), still under
construction, will be used to observe radio emission with somewhat
lower frequency than JCMT or CSO, but with much greater detail.
Finally, there is a radio receiver which is part of the National Radio
Astronomical Observatory (NRAO) Very Large Baseline Array (VLBA).
This telescope, which observes radiation that is only about 100 times
higher frequency than your car radio, is connected to 9 other
telescopes throughout mainland, and as far away as the U.S. Virgin
Islands. Together these telescopes act as a single radio telescope,
producing images with a resolution one hundred times higher than even
the Hubble Space Telescope.
One of the many special aspects of Mauna Kea is the combination of
high altitude and very dry air. It turns out that water vapor is a
serious problem for observations in the infrared and microwave
regions. Since the air above Mauna Kea is so dry, this site is one of
the best places in the world to perform observations of this sort.
The two of the other places best suited to these observations are
extremely remote: Antarctica and a high desert plateau in Chile called
the Atacama. For more exotic types of observations, it is necessary
to go to the ultimate observing location, but also the ultimately
remote site of outer space.
For more on these topics, see the web pages of the telescopes
mentioned above, links to which can be found on the web pages of the
University of Hawaii Institute for Astronomy:
www.ifa.hawaii.edu
Eugene Magnier