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:

Eugene Magnier