Telescopes - Different Wavelengths

Earth's atmosphere provides one of the largest limits to a telescope's resolving power. Large astronomical are capable of much better resolution than atmospheric turbulence will allow. We discussed the fact that, by themselves, stars don't twinkle. If the Space Station crew looked out their window and could see the stars (with all the interior lights in the Station), they would NOT see twinkling. That romantic twinkling of the stars is purely an atmospheric effect. Astronomers call it "seeing." Bad seeing means lots of twinkling. If the seeing is very good, stars overhead will be seen to NOT twinkle.

Atmospheric opacity in wavelength bands other than visible light plus the turbulence are the reasons astronomers put telescopes in orbit. The Hubble Space Telescope has a resolving power around twenty times better than ground-based telescopes (in visible light). The latest infra-red telescope, known as the Spitzer Space Telescope, is in solar orbit, sharing the Earth's orbit. The Chandra X-ray Telescope is in Earth orbit. Space is even better than any high mountain - there is NO atmosphere to bother with.

Ground-based telescopes are not down and out, however. A new technique, called adaptive optics, is showing great promise. It involves using high-speed computer controls to either wobble or deform a small mirror in the light path to counteract the turbulence and sharpen the image. This was first developed by the military.

There is a modern technique that has allowed imaging that was not possible earlier. Even amateur astronomers are using video (even webcams) to gather images. Video produces 30 frames (images) per second, so just a few minutes of video contains a LOT of images. There is one image every 1/30 second. If the atmosphere settles down and steadies for a few 30ths of a second, the video recording will have a few very sharp images. The observer simply locates the sharp images and stacks them with something like Photoshop. The result is planetary images of spectacular sharpness.

We went on to generalize the description of a telescope; it consists of a concave reflector that collects electromagnetic energy from distant objects and brings it to a focus. Now we are not restricted to light. The important detail for us is that the reflector (mirror) will be made of whatever is appropriate for the wavelength desired. A radio wave reflector for longer wavelengths can be a metal mesh - not solid. This reduces its wind resistance and its weight. A nice shiny silver or aluminum coating that works fine in visible light might not do well in infra-red.

The size of the reflector will be appropriate. Radio energy from distant objects is weak and the wavelength is long. This calls for VERY large reflectors; the largest fully steerable (aimable) reflector is 105 meters (350 feet) in diameter. It is located near Green Bank, West Virginia. The largest radio telescope is 1000 feet in diameter. It is located near Arecibo in Puerto Rico. A very new telescope, known as ALMA (Atacama Large Millimeter Array) is located on the Chajnantor Plain in the Andes Mountains of Chile.

Radio astronomers have one thing going for them - the sky is basically dark at radio wavelengths. This means that they can observe the sky day and night; they are not limited to night only as are optical astronomers (ground-based ones anyway). A radio telescope can be collecting data 24 hours a day, interrupted only by weather conditions that do not allow operation of the telescope.

The Arecibo 1000-foot radio telescope is interesting in that the dish cannot move to follow an object as the earth rotates. The receiving complex, suspended by three cables, actually moves to track the object.

Interferometry is the process of combining the signals from two or more radio telescopes separated by some distance to synthesize a virtual telescope of size equal to that distance. THe VLA in New Mexico (Figure 3.30) is a very famous and important interferometric array. A world-wide arrangement called the VLBA (Very Long Baseline Array) combines data from radion telescopes separated by thousands of kilometers to produce resolving power fifty times better that Hubble can do in visible light.

The Chandra X-Ray telescope is a reflecting telescope in the true sense. It uses a reflecting mirror to focus the x-rays. This might seem strange, as x-rays are known for penetrating things. The x-ray mirrors take advantage of reflection at a low angle. It produces focussed images in x-rays. See Figure 3.27 in your book for an illustration.

Infra-red telescopes pose an engineering challenge - controlling their temperature. Consider what produces infra-red radiation - objects that are glowing because they are warm. Infra-red astronomers are interested in seeing objects that are relatively cool and do not emit much visible light. Suppose you want to see the faint infra-red glow of interstellar dust at 100K. If your telescope is at 200K from sunlight, it will glow brighter than the dust you want to see! It's like building an optical telescope that is lined with light bulbs then trying to observe with the lights on! The only way to solve this problem is to equip the infra-red telescope with a liquid helium cooling system to chill the entire telescope down to 4K! That way the telescope is cooler than the objects to be observed.

Note about the energy of light: the energy of a photon increases as the frequency increases. Recall that, as frequency increases, wavelength decreases. This means that shorter wavelength light is higher energy light.

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