Chapter 10: Detecting Radiation from Space

The 4 main telescopes of the VLT, and the 4 auxiliary telescopes in Paranal, Chile

For most of human history, we have learned about the universe with our eyes. Around 400 years ago Dutch opticians learned to place two lenses together to make a telescope. The evidence is not convincing, but a Dutchman called Hans Lippershey may have been the first person to construct a telescope. He at least filed a patent, though there were several others who experimented with basic telescopes around the same time. Galileo used this new invention to magnify and sharpen our view of the heavens and provide vital evidence in support of the Copernican hypothesis. Astronomers have been building larger and larger telescopes ever since. Astronomy has been transformed again in the past few decades. Advances in technology have changed the way we detect radiation and have enabled us to see the full range of the electromagnetic spectrum.

Hubble Space Telescope as seen from the Space Shuttle Atlantis

The important features of a telescope have not changed since the time of Galileo. The most important function is to collect light. This is controlled by the collecting area. Telescopes have the ability to gather light and reveal fainter objects than the naked eye can see. When light from a distant planet or star reaches Earth, a certain number of photons strike each square centimeter of Earth each second. The pupil of the eye has a diameter of less than half a centimeter and can receive only a limited number of photons per second. But a telescope collects the photons that strike a lens or mirror many centimeters across. When all of this light is brought to a focus it produces a much greater level of illumination. The light-gathering power depends on the collecting area. The collecting area increases in proportion to the square of the diameter of the light-gathering device. Increasing the light gathering power helps us collect more information.

Amateur Refracting Telescope

A second function of a telescope is its resolution or the ability to discriminate fine detail. Whereas the eye can resolve angular details only a few minutes of arc across, a telescope might show details a hundred times smaller, or only a second of arc across. The resolution of a telescope is proportional to the ratio of the wavelength of the light being measured to the diameter of the telescope. In equation form, the resolution in arc seconds is 250,000 × (Wavelength / Diameter), where the wavelength and diameter can be expressed in any units as long as they are the same. Larger telescopes can resolve smaller angles, and are said to have higher resolution. If you use this equation for the human eye, which has a pupil size of about 4 millimeters, the resolution is 200 seconds or arc, or about 3 arc minutes. The Moon is 30 arcminutes across (half a degree), so the smallest features you can see on the Moon are about 1/10 of its diameter. Now use the equation for a telescope with a diameter of 4 meters. The aperture is a thousand times bigger than the eye, so the resolution should be a thousand times better, about 0.02 seconds of arc.

Amateur Reflecting Telescope

However, in practice, the blurring of incoming light due to turbulent motions in the Earth's atmosphere at most locations reduces the resolution to about 1 second of arc. The very best high observatory sites in Chile and Hawaii may have "seeing" three times better, or 0.3 seconds of arc. Astronomers use the term seeing to describe the sharpness of images before they reach the telescope. Many backyard telescopes are large enough to resolve to this one second of arc limit. In fact, telescopes larger than 30 cm in diameter (about a foot) are unable to deliver images as good as their optics can deliver in principle; their resolution is in a sense "wasted." For this reason, astronomers go to the great expense of launching telescopes into Earth orbit to be immune from the atmosphere. They had also devised clever methods to correct for the blurring effect of the atmosphere and so deliver the theoretical resolution of their telescope optics. To get a sense of this level of angular resolution, remember that one second of arc is the angle subtended by a dime at a distance of 1.5 miles.

Very Large Array radio telescopes in New Mexico, USA

Telescopes designed for visual observation have another function, which is to magnify an object; magnification is the apparent angular size of a distant object seen through the eyepiece relative to its apparent size seen by the naked eye. If a telescope makes something look 10 times larger, we say it has a magnification of 10×. Note that magnification is not a fundamental property like the first two we described. When an image is magnified, the blurring is magnified too. Also, a magnified image is dimmer since the same amount of collected light is spread over a larger area. Early telescopes were designed entirely for observers to look through. In modern professional telescopes, the human eye is replaced by instruments that can make more precise measurements. Modern astronomers rarely look through their giant telescopes!