Reading: "The Telescope" by Chris Impey
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 Lippershey may have been the first person to construct a telescope. 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.
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 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.
A second function of a telescope is its resolution -- 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, you would see that a telescope with a diameter of 4 meters should have a resolution of 0.02 second of arc. However, in practice, the blurring of incoming light due to turbulent motions in the Earth's atmosphere at most sites reduces the resolution to about 1 second of arc. Many backyard telescopes are large enough to resolve to this one second of arc limit. 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.
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. Thus modern astronomers rarely look through their giant telescopes!
Telescopes have a similar function regardless of the wavelength of radiation collected. Radio, infrared, optical, ultraviolet, and X-ray telescopes all must gather radiation and bring it to a focus to form an image. Detectors are placed at the focus of a telescope. Again, regardless of the wavelength of observation, a detector should register the photons at the focus as efficiently as possible.
Author: Chris Impey
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 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.
A second function of a telescope is its resolution -- 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, you would see that a telescope with a diameter of 4 meters should have a resolution of 0.02 second of arc. However, in practice, the blurring of incoming light due to turbulent motions in the Earth's atmosphere at most sites reduces the resolution to about 1 second of arc. Many backyard telescopes are large enough to resolve to this one second of arc limit. 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.
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. Thus modern astronomers rarely look through their giant telescopes!
Telescopes have a similar function regardless of the wavelength of radiation collected. Radio, infrared, optical, ultraviolet, and X-ray telescopes all must gather radiation and bring it to a focus to form an image. Detectors are placed at the focus of a telescope. Again, regardless of the wavelength of observation, a detector should register the photons at the focus as efficiently as possible.
Author: Chris Impey
Astropedia by Astropedia/Chris Impey is licensed under a Creative Commons Attribution-NonCommercial-NoDerivs 3.0 Unported License.
Last modified: Monday, August 30, 2021, 9:47 AM