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Chapter 6Telescopes: Portals of Discovery
Much of this talk is © 2006 Pearson Education Inc, publishing as Addison-Wesley - Do not distribute beyond Physics 5
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How does your eye form an image?
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Refraction• Refraction is the
bending of light when it passes from one substance into another
• Your eye uses refraction to focus light
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Example: Refraction at Sunset
• Sun appears distorted at sunset because of how light bends in Earth’s atmosphere
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Focusing Light
• Refraction can cause parallel light rays to converge to a focus
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Focusing Light
• A camera focuses light like an eye and captures the image with a detector
• The CCD detectors in digital cameras are similar to those used in modern telescopes
Digital cameras detect light with charge-coupled devices (CCDs)
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What are the two most important properties of a telescope?
1. Light-collecting area: Telescopes with a larger collecting area can gather a greater amount of light in a shorter time.
2. Angular resolution: Telescopes that are larger are capable of taking images with greater detail.
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Light Collecting Area
• A telescope’s diameter tells us its light-collecting area: Area = π(diameter/2)2
• The largest telescopes currently in use have a diameter of about 10 meters
• Bigger is better!!
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Thought QuestionHow does the collecting area of a 10-meter telescope compare with that of a 2-meter
telescope?
a) It’s 5 times greater.b) It’s 10 times greater.c) It’s 25 times greater.
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Thought QuestionHow does the collecting area of a 10-meter telescope compare with that of a 2-meter
telescope?
a) It’s 5 times greater.b) It’s 10 times greater.c) It’s 25 times greater.
Area = π(diameter/2)2
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Angular Resolution• The minimum angular
separation that the telescope can distinguish.
• Depends on both separation and distance to us
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Angular Resolution• Ultimate limit to
resolution comes from interference of light waves within a telescope.
• Larger telescopes are capable of greater resolution because there’s less interference
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Angular Resolution• The rings in this image
of a star come from interference of light waves.
• This limit on angular resolution is known as the diffraction limit
Close-up of a star from the HubbleSpace Telescope
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Thought QuestionSuppose two stars are separated in the sky by
0.1 arc-second. If you look at them with a telescope with an angular resolution of 0.01 arc-
second, what do you see?
a) Two distinct stars.b) One point of light that is the blurred image
of both stars.c) Nothing at all.
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Thought QuestionSuppose two stars are separated in the sky by
0.1 arc-second. If you look at them with a telescope with an angular resolution of 0.01 arc-
second, what do you see?
a) Two distinct stars.b) One point of light that is the blurred image
of both stars.c) Nothing at all.
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What are the two basic designs of telescopes?
• Refracting telescope: Focuses light with lenses
• Reflecting telescope: Focuses light with mirrors
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Refracting Telescope
• Refracting telescopes need to be very long, with large, heavy lenses
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Reflecting Telescope
• Reflecting telescopes can have much greater diameters
• Most modern telescopes are reflectors
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All modern telescopes are reflecting telescopes because:
• Only 1 precise surface needs polishing– (as opposed to two surfaces)
• Possible to support mirror on back of glass– (as opposed to being only held by their edges)
• Large lens at bottom of telescope– (as opposed to to at the top of the telescope)
• Possible to reduce chromatic aberration– (lenses bringing different colors of light into focus at
slightly different places)
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Mirrors in Reflecting Telescopes
Twin Keck telescopes on Mauna Kea in Hawaii
Segmented 10-meter mirror of a Keck telescope
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What do astronomers do with telescopes?
• Imaging: Taking pictures of the sky• Spectroscopy: Breaking light into spectra• Timing: Measuring how light output varies
with time
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Imaging• Astronomical
detectors generally record only one color of light at a time
• Several images must be combined to make full-color pictures
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Imaging• Astronomical
detectors can record forms of light our eyes can’t see
• Color is sometimes used to represent different energies of nonvisible light
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Spectroscopy• A spectrograph
separates the different wavelengths of light before they hit the detector
Diffractiongrating breakslight intospectrum
Detectorrecordsspectrum
Light from only one starenters
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Just like a prism
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Spectroscopy• Graphing relative
brightness of light at each wavelength shows the details in a spectrum
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Timing
• A light curve represents a series of brightness measurements made over a period of time
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Discuss with someone near you• What are the two most important properties of a
telescope?– ??
– ??
• What are the two basic designs of telescopes?– ??– ??
If no one is near you, move next to someoneGroups of two please
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Discuss with someone near you• What are the two most important properties of a
telescope?– Collecting area determines how much light a
telescope can gather– Angular resolution is the minimum angular
separation a telescope can distinguish• What are the two basic designs of telescopes?
– Refracting telescopes focus light with lenses– Reflecting telescopes focus light with mirrors
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1. Light Pollution
• Scattering of human-made light in the atmosphere is a growing problem for astronomy
How does Earth’s atmosphere affect ground-based observations?
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2. Twinkling and Turbulence
Turbulent air flow in Earth’s atmosphere distorts our view, causing stars to appear to twinkle
Star viewed with ground-based telescope
Same star viewed with Hubble Space Telescope
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Adaptive Optics
Rapidly changing the shape of a telescope’s mirror compensates for some of the effects of turbulence
Without adaptive optics With adaptive optics
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• The best ground-based sites for astronomical observing are:– Calm (not too windy)– High (less atmosphere to see through)– Dark (far from city lights)– Dry (few cloudy nights)
Summit of Mauna Kea, Hawaii
ie: atop remote mountains
Where should we build telescopes?
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Why do we put telescopes into space?
• Escape from atmospheric distortion (seeing)
• Escape from atmospheric airglow and light pollution
• Observe other regions of electromagnetic spectrum
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Transmission in Atmosphere
• Only radio and visible light pass easily through Earth’s atmosphere
• We need telescopes in space to observe other forms
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How can we observe nonvisible light?
• A standard satellite dish is essentially a telescope for observing radio waves
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Radio Telescopes
• A radio telescope is like a giant mirror that reflects radio waves to a focus
• Wavelengths of light much longer than visible light
• Irregularities should be less than 1/5 the wavelength of light being focused
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X-Ray and Gamma Ray Telescopes• X-ray
telescopes also need to be above the atmosphere
• As do Gamma Ray telescopes
Chandra
Compton
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How can multiple telescopes work together?
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Interferometry
• Interferometery is a technique for linking two or more telescopes so that they have the angular resolution of a single large one
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Interferometry
• Easiest to do with radio telescopes
• Now becoming possible with infrared and visible-light telescopes
Very Large Array (VLA)