chapter 5 telescopes: “light bucket”
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Chapter 5 Telescopes: “light bucket”. Energy Output. UV. IR. Wavelength. Telescopes have three functions. 1.Gather as much light as possible: LGP ∝ Area = π R 2 Why?. The light gathering power is proportional to the area. Telescopes have three functions. - PowerPoint PPT PresentationTRANSCRIPT
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Chapter 5Telescopes: “light bucket”
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Telescopes have three functions
1.Gather as much light as possible: LGP ∝ Area = πR2 Why?
UV IR
Ene
rgy
Out
put
Wavelength
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The light gathering power is proportional to
the area
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Telescopes have three functions
2. Resolve objects: Θ = 0.25 (λ/D)Why?In astronomy, we are always concerned with angular measurement
“close together” means “separated by a small angle on the sky,”
angular resolution is the factor that determines our ability to see fine structure
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Telescopes have three functions
3. Magnify EXTENDED objects. Why?
Extended objects: The most surprising property is that extended objects do not get brighter in a scope.
Also the contrast does not change (actually it only can get worse).
The only thing that changes is that the object itself gets bigger by magnification
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Reflection is the
bouncing of light off a surface
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How do we see an object Light rays coming from one point of
an object are focused at one point at the back of your eye
(retina).
Different points from an object are focused at different points of your retina.
So we see an image of the object produced by the lense in our eyes at the back of our eye.
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A telescope is not a microscope• Microscope’s function is magnification
• Telescope’s function is gathering as much light as possible: Large lenses
• Large lenses do not result in larger magnification
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The refracting telescope uses two lenses.
Where is the image of the star?
Light Bending
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Problem with refraction
Different color light is refracted at different angles, causing a distortion of the image
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Refractors suffer from Chromatic Aberration
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Dispersive refraction leads to chromatic aberration:
More on chromatic aberration
CLL: Diffraction Rings
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Chromatic Aberration
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Images can be formed through reflection or refraction
Reflecting mirror
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Image formation
Where do you stand to observe the object?What’s the problem with that?How to overcome the problem?
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Types of reflecting telescopes:
To get around the blocking of light when you stand in the focal plane, different telescope types were invented
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Do reflecting optical devices suffer from chromatic aberration
A) Yes, different color light reflects at different angles and therefore focuses at different points.
B) No, different color light reflects always at the same angle as it incidents, causing no chromatic aberration.
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But: Spherical mirrors suffer from Spherical
Aberration
Parallel light from different locations of the mirror is not exactly focused at one point
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Spherical aberration
can be eliminated
by a parabolic shape or a corrector
plate
CLL: Focus of a Cassegrain reflector
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Optical TelescopesThe Keck telescope, a modern research telescope:
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Optical Telescopes
The Hubble Space Telescope has a variety of detectors:
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Discovery 5-1: The Hubble Space Telescope
The Hubble Space Telescope’s main mirror is 2.4 m in diameter and is designed for visible, infrared, and ultraviolet radiation
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Discovery 5-1: The Hubble Space Telescope
Here we compare the best ground-based image of M100, on the left, with the Hubble image on the right
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In astronomy, we are always concerned with angular measurement
“close together” means “separated by a small angle on the sky,”
angular resolution is the factor that determines our ability to see fine structure
Protractor-string Example
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Light-gathering power: Improves detailBrightness proportional to square of radius of mirrorBelow: (b) was taken with a telescope twice the size of (a)
Telescope Size
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Resolving power: When better, can distinguish objects that are closer togetherResolution is proportional to wavelength and inversely proportional to telescope size—bigger is better!
Telescope Size
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Resolution Want to resolve objects which are close
together. Objects that are close together are
separated by a smaller angle than objects that are at the same distance but further apart.
Θ = 2.06 X 105 (λ/D)
Smaller the better
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How to improve the resolution?• A) Increase the size of the mirror• B) Decrease the size of the mirror• C) Use light with smaller wavelength• D) Use light with larger wavelength
Smaller the better
For comparison, the angular resolution of the human eye in the middle of the visual range is about 0.5'.
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What limits a telescope’s resolution?
One important factor is diffraction, the tendency of light, and all other waves to bend around corners
Diffraction introduces a certain “fuzziness,” or loss of resolution
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Effect of improving resolution:(a) 10′; (b) 1′; (c) 5″; (d) 1″
5.2 Telescope Size
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Image acquisition: Charge-coupled devices (CCDs) are electronic devices, can be quickly read out and reset
5.3 Images and Detectors
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5.3 Images and DetectorsPhotometry is a technique of astronomy concerned with measuring the flux, or intensity of an astronomical object's electromagnetic radiation
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Image processing by computers can sharpen images
5.3 Images and Detectors
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Atmospheric blurring: Due to air movements5.4 High-Resolution Astronomy
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“Seeing”
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Solutions:• Put telescopes on mountaintops,
especially in deserts• Put telescopes in space
5.4 High-Resolution Astronomy
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Active optics: Control mirrors based on temperature and orientation
5.4 High-Resolution Astronomy
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Adaptive optics: Track atmospheric changes with laser; adjust mirrors in real time
5.4 High-Resolution Astronomy
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5.4 High-Resolution AstronomyThese images show the improvements possible with adaptive optics:
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Radio telescopes:• Similar to optical reflecting telescopes• Prime focus• Less sensitive to imperfections (due to longer
wavelength); can be made very large
5.5 Radio Astronomy
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Largest radio telescope: 300-m dish at Arecibo5.5 Radio Astronomy
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Longer wavelength means poor angular resolutionAdvantages of radio astronomy:• Can observe 24 hours a day
• Clouds, rain, and snow don’t interfere
• Observations at an entirely different frequency; get totally different information
5.5 Radio Astronomy
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Interferometry:• Combine information from several widely spread
radio telescopes as if they came from a single dish
• Resolution will be that of dish whose diameter = largest separation between dishes
5.6 Interferometry
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5.6 InterferometryInterferometry involves combining signals from two receivers; the amount of interference depends on the direction of the signal
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Can get radio images whose resolution is close to opticalInterferometry can also be done with visible light but is much more difficult due to shorter wavelengths
5.6 Interferometry
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Infrared radiation can image where visible radiation is blocked; generally can use optical telescope mirrors and lenses
5.7 Space-Based Astronomy
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Infrared telescopes can also be in space; the image on the left is from the Infrared Astronomy Satellite
5.7 Space-Based Astronomy
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The Spitzer Space Telescope, an infrared telescope, is in orbit around the Sun. These are some of its images.
5.7 Space-Based Astronomy
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Ultraviolet observing must be done in space, as the atmosphere absorbs almost all ultraviolet rays.
5.7 Space-Based Astronomy
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X-rays and gamma rays will not reflect off mirrors as other wavelengths do; need new techniquesX-rays will reflect at a very shallow angle and can therefore be focused
5.7 Space-Based Astronomy
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X-ray image of supernova remnant
5.7 Space-Based Astronomy
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Gamma rays cannot be focused at all; images are therefore coarse
5.7 Space-Based Astronomy
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Much can be learned from observing the same astronomical object at many wavelengths. Here, the Milky Way:
5.8 Full-Spectrum Coverage
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• Refracting telescopes make images with a lens• Reflecting telescopes with a mirror• Modern research telescopes are all reflectors• CCDs are used for data collection• Data can be formed into image, analyzed
spectroscopically, or used to measure intensity• Large telescopes gather much more light,
allowing study of very faint sources• Large telescopes also have better resolution
Summary of Chapter 5
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• Resolution of ground-based optical telescopes is limited by atmospheric effects
• Resolution of radio or space-based telescopes is limited by diffraction
• Active and adaptive optics can minimize atmospheric effects
• Radio telescopes need large collection area; diffraction limited
• Interferometry can greatly improve resolution
Summary of Chapter 5 (cont.)
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• Infrared and ultraviolet telescopes are similar to optical
• Ultraviolet telescopes must be above atmosphere
• X-rays can be focused, but very differently than visible light
• Gamma rays can be detected but not imaged
Summary of Chapter 5 (cont.)