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Courtesy of UIT Team/NASA
Chapter 5
Telescopes
Courtesy of ESO
Windows to the
Universe
The Crab nebula as seen in visible light
The Crab nebula as seen in ultraviolet
© 2004 Jones and Bartlett Publishers
5-1 Refraction and Image Formation
1. Light travels in a straight line as long as it
remains in the same medium (i.e., the material
that transmits light).
2. Refraction is the bending of light as it crosses
the boundary (interface) between two materials
in which it travels at different speeds.
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Figure 5.02a: As a light beam crosses the interface between different
media, it bends due to the change in the speed of propagation.
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3. The amount of refraction is determined by two
factors:
(a) Relative speeds of light in the two materials
(e.g., air and glass).
(b) Angle between a light ray and the interface;
the smaller the angle between a light ray and the
interface, the more the light bends on passing
through the interface.
4. Image is the visual counterpoint of an object,
formed by refraction or reflection of light from
the object.
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5. Focal point (of a converging lens or mirror) is
the point at which light from a very distant
object converges after being refracted or
reflected.
6. Focal length is the distance from the center of a
lens or a mirror to its focal point.
Figure 5.03: (a) A lens bends incoming rays of light toward a single point.
(b) Light from different stars are focused into separate images.
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5-2 The Refracting Telescope
1. Objective is the main light-gathering element—
lens or mirror—of a telescope. It is also called the
primary.
2. An eyepiece (which may be a combination of
lenses) added just beyond the focal point of the
telescope’s objective acts as a magnifier to
enlarge the image.
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Chromatic Aberration
1. Dispersion is the separation of light into its
various wavelengths upon refraction.
2. Chromatic aberration is a defect of optical
systems that results in light of different colors
being focused at different places.
The resulting image will be fuzzy at the edges.
3. An achromatic lens (or achromat) is an optical
element that has been corrected so that it is free
of chromatic aberration.
This is done by combining two or more lenses made of
different kinds of glass.
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Figure 5.06a: Chromatic aberration
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5-3 The Powers of a Telescope
Angular Size and Magnifying Power
1. Angular size of an object is the angle between
two lines drawn from the viewer to opposite
sides of the object.
2. Magnifying power (or magnification) is the ratio
of the angular size of an object when it is seen
through the instrument to its angular size when
seen with the naked eye.
Magnifying power:
M = fobjective/feyepiece
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3. Long focal length eyepieces produce less
magnification; short focal length eyepieces
produce more magnification.
4. Field of view is the actual angular width of the
scene viewed by an optical instrument.
5. As magnification increases the field of view
decreases.
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Light-Gathering Power
1. Light-gathering power is a measure of the
amount of light collected by an optical
instrument.
2. Light-gathering power is related to the size of the
objective, which is usually given as a diameter.
Remember that the area of a circle is proportional to
the (diameter)2.
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Resolving Power
1. Diffraction is the spreading of light upon passing
the edge of an object.
2. Resolving power (or resolution) is the smallest
angular separation detectable with an
instrument. It is a measure of an instrument’s
ability to see detail.
3. The resolving power of a human eye is about 1
arcminute (1/60 of a degree).
A 15-cm (6-inch) telescope has a maximum resolving
power of 1 arcsecond (1/3600 of a degree).
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4. Astronomical seeing is the blurring and twinkling of
the image of an astronomical light source caused by
the Earth’s atmosphere.
5. Seeing is the best possible angular resolution that
can be achieved.
6. Because of atmospheric turbulence (which causes
the stars to twinkle), even the largest Earth-based
telescopes have a practical resolving power of
between 1 and 0.25 arcsecond.
7. Operating above the atmosphere, the Hubble Space
Telescope has a resolving power of 0.1 arcsecond or
better.
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Figure 5.11: M-13 is a cluster of more than 100,000 stars about 23,000 ly
away orbiting the center of our Galaxy.
Courtesy of Gemini Observatory and Canada-France-Hawaii
Telescope/Coelum/Jean-Charles Cuillandre
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5-4 The Reflecting Telescope
1. An inwardly curved—or concave—mirror can bring
incoming light rays to a focus and is used to
construct reflecting telescopes.
2. In the late 1660s, Isaac Newton invented a special
focal arrangement for a reflecting telescope by
placing a small flat mirror in front of the objective
mirror to deflect light rays out to the eyepiece.
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Figure 5.12a: The focal point is defined as the point where incoming rays
that are parallel to the axis of the mirror converge.
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Figure 5.12b: The Newtonian focal arrangement.
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3. Reflectors can be made larger (and less
expensively) than refractors because:
(a) There are fewer surfaces to grind, polish, and configure
correctly.
(b) Reflecting mirrors do not exhibit chromatic aberration
as do lenses.
(c) Light doesn’t transmit through a mirror so
imperfections in the glass are not critical.
(d) Mirrors can be supported on their backs, thus
minimizing shape deformations due to gravity; lenses must
be supported along their rims.
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Large Optical Telescopes
1. A Newtonian focus reflecting telescope has a
plane mirror mounted along the axis of the
telescope so that the mirror intercepts the light from
the objective mirror and reflects it to the side.
2. A Cassegrain focus reflecting telescope has a
secondary convex mirror that reflects the light back
through a hole in the center of the primary mirror.
3. Prime focus is the point in a telescope where the
light from the objective is focused (i.e., the focal
point of the objective).
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4. The Coude design allows for large and heavy
equipment to be set at the focal point, outside
the main telescope tube; here light reflects off
three mirrors before it exits.
Figure 5.16
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5. For best viewing conditions large telescopes are
located on top of mountains in dry, clear climates.
Figure 5.17a: The four telescopes of the Very Large Telescope
Courtesy of ESO - (The) European Organization for Astronomical
Research in the Southern Hemisphere
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Active and Adaptive Optics
1. Active optics is a technology that relies on a
system that monitors and changes the shape of a
telescope’s objective to produce the best image.
2. Adaptive optics is a technique that improves image
quality by reducing the effects of astronomical
seeing.
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Telescope Accessories
1. Camera with photographic plates.
2. Charge-coupled device (CCD) is an electronic
“film” that serves as a light detector. It works by
collecting electrons excited into higher energy
states when the detector is struck by incident
photons. The data collected is formed into images
by a computer.
3. Photometry is the measurement of light intensity
from a source, either the total intensity or the
intensity at each of various wavelengths. Early
photometers were like a camera’s light meter;
modern photometers use a CCD for greater speed
and accuracy.
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4. Spectral analysis uses a spectrometer—an
instrument that separates electromagnetic
radiation according to wavelength. A
spectrograph is a visual record of the spectrum
taken by a spectrometer.
5. A spectrometer uses a diffraction grating—a
device that uses the wave properties of EM
radiation to separate the radiation into its
various wavelengths.
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5-5 Radio Telescopes
1. Compared to visible light, radio waves from a star
have less intensity; also, their longer wavelengths
lead to images of smaller resolution.
2. For better resolution in detecting radio waves, very
large dishes are required.
• The dish surfaces do not have to be as smooth as
glass mirrors.
• Even though longer wavelengths diffract more when
going through an opening, they don’t require as
smooth a surface for reflection.
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Figure 5.20: The radio telescope near Arecibo, Puerto Rico, is the world's
largest.
The Arecibo Observatory is part of the National Astronomy and Ionosphere Center, which is operated by Cornell University under a
cooperative agreement with the National Science Foundation
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Figure 5.22: A graph, a contour map, and a color-enhanced contour map
from scans of a radio telescope
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Figure 5.23b: A Spitzer infrared telescope telescope view of Andromeda
Galaxy.
Courtesy of JPL-Caltech/K. Gordon (University of Arizona)/NASA
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Figure 5.23c: A radio image of Andromeda shows that most radio waves
are emitted from its spiral arms and from its center.
Courtesy of Max-Planck-Institut fur Radioastronomie, Bonn (R. Beck,
E.M. Berkhuijsen and P. Hoernes)
© 2004 Jones and Bartlett Publishers
5-6 Interferometry
1. Interferometry is a procedure that allows a number of
telescopes to be used as one by taking into account the time
at which individual waves from an object strike each
telescope.
2. Interferometry is possible because extremely accurate
atomic clocks allow for precise timing of the signals received
by radio telescopes from a distant object.
3. The farther apart the telescopes, the better the resolution.
The VLBA has a resolution of a fraction of a milliarcsecond, 10,000
times better than earth-bound optical telescopes.
4. Interferometry can also be employed with the newest optical
telescopes, such as the VLT in Chile or CHARA on Mount
Wilson.
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Figure 5.26: The Very Large Array radio telescopes
Courtesy of NRAO/AUI
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Figure 5.25: Interferometry
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Figure 5.24: two small radio telescope dishes replace one big dish.
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5-7 Detecting Other
Electromagnetic Radiation
1. Near infrared—1200 nm to 40,000 nm—can be
detected from high, dry mountain tops such as
Mauna Kea in Hawaii; water vapor is the main
absorber of infrared light.
2. Far infrared—greater than 40,000 nm—can be
detected from aircraft (e.g., the SOFIA project,
NASA’s Airborne Observatory).
3. Infrared telescopes must be cooled so heat (IR
radiation) from the surroundings does not mask the
signals received from space.
4. Ozone is the chief absorber of wavelengths shorter
than about 400 nm. Ultraviolet, X-ray and gamma-ray
telescopes must be located in space.
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Figure 5.27: Spitzer’s infrared image of the W5 star forming region
compared to a visible-light picture of the same region
Infrared, Courtesy of NASA/JPL-Caltech/L. Allen (Harvard-Smithsonian CfA); Visible, Courtesy of CIT, DSS
© 2004 Jones and Bartlett Publishers
Tools of Astronomy: The Hubble Space
Telescope
1. HST is mainly an optical telescope, with a 2.4-m
primary mirror, but is designed to observe across
the spectrum from near infrared to near ultraviolet
(115–2500 nm).
2. The HST, after a number of servicing missions
including a successful repair in 1993, is now
functioning at design specifications.
Its successor is the James Webb Space Telescope,
schedule to launch in 2011.
© 2004 Jones and Bartlett Publishers
© 2004 Jones and Bartlett Publishers