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Telescopes
• Galileo first used a telescope to observe the sky in
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1610
The main function of a telescope is to gather faint
light from an astronomical source and to focus
that light into an image
The light gathering ability of a telescope is
defined by its diameter or aperture
Before the 20th century, astronomer looked
through the telescope with their eyes
Today the images are recorded photographically
and electronically
ISP 205 - Astronomy Gary D. Westfall
Lecture 7
1
Types of Telescopes
• A telescope made with lenses is called a refracting
telescope
Objective
Eyepiece
• A telescope made
with mirrors is called
a reflecting telescope
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Modern astronomical
telescopes are
reflecting
ISP 205 - Astronomy Gary D. Westfall
Lecture 7
2
Properties of Images
• The brightness of an image is determined by the area of
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the collector
The brightness of an image is proportional to the aperture
squared
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A = πr2
• The main use of large aperture telescopes is to gather as
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much light as possible from faint sources
Resolution refers to the fineness of details in an image
Atmospheric instability limits most ground based
telescopes to a resolution of 1 arc-sec
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Angle subtended by a quarter at a distance of 5 km
• Some locations have better “seeing”
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Mauna Kea, 0.3 arc-sec
ISP 205 - Astronomy Gary D. Westfall
Lecture 7
3
Optical Detectors and Instruments
• Telescopes collect and focus light
• Telescopes need excellent light detectors
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The human eye is not a good light detector for
astronomical purposes
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Short integration time
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Information recorded only in astronomer’s brain
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Faint images need long collection times
Enough said!
Specific measurements are required
Imaging
 Brightness
 Spectroscopy

ISP 205 - Astronomy Gary D. Westfall
Lecture 7
4
Photographic and Electronic Detectors
• In the 20th century, photographic detection was the prime
method of collecting light
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Excellent spatial resolution
Poor quantum efficiency (1%)
Logarithmic response to light
Film must by analyzed by hand
• In recent times, electronic detectors have begun to
replace film
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Charge-coupled devices (CCD)
Acceptable spatial resolution
Excellent quantum efficiency (60%)
Linear response to light
Data recorded directly to computer
ISP 205 - Astronomy Gary D. Westfall
Lecture 7
5
Observing with Nonvisible EM Radiation
• Astronomers also observe the sky with different
wavelength EM radiation
• Different wavelengths carry a wealth of
information have their challenges

Infrared
Huge background from ambient heat
 Detection difficult
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X-rays and -rays
Must be done outside the Earth’s atmosphere
 Focusing and detection difficult
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Radio waves
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Require large detectors
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Long wavelengths!
ISP 205 - Astronomy Gary D. Westfall
Lecture 7
6
Spectroscopy
• By analyzing the wavelength of the gathered light as well
as creating an image, details of the astronomical object
can be extracted
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Temperature
Abundances of elements
Red shift (velocity of recession)
Information about rotation
ISP 205 - Astronomy Gary D. Westfall
Lecture 7
7
Picking the Best Observing Sites
• The performance of an optical telescopes depends on its
location
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Weather, clouds, fog, etc.
Humidity causes absorption of infrared
Light pollution
Stability of atmosphere
• Best sites are
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High
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Dark
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Isolated from civilization
Dry
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Mountain tops
Deserts
Examples
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Chile, Arizona, Hawaii
ISP 205 - Astronomy Gary D. Westfall
Kitt Peak National Observatory
Tuscon, Arizona
Lecture 7
8
Major New Telescopes
• Kitt Peak National Observatory
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Arizona, Chile
• European Southern Observatory
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Chile
• Keck Observatory
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Keck
Observatory
Hawaii
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Twin 10 m telescopes
• European Very Large Telescope (VLT)
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Four 8 m telescopes
Chile
SOAR
Observatory
• SOAR
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Southern Astrophysical Research
ISP 205 - Astronomy Gary D. Westfall
Lecture 7
9
Information from Radio Telescopes
• Cosmic radio waves carry information about distant
objects
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In this picture, a galaxy is ejecting matter
ISP 205 - Astronomy Gary D. Westfall
Lecture 7
10
Radio Telescopes
• Radio telescopes need to be large because of the
long wavelength of radio waves and to collect as
much signal as possible
• Radio waves are reflected by electrically
conducting surfaces and the signal is collected by
sensitive radio receivers
ISP 205 - Astronomy Gary D. Westfall
Lecture 7
11
Observations outside the Atmosphere
• Infrared
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On jets, balloons
Satellite, Infrared Astronomical
Satellite (IRAS)
• Gamma rays
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Compton Gamma Ray
Observatory
• X-rays
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Chandra X-ray Observatory
ISP 205 - Astronomy Gary D. Westfall
Lecture 7
12
Hubble Space Telescope
• The Hubble Space Telescope
was launched April 24, 1990
by the space shuttle
Discovery.
ISP 205 - Astronomy Gary D. Westfall
Lecture 7
13
Story of Hubble Space Telescope
• As told by NASA
QuickTime™ and a
Sorenson Video decompressor
are needed to see this picture.
ISP 205 - Astronomy Gary D. Westfall
Lecture 7
14
Movies from Hubble
• The HST took a series of pictures of Neptune
QuickTime™ and a
decompressor
are needed to see this picture.
ISP 205 - Astronomy Gary D. Westfall
Lecture 7
15
HST Looking Deep
• The Hubble Space Telescope looks deep into the
universe, as told by NASA
QuickTime™ and a
Sorenson Video decompressor
are needed to see this picture.
ISP 205 - Astronomy Gary D. Westfall
Lecture 7
16