Light & Telescopes
(Chapter 5)
All of what we know and understand
about the stars is the result of observation
and analysis of light.
Student Learning Objectives
• Describe the properties of light
• Identify types and properties of telescopes
What are the properties of light?
Electromagnetic waves
carry information as
well as energy.
Light speed is constant
in a vacuum (in space)
• 3 x 108 m/s
• 186,000 miles/sec
• 670 million mph
The energy we are now observing from objects in
space was generated at some time in the past.
Sun
Proxima Centauri
Sirius
Andromeda Galaxy
8 min.
4.2 LY
8.6 LY
2.5 x 106 LY
Practice:
In a science fiction movie, when a spaceship
explodes, we see and hear the explosion
instantly. What are the mistakes in this scenario?
Wave Properties
Electromagnetic waves have a wavelength (l)
and frequency (n).
c = ln
 Electromagnetic radiation
exhibits wave-particle
duality.
 Photons
 No mass
 Pure energy
 Each photon of light
carries energy.
E = hc
l
E = hn
 The more photons, the
higher the intensity.
Electromagnetic Radiation
l n
E
Gamma Rays
X-Rays
Ultraviolet
Visible
Infrared
Radio
http://www.chromoscope.net/
http://spaceplace.nasa.gov/ir-photo-album/en/
We interpret particular wavelengths of visible light as
particular colors!
Red
7000 Å
Cooler
Less Energy
5500 Å
Blue
4000 Å
Hotter
More Energy
ROY G BV
Some of the Information in The Light
• Energy output
• Temperature
• Chemical composition
• Distance
• Relative Motion
Atmospheric Windows
Practice
1) What is the speed of a radio wave?
2) Which wavelength is longer?
a. 95.1 MHz
b. 1220 MHz
More Practice
3) Which star would be hotter, a blue star or a
red star?
4) What affects the intensity of the light we
receive from stars?
What are the main features of optical telescopes?
Reflection is a bounce
(mirrors)
 Refraction is a change in
velocity (lenses)
Reflecting Telescope
 Reflecting telescopes utilize a mirror.
Refracting Telescope
 Refracting telescopes utilize a
lens.
The objective
or primary
lens/mirror of
the telescope is
the most
important part.
Newtonian Reflector Mirror
Refractor Issues
All refractors produce chromatic aberration
(“color deviation or distortion”).
Interesting Note: A very long focal length will minimize
chromatic aberration. This is why the early refracting
telescopes (like Galileo’s) were made very long.
Reflector Issues
Reflectors may produce a fuzzy image due to
spherical aberration (curvature distortion).
Hubble Images
Advanced optical telescopes use floppy mirror
segments controlled by a computer.
Why?
Practice
1) What is the main purpose of a telescope?
2) Where are the telescopes that astronomers
use?
3) Are there any telescopes that can be used
during the day?
Mauna Kea, Hawaii
(Elevation 14,000 Feet)
What are the functions of telescopes?
Light Gathering Power
Ability of the telescope to collect photons.
LGP = (Diameter)2
Resolving Power
Ability of the telescope to “see” fine detail, and
distinguish individual objects from each other
a = (2.06 x 105) l
Diameter
Larger D and Smaller l
sharper images
An image of two
stars through a
telescope with low
angular resolution
An image of two
stars through a
telescope with high
angular resolution.
Image Credit: Richard Bloch
Magnifying Power
Telescopes enlarge the image.
Atmosphere & Light affect
observations.
Mars with naked eye
Practice
1) Which telescope would allow you to see
fainter objects?
a. Small diameter telescope
b. Large diameter telescope
c. Either as long as there is good magnification
2) What are the advantages of space based
telescopes?
What is a radio telescope?
A large parabolic dish
gathers radio waves and
reflects this energy to a
central focus.
Star formation regions
SETI program
Interferometers
A single radio telescope has
very poor angular resolution.
An interferometer is a set of
radio telescopes connected
together.
 Constructive light Interference
NRAO
Detectors
Photometers: count
photons
Measure Intensity
 CCD - Charge-Coupled
Device: produce electronic
images with millions of
light detectors (pixels on a
small silicon wafer)
Measure Intensity per Pixel
Camera!
Spectrographs: use a
grating to split light into
individual wavelengths
White Light → Rainbow
Adaptive optics: is a
computer program that
corrects for bad seeing
Removes blurring caused
by atmosphere
Sun's Visible Spectrum
Image Credit: Canada-France-Hawaii Telescope
Image Credit: Gemini Observatory
Practice
1) What do people use everyday that has a CCD
camera?
2) Optics are lenses and mirrors. How do
adaptive optics differ from regular optics?
3) What are two tools that astronomers can use
to produce the clearest image possible?