ASTR 103--Week 3
14 Sep 2000
ASTR103, GMU, Dr. Correll
1
Light and Telescopes
• The Nature of Light
• Optics and Telescopes
• Radio Astronomy--and Beyond
14 Sep 2000
ASTR103, GMU, Dr. Correll
2
The Nature of Light
• Early discoveries
– White light comprised of a spectrum of colors
• Isaac Newton demonstrated colors inherent in the light,
not added by the prism
14 Sep 2000
ASTR103, GMU, Dr. Correll
3
The Nature of Light
– Light travels at a finite
speed, c, not
instantaneously
• Romer measures delay
in Jupiter’s lunar eclipses
c  3.0x105 km/sec
 1.86x105 mi/sec
• 7 minutes from Sunlight
to reach Earth
• 0.13 seconds for light to
go around the world
14 Sep 2000
ASTR103, GMU, Dr. Correll
4
Light--Waves or Particles?
• Newton believed light a particle phenomenon
• Thomas Young shows light behaves like waves
(1801)
14 Sep 2000
ASTR103, GMU, Dr. Correll
5
Electromagnetic Waves
• Waves are described by
–
–
–
–
wavelength,  (lambda)
frequency,  (nu)
speed, v =
and they transport energy
• Electromagnetic waves result from the motion of
electrons
14 Sep 2000
ASTR103, GMU, Dr. Correll
6
Light--Waves or Particles?
• Albert Einstein shows
that light contains energy
in discrete packets
h
e-
– Photons--particles of light
and energy
• Photon energy increases
with frequency,
decreases with
wavelength
14 Sep 2000
ASTR103, GMU, Dr. Correll
E  h 
hc

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Electromagnetic Spectrum
High energy photons
nm--nanometer--equal to
10-9 meters
Low energy photons
14 Sep 2000
ASTR103, GMU, Dr. Correll
8
Electromagnetic Spectrum
• Different spectral types
correspond to different
physical phenomenon
– Gamma rays and X rays
generated by extreme
energy
– visible light by atomic and
molecular processes
– infrared light generated by
heat and molecular
processes
– radio waves generated by
motion of free electrons
and ions
14 Sep 2000
ASTR103, GMU, Dr. Correll
9
Atmospheric Windows
• Most electromagnetic energy absorbed by the atmosphere,
but radio waves and visible light pass through
14 Sep 2000
ASTR103, GMU, Dr. Correll
10
Light and Telescopes
• The Nature of Light
• Optics and Telescopes
• Radio Astronomy--and Beyond
14 Sep 2000
ASTR103, GMU, Dr. Correll
11
Astronomical Telescopes
• Two basic types:
– Refracting-through a glass
lens
– Reflecting--from a
mirror
14 Sep 2000
ASTR103, GMU, Dr. Correll
12
Refraction
• Refraction--light bends, or changes direction, when
crossing an interface between different media, for
example, air and glass
14 Sep 2000
ASTR103, GMU, Dr. Correll
13
Lenses
• Thus a lens collects
light from a distant
object and
concentrates it at a
focused image
– every lens has a
characteristic focal
length
14 Sep 2000
ASTR103, GMU, Dr. Correll
14
Refracting Telescope
• Essential elements of a refracting telescope
14 Sep 2000
ASTR103, GMU, Dr. Correll
15
Telescope Performance
• Brighten (Aperture)
– Larger objective lenses and longer exposure
times gather more light
• Resolve (Resolving Power)
– Larger objective lenses and properly curved
surfaces improve resolution
• Magnify (Magnification)
– M=focal length of objective divided by focal
length of eyepiece
– Least important measure of performance
14 Sep 2000
ASTR103, GMU, Dr. Correll
16
Resolution Reveals Details
14 Sep 2000
ASTR103, GMU, Dr. Correll
17
Complications with Refractors
• Chromatic Aberration-different colors of light
refract at different
angles and thus have
different focal points
14 Sep 2000
ASTR103, GMU, Dr. Correll
18
Complications with Refractors
• Spherical Aberration--best shape for lens
surface is parabolic; this is difficult to make
14 Sep 2000
ASTR103, GMU, Dr. Correll
19
Reflecting Telescopes
• Reflecting mirrors
do not suffer
chromatic
aberration since
all colors reflect at
the same angles
• Spherical
aberrations must
still be corrected
14 Sep 2000
ASTR103, GMU, Dr. Correll
20
Reflecting Telescopes
• Some standard configurations
14 Sep 2000
ASTR103, GMU, Dr. Correll
21
Reflecting Telescopes
• Typical home
telescope
14 Sep 2000
ASTR103, GMU, Dr. Correll
22
Atmospheric distortion
• Visible light propagates
through the atmosphere,
but is distorted
– Twinkle, twinkle little star…
– Telescopes in space don’t
suffer this problem
– Hubble Space Telescope
• Ground-based telescopes
can now be corrected with
flexible optics and
computer corrections to
remove distortion
14 Sep 2000
ASTR103, GMU, Dr. Correll
23
Atmospheric distortion
Ground
14 Sep 2000
Ground/Adaptive
ASTR103, GMU, Dr. Correll
HST
24
Image Recording
CCD
• Sketches
• Film
• Electronic Data
14 Sep 2000
ASTR103, GMU, Dr. Correll
25
Light and Telescopes
• The Nature of Light
• Optics and Telescopes
• Radio Astronomy--and Beyond
14 Sep 2000
ASTR103, GMU, Dr. Correll
26
Radio Telescopes
• Radio Waves have
long wavelenths (cm
and m)
– objectives need to be
big, but not polished
– “image” data recorded
with antenna instead
of CCDs
• First radio telescopes
in 1930s by Karl
Jansky at Bell Labs
14 Sep 2000
ASTR103, GMU, Dr. Correll
27
Radio Telescopes
• By interfering, or
combining wave
patterns, of signals
from multiple radio
telescopes much
higher resolution
images can be made
– makes a “synthetic”
objective much larger
than each part
– VLA in New Mexico (at
right)
– VLBA from around the
globe
14 Sep 2000
ASTR103, GMU, Dr. Correll
28
Radio Telescopes
14 Sep 2000
ASTR103, GMU, Dr. Correll
29
Other wavelengths
• Infrared from high
mountain tops,
rockets, or satellites
• UV, X-ray and
gamma rays from
satellites
14 Sep 2000
ASTR103, GMU, Dr. Correll
30
Different perspectives
• Comparing information from
different spectra gives a
better understanding of
what’s going on
14 Sep 2000
ASTR103, GMU, Dr. Correll
31
Questions for study
• How are radio telescopes similar and different
from visible light telescopes?
14 Sep 2000
ASTR103, GMU, Dr. Correll
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