Transcript Foundation 1 - Discovering Astronomy

Light and Telescopes
What do you think?
• What is the main purpose of a telescope?
• Why do stars twinkle?
What is Light and Why Would
Astronomers Want to Study the
Properties of Light?
• Sometimes we say light is made of waves
• Sometime we say light is made of particles called
photons
• Moves very fast, at 186 000 miles
per second
• 300,000 km per second
• consider a prism ...
If you pass white light through a prism, it
separates into its component colors.
long wavelengths
short wavelengths
R.O.Y. G. B.I.V
spectrum
Analyzing The Properties of
Light
• Visible Light is but one part of the entire
electromagnetic (EM) spectrum.
• EM Spectrum includes all kinds of light
•
•
•
•
•
•
•
radio waves (all light moves at the same speed micro waves
light speed)
infrared light
visible light
ultra violet light
(some light photons have
x rays
shorter wavelengths
gamma rays
and more energy than others)
Visible light is only one
type of electromagnetic
radiation emitted by
stars
Each type of EM
radiation travels at
exactly the same speed the speed of light!
Not all EM radiation can penetrate Earth’s atmosphere.
Astronomers use different instruments to look at
light of different wavelengths - sometimes, we even
have to go above Earth’s atmosphere.
SOFIA - the Stratospheric Observatory for Infrared Astronomy
Observations at other wavelengths are
revealing previously invisible sights
UV
Ordinary
visible
infrared
Map of
Orion
region
Consider Orion in Different Wavelengths of Light!
http://www.cnn.com/2001/LAW/02/20/scotus.heatdetector.01.ap/index.html
Hubble Space Telescope Views of Orion Nebula showing stars hidden in clouds
http://oposite.stsci.edu/pubinfo/pr/97/13/A.html
TODAY’S Sun as seen in visible light from Earth and
from space in X-rays by satellites
http://solar.physics.montana.edu/tslater/real-time/
Observations at wavelengths other than visible
light are revealing previously invisible sights
Visible light image
radio wavelength image
High Energy Gamma Rays - Compton Gamma Ray Observatory (GRO) Satellite
The Sky’s emission of Gamma Rays
But, we receive GRBs from every direction !!
The fact that GRBs come from every direction imply that
GRBs don’t come from our galaxy, but from other galaxies
spread in every direction!
Radio wavelength observations
are possible from Earth’s surface
The Very Large Array (VLA) in New Mexico
Different types of EM radiation
require different types of telescopes
• A refracting telescope uses a lens to
concentrate incoming light
• A reflecting telescope uses mirrors to
concentrate incoming starlight
Analyzing Spectra:
The Properties of Light
• Visible Light is one part of the electromagnetic
(EM) spectrum.
• EM radiation is described as a wave with
wavelengths in the range 1x10-15 m to 100 m.
• Visible light is only a tiny small part of the entire
electromagnetic spectrum.
• Little bits of light are called photons.
Dividing Light Into a Spectrum
Astronomers separate out light into its
individual components using a
diffraction grating or using a prism then they analyze each part
independently!
blue
4600 A
81
Filter
Detector
blue 4600 A
green 5300 A
81
85
Filter
Detector
blue 4600 A
green 5300 A
yellow 5800 A
81
85
83
Filter
Detector
blue 4600 A
green 5300 A
yellow 5800 A
orange 6100 A
81
85
83
78
Filter
Detector
blue 4600 A
green 5300 A
yellow 5800 A
orange 6100 A
red
6600 A
81
85
83
78
70
The spectrum is continuous.
Filter
Detector
UV
IR
Spectra
• Most light sources contain energy in lots
of different wavelengths.
• We can measure the brightness in various
wavelength bands--the result is called the
spectrum.
• The spectrum (total character of light
emitted) can tell us a lot about a source.
A refracting
telescope
uses a lens to
concentrate
incoming
light
Similar to a
magnifying glass
A larger
objective lens
provides a
brighter (not
bigger) image
lenses
reverse
images
Three main functions of a telescope
• Brighten
(called light gathering power)
• See fine detail
(called resolution)
and least important,
• Magnify
magnification = (objective lens focal length / eyepiece lens focal length)
Functions of a Telescope
• To gather light.
– want a large objective
– range of few inches to 10 meters!!
• To resolve fine detail.
– limited by size and atmospheric “seeing”
• To magnify
– least important
– about 50x per inch of aperture (rule of thumb)
Refracting telescopes have
drawbacks
• Spherical aberration
• Chromatic aberration
Special achromatic compound lenses and lens
coatings can often fix this aberration
Refracting telescopes have drawbacks
• Spherical aberration
• Chromatic aberration
• Sagging due to gravity
distorting the lens
• Unwanted refractions
• opaque to certain
wavelengths of light
Yerkes Observatory - 40-inch Refracting Telescope:
The Largest Refracting Telescope in the World
Reflecting telescopes use mirrors to
concentrate incoming starlight
Newtonian Focus
Prime Focus
Cassegrain focus
coude’ focus
Astronomer’s face two major
obstacles in observing the stars
• Light Pollution from Cities
• Effects of Twinkling from Earth’s atmosphere
Tucson, Arizona in 1959 and 1980
Earth’s atmosphere hinders astronomical research
Image of stars taken
with a telescope on
the Earth’s surface
Same picture taken with
Hubble Space Telescope
high above Earth’s blurring
atmosphere
Rapid changes in the density of
Earth’s atmosphere cause passing
starlight to quickly change
direction, making stars appear to
twinkle.
Advanced technology is
spawning a new generation of
equipment to view the universe
• CCDs (charge-coupled devices)
• Large telescopes on remote mountain tops
– Mauna Kea in Hawaii
– Cerro Pachon in Chile
• Adaptive Optics to counteract the blurring
of Earth’s atmosphere
• Orbiting space observatories
A Charge-Coupled Device
(CCD)
Ordinary Photographs vs. CCDs
Film (negative)
CCD (negative)
CCD (positive)
Same integration (I.e. exposure) time.
Different quantum efficiency: Film 1% CCD 70%
Matching 10-m, multiple
mirror Keck Telescopes in
Hawaii with adaptive optics
High above
Earth’s
atmosphere,
the Hubble
Space
Telescope
provides
stunning
details about
the universe
What did you think?
• What is the main purpose of a telescope?
A telescope is designed to collect as much light
as possible. It also improves resolution and
magnifies images.
• Why do stars twinkle?
Rapid changes in the density of Earth’s
atmosphere cause passing starlight to change
direction, making stars appear to twinkle.