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An Introduction to Astronomy
Part III: Light and Telescopes
Lambert E. Murray, Ph.D.
Professor of Physics
The Electromagnetic Spectrum
Visible light is only a small region of the
electromagnetic spectrum.
Types of Spectra

Continuous Spectra
– This “blackbody” spectrum arises from the heating of
an object.
– The temperature of the object determines its color.

Emission Line Spectra
– This arises when electrons loose energy and emit
radiation at wavelengths that are specific to the
chemical makeup of the substance.

Absorption Line Spectra
– This is the opposite of emission spectra - it arises when
electrons gain energy and absorb radiation at
wavelengths that are specific to the chemical makeup of
the substance.
Color Temperature and Stars
Color Spectra
Types of Spectra
Hydrogen Absorption Spectra
Atomic Structure
Atoms consist of a very small, heavy nucleus
made up of protons and neutrons surrounded by a
“cloud” or electrons.
 Atoms are not charged – they have the same
number of protons as electrons.
 The chemical nature of atoms is determined by the
number of protons.
 A neutron is approximately the same size as a
proton, but has no charge.
 Atoms with the same number of protons, but
different numbers of neutrons are called isotopes.
 Some isotopes are radioactive.

Basic Atomic Structure
Absorption and Emission
Excitation and Ionization
Spectroscopic Notation of Ions
OI is the designation of neutral oxygen
 OII means one electron has been removed due to
ionization
 OIII means two electrons have been removed by
ionization
 In a hot gas we may find highly ionized elements,
such as Fe XIV.
 Each ionic species has its own unique spectrum.

Radioactive Isotopes
 Many
elements have different isotopes
 Each isotope has the same chemical
properties, but different numbers of
neutrons in the nucleus.
 Some isotopes are unstable (radioactive).
 Radioactive isotopes change from one
elementary species to another according to a
radioactive decay rate characteristic of the
parent isotope.
Radioactive Decay
Color Spectra
Color and Temperature
Color Temperature and Wein’s Law
Wein’s Law states that the wavelength at which most of the light
energy is emitted is given by the equation:
lmax=1/T
The Stephan-Boltzmann Law
The Inverse-Square Law
Scattering of Blue Light by Dust
The Doppler Shift
 This
is the shift of wavelength and
frequency as the source and receiver move
toward or away from each other.
 A red-shift occurs when the source and
receiver are moving away from each other.
 A blue-shift occurs when the source and
receiver are moving toward each other.
Using the Doppler Shift
Possible Emission Spectrum of a
Star
Proper Motion and Radial Motion
Radiation from the Sun
The total amount of energy incident upon the
earth’s outer atmosphere is called the solar
constant: 0.139 watt/sq-cm (about 10 inches
square would give 100 watts). This is the total
intensity for all wavelengths combined.
 As we have already pointed out, solar radiation in
certain wavelengths is greater than in others and
depends upon the surface temperature of the Sun.

Solar Spectrum
Optical Telescopes
 There
are two principle types of optical
telescopes:
– Refractors
– Reflectors
 Refracting
telescopes use lenses to bend and
focus the light.
 Reflecting telescopes use mirrors to reflect
and focus the light.
An Astronomical Refracting Telescope
A Refracting Telescope
This is a picture of the world’s largest refracting telescope, found in
the University of Chicago’s Yerkes Observatory in Williams Bay,
Wisconsin.
A Reflecting Telescope
Principle Objectives of the Telescope
 A telescope
has two principle objectives:
– To gather as much light from a star, planet, or
planetary satellite as possible so that the image
appears as bright as possible.
– To produce a magnified image
 Magnification
may make the object appear bigger
 Magnification increases the ability to resolve
different features which are close together.
 Both
the light gathering capacity of a
telescope and its ability to resolve small
features depend upon the diameter of the
objective lens or mirror of the telescope.
Magnification
Increasing the size of the telescope increases the brightness and the
resolution of the image.
Improved Resolution with and
Increase in Primary Mirror Size
Principle Disadvantages of Refracting
Telescopes

The light must pass through the lenses, thus the size of the
lenses are limited because of the way they must be
supported (around the edge).
– If the lenses are too big, gravity will make them sag and distort the
image.
– Long focal length lenses are thinner, so the largest diameter
refracting telescopes have very long focal lengths.

A single lens will bend different wavelengths (colors) by
slightly different amounts, creating chromatic aberration.
This can be corrected using special lenses – but these are
much more difficult to manufacture and must be thicker
and heavier, so that quality achromatic lenses must be
smaller in size to prevent them from sagging.
Correcting Chromatic Aberration
Two different kinds of glass, with different refractive indexes must
be used, and the lens curvatures must be properly matched.
Principle Advantages of A Reflecting
Telescope
 A concave
mirror can be supported over its
entire back surface dimension. Such a mirror
can therefore be made much larger than the
largest refracting telescopes. However, if the
size of the mirror is too large, gravitational
distortions will occur when the mirror’s
orientation is changed.
 Since light does not pass through a mirror,
mirrors are not subject to the problem of
chromatic aberrations.
Principle Disadvantages of
Reflecting Telescopes
 The
focal point of a concave mirror is in
front of the mirror – in the light path. Thus,
the entire area of the mirror cannot be
utilized to capture light from the object
observed.
 It sometimes creates difficulties to locate
the observer at the focal point of the mirror.
Types of Reflectors
Schematic of the Hale Reflecting Telescope
An example of
a prime-focus
reflecting
telescope.
Newtonian Reflecting Telescope
A Hybrid Telescope
Spherical Mirrors Exhibit Spherical
Aberration
Inexpensive reflectors have spherical mirrors, and thus suffer from
spherical aberration. Spherical lenses also exhibit spherical
aberrations.
Research-Grade Telescopes Use
Parabolic Mirrors
Atmospheric Influence on Astronomy

The atmosphere greatly influences what can be
studied from the Earth’s surface.
– The atmosphere is not transparent in certain regions of
the spectrum.

There are several “windows” to the heavens
– An optical window
– An infrared window
– A radio window
– Turbulence in the atmosphere causes stars to “twinkle”
and reduces the resolution of the telescopes.
– “Seeing” refers to atmospheric turbulence.
– Light pollution limits our ability to see faint objects.

To overcome this limitation, we send satellites
beyond the earth’s atmosphere.
Absorption by
Gases in the
Atmosphere
Oxygen and Ozone absorb
strongly in the ultraviolet,
while Carbon Dioxide and
Water Vapor absorb
strongly in the infrared.
Saturn
in:
Visible
Radio
The Hubble Space Telescope
Hubble Images vs. Ground-Based
Images
Hubble Images
Color Images
Toward the Next Generation
Telescopes
One way to make larger reflecting telescopes is to
make many smaller mirrors act as a single, much
larger mirror.
 These hybrid reflectors have electronic
adjustments for each individual mirror.
 New techniques are allowing us to change the
position of each individual element of these multielement telescopes by small amounts to counteract
the effects of the atmosphere.

Multiple
Mirrors can be
Utilized to Act
as a Single,
Large Mirror
visible
radio
X-ray
infrared
Gamma ray
End of Part III