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:

max=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.

Movie

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.

Saturnin:

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 multi-element 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

infrared

X-ray

Gamma ray

End of Part III