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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
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An Introduction to AstronomyPart 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.
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.
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.
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
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. The Doppler Shift
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.
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.
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.
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.
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.
Schematic of the Hale Reflecting Telescope An example of a prime-focus reflecting telescope.
