Chapter 5
Astronomical Instruments
How do we learn about
objects too far away for
spacecraft?
How do telescopes work?
Do all telescopes use
light?
Why are telescopes
located on mountains?
Why are some telescopes
in orbit around the Earth?
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5.1 Telescopes
• Telescopes are devices for “seeing”
distant objects.
– To see faint objects, collect more light by
using a bigger aperture.
– Magnification is much, much less important.
• Two types of optical telescopes:
– Refracting
– Reflecting
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Refracting Telescopes
• Use a large lens as the primary
light collecting element.
• Common in older telescopes.
• Chromatic aberrations and
ultraviolet cutoff.
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Reflecting Telescopes
• Use a large mirror as
the primary light
collecting element.
• Easier to make a
large mirror than a
large lens.
• Modern telescopes
are reflectors.
• Reduced chromaticity
problems.
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Refracting versus Reflecting
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Prime, Newton, and Cassegrain
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5.2 Optical Detectors and
Instruments
• Second greatest advance in astronomy was the use
of photographic film
– Longer integration time than the eye.
– Images available for later (re)inspection.
• The 3 uses of telescopes:
– Imaging
– Measuring brightness and color
– Spectroscopy
• All benefit from longer integration time.
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Charge-Coupled Device
• A modern advance
is the use of chargecoupled devices or
CCD’s.
• These are the
“digital film” of
digital cameras.
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5.3 Optical and Infrared
Observatories
• Ground based telescopes are housed in domes or
other structures to protect them when not in use.
• They are often located on remote mountains, away
from lights, and smog, and above humid and
turbulent air.
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NOAO on Kitt Peak, AZ
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South
African
Large
Telescope
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European Southern
Observatory
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Choice of Telescope Sites
• Weather: clouds, wind, and rain limit observing
time.
• Water vapor: water vapor in the atmosphere filters
out infrared wavelengths.
• Darkness: city light can overwhelm faint stars.
• Turbulence: turbulent air blurs images.
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Observatory Sites
• Newest telescopes are sited in the Chilean Andes,
desert peaks of Arizona, Canary Islands, and Mauna
Kea in Hawaii.
• Modern telescopes can use segmented mirrors,
adaptive optics, and interferometry.
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Observing Other Radiation
• To learn the most about planets, stars, and galaxies,
astronomers use as many types of radiations as
possible.
–
–
–
–
Radio
Infrared and ultraviolet
X-rays
Gamma rays
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5.4 Radio Telescopes
• Many objects emit radio waves, from long wave to
microwaves.
• Radio telescopes generally look like large satellite
dishes.
• They can operate during the day and night.
• Large facilities in Puerto Rico, West Virginia, and
New Mexico.
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The Very Large Array (VLA)
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The VLBA Spans A Continent
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Radio Image of Quasar Jets
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5.5 Observatories Outside the
Earth’s Atmosphere
• Some wavelengths of radiation are absorbed by
Earth’s atmosphere (gamma rays, x-rays, and
ultraviolet).
• Telescopes for these wavelengths must be put in
space.
• The Hubble telescope demonstrates the advantages
for an optical telescope to be above the atmosphere.
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The Hubble Space Telescope
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The Swift Satellite (GRB’s)
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