Chapter 5

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Chapter 5Astronomical 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 charge-coupled 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)