introduction to radio astronomy updated february 2009

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Introduction to Radio Astronomy Updated February 2009

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Introduction to Radio Astronomy

Updated February 2009

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The Visible Sky, Sagittarius Region

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The Radio Sky

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Optical and Radio can be done from the ground!Optical and Radio can be done from the ground!

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OutlineThe Discovery of Radio Waves

Maxwell, Hertz and Marconi

The Birth of Radio AstronomyJansky and Reber

Tools of Radio AstronomyWhat we use to detect radio

Sources of Radio EmissionEverything!

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James Clerk Maxwell

Tied together theories of electricity and magnetism (Maxwell’s equations) to

derive the electromagnetic theory of light

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• Electric and magnetic fields oscillate together with the same frequency and period

• Electromagnetic waves do not require a medium!

• The velocity and wavelength spectrum are defined:

c = f

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Heinrich Hertz

Constructed a circuit to induce electric and magnetic field oscillations. This circuit transmitted electromagnetic waves to a

nearby capacitive loop receiver

Heinrich Hertz's first transmitter, 1886

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Karl Guthe Jansky

Founder of Radio Astronomy

Hired by Bell Labs in the late 1920’s, Jansky’s mission was to find sources of radio interference

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• Jansky constructed a directional 20.5 MHz antenna on a turntable to locate radio noise source positions

• Sources of noise– Nearby storms

– Distant storms

– A faint hiss that returned every 23 hours 56 minutes

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In 1933, Jansky identified this source of noise as the

center of our galaxy, in Sagittarius

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Grote ReberRadio Astronomy Pioneer

• After Jansky’s project ended, Bell Labs was not interested in studying radio astronomy

• Reber continued Jansky’s original work, by constructing his own radio telescope in 1937

• Provided the first maps of the radio sky at 160 and 480 MHz

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Reber’s 31.4 ft parabolic reflector

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Reber’s contour maps of the Milky Way, at 160 and 480 MHz

Synchrotron or Non-thermal

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Colors of light we can’t see…

• Ionizing Radiation – UV– X-Rays– Gamma Rays

• Non-Ionizing Radiation– IR– Microwave– Radio

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Multi-wavelength Astronomy

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Astronomy expands to the entire spectrum.

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FCC allotment of Radio Spectrum

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Tools of Radio Astronomy

• Your car radio is an example of a simple antenna and receiver

• Radio waves actually cause free electrons in metals to oscillate!

• Radio receivers amplify these oscillations, so, radio telescopes measure the voltage on the sky

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Formation of Radio Waves

• Thermal Radiation

• Synchrotron Radiation– Relativistic e- in magnetic fields

• Bremstrahlung– “Breaking Radiation” e- /ion collisions

• Maser– Microwave Laser e- oscillations in molecular clouds

• Atomic Transitions (emission spectra)– Hydrogen e- spin flip

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Formation of 21cm Radio waves(1420 MHz)

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Reception of Radio Waves

• Radio waves cause oscillation of free e- in metals

• Dish reflector antennas localize the source and exclude background noise

• Radio signal intensity is measured as voltage

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Telescopes for visible lightTelescopes for visible light

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The 140 Foot Telescope

Green Bank, WV

receiver

parabolic reflector

control room

Radio TelescopeRadio Telescope

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Reception of Radio Waves

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Reception of Radio Waves

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Receiver Feed Horn

Amplifier

Mixer

SpectrometerAntennaControl

Control Computer

Example signal pathof a radio telescope

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Spectrometer Output• Spectrum: brightness vs. radio frequency• Continuum: total brightness over all frequencies

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• Radio waves are VERY weak!

• Radio brightness measured in units of Janskys

1 Jansky (Jy) = 10-26 W/m2/Hz

• Typical sources:– Sun: 10,000’s of Jy– Brightest Supernova Remnant: 1000’s of Jy– Active Galactic Nuclei: 10-100 Jy

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The Ideal Radio Telescope

• Directional antennae, such as those with reflectors, isolate the radio power from single sources to reduce confusing radiation from others

• Low temperature receivers are more sensitive

• Large collecting areas increase gain and resolution

• Resolution: roughly 57.3 /D degrees (: observing wavelength, D: diameter of aperture)

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• Optical telescopes have an advantage on radio telescopes in angular resolution

• A one meter optical telescope has a resolution of 0.1 seconds of arc.

• Since radio telescopes cannot be built large enough to match optical resolution, they can be combined as an interferometer to emulate a large single dish

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At 21-cm wavelengths, PARI’s 26-m and Smiley (4.6 m) have resolutions of 0.5 and 2.5 degrees respectively

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12 Meter

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Greenbank (WV) 100-m telescope in has a resolution of 7 arc-minutes

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300-m telescope in Arecibo, Puerto Rico (resolution – 2.4 arcminutes)

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Very Large Array, resolution – 1.4 arc seconds

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10 Antennas of the Very Long Baseline Array (resolution – 5 milli-arcseconds)

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Time for Radio Astronomy Observing…