Radio Astronomy ASTR 3010 Lecture 25. Intro to Radio Astronomy Concepts - Amplifiers - Mixers...
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Transcript of Radio Astronomy ASTR 3010 Lecture 25. Intro to Radio Astronomy Concepts - Amplifiers - Mixers...
Radio Astronomy
ASTR 3010
Lecture 25
Intro to Radio Astronomy
• Concepts - Amplifiers - Mixers (down-conversion) - Principles of Radar - Radio Astronomy basics: System temperature, Receiver temperature Brightness temperature, The beam (q = l / D) [ its usually BIG] Interferometry (c.f. the Very Large Array – VLA) Aperture synthesis
History of Radio Astronomy (the second window on the Universe)
• 1929 - Karl Jansky (Bell Telephone Labs)• 1030s - Grote Reber• 1940s - WWII, radar - 21 cm (Jan Oort etc.)• 1950s - Early single dish & interferometry - `radio stars’, first map of Milky Way - Cambridge surveys (3C etc)• 1960s - quasars, pulsars, CMB, radar, VLBI aperture synthesis, molecules, masers (cm)• 1970s - CO, molecular clouds, astro-chemistry (mm)• 1980s /90s – CMB anisotropy, (sub-mm)
NRAO/AUI/NSF 4
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Optical and Radio can be done from the ground!
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Radio Telescope
Optical Telescope
Nowadays, there are more similarities between optical and radio telescopes than ever before.
Outline
• A Simple Heterodyne Receiver System– mixers and amplification
• Observing in the Radio– resolution– brightness temperature
• Radio Interferometry• Aperture synthesis
Df = 1850 Hz
f trans
Freflect = f trans + / - Df
Mixing: Adding waves together
Mixers
signal inLO
local oscillator
w1 w2
signal outw1+w2 andw1-w2
A mixer takes two inputs: the signal and a local oscillator (LO).
The mixer outputs the sum and difference frequencies.
In radio astronomy, we usually filter out the high frequency (sum) component.
Mixers
frequency
sign
al LO
originalsignal
mixedsignal
0 Hz
Mixers
frequency
sign
al LO
originalsignal
mixedsignal
The negative frequencies in the difference appear the same as a positive frequency.
To avoid this, we can use “Single Sideband Mixers” (SSBs) which eliminate the negative frequency components.
0 Hz
W-band (94 GHz,4 mm) amplifier
A Simple Heterodyne Receiver
low noiseamplifier
filter
receiver horn
LO
tunablefilter
signal @ 1420 MHz
1570 MHz
1420 MHz
tunableLO
~150 MHz
Analog-to-DigitalConverter
Computer
+ +
outputs a power spectrum
150 MHz
Observing in the Radio I
• We get frequency and phase information, but not position on the sky– 2D detector
• A CCD is also a 2D detector (we get x & y position)
Observing in the Radio II:Typical Beamsize (Resolution)
• i.e. The BURAO 21 cm horn (D ~ 1 m)
Observing in the Radio II• i.e. The NRAO GBT (D ~ 100 m)
at 21cm = 1.420 GHz
at 0.3 cm = 100 GHz
Observing in the Radio II• i.e. The Arecibo Telescope (D ~ 300 m)
at 21cm = 1.420 GHz
at 0.3 cm = 100 GHz
Observing in the Radio III:Brightness Temperature
Flux: erg s-1 sr-1 cm-2 Hz-1 (1023 Jy)Bu(T): erg s-1 sr-1 cm-2 Hz-1 (1023 Jy)
We can use temperature as a proxy for flux (Jy)
Conveniently, most radio signals have hu/kT << 1, so we can use the Raleigh-Jeans approximation
Bu(T) = 2kT/l2
Thus, flux is linear with temperature
Antenna Temperature
• Brightness temperature (TB) gives the surface temperature of the source (if it’s a thermal spectrum)
• Antenna Temperature (TA):
if the antenna beam is larger than the source, it will see the source and some sky background, in which case TA is less than TB
• Noise in the system is characterized by the system temperature (Tsys)– i.e. you want your system temperature (especially in the first
amp) to be low
TB = Ful2/2k
TA ~ TB Ws/Wb
Radio Interferometry
+
Q
East
positional
phase delay
to source
Q
Two Dish Interferometry
• The fringe pattern as a function of time gives the East-West (RA) position of the object
• Also think of the interferometer as painting a fringe pattern on the sky– the source moves through this pattern, changing
the amplitude as it goes
Aperture Synthesis
• A two dish interferometer only gives information on the E-W (RA) structure of a source
• To get 2D information, we want to use several dishes spread out over two dimensions on the ground
Radio Telescope ArraysThe VLA:An array of 27 antennas with 25 meter apertures
maximum baseline: 36 km
75 Mhz to 43 GHz
Very Large Array radio telescope (near Socorro NM)
VLBA
Radio Telescope ArraysALMA:An array of 64 antennas with 12 meter apertures
maximum baseline: 10 km
35 GHz to 850 GHz
The U-V Plane
• Think of an array as a partially filled aperture– the point source function (PSF) will have complicated
structure (not an airy disk)– the U-V plane shows what part of the aperture is
filled by a telescope– this changes with time as the object rises and sets– a long exposure will have a better PSF because there
is better U-V plane coverage (closer to a filled aperture)
The U-V plane
a snapshot of the U-V plane(VLBA)
U-V coverage in a horizon to horizon exposure
Point Spread FunctionThe dirty beam : the diffraction pattern of the array
Examples of weighting
Dirty Beams:A snapshot (few min) Full 10 hrs VLA+VLBA+GBT
Image Deconvolution
• Interferometers have nasty PSFs• To get a good image we “deconvolve” the
image with the PSF– we know the PSF from the UV plane coverage– computer programs take a PSF pattern in the
image and replace it with a point– the image becomes a collection of point sources
UV Plane Coverage and PSF
images from a presentation by Tim Cornwell (given at NRAO SISS 2002)
UV Plane Coverage and PSF
images from a presentation by Tim Cornwell (given at NRAO SISS 2002)
Image Deconvolution
images from a presentation by Tim Cornwell (given at NRAO SISS 2002)
What emits radio waves?
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Recipe for Radio Waves
1. Hot Gases
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Electron accelerates as it passes near a proton.
EM waves are released
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2. Atomic and molecular
transitions (spectral lines)
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Recipe for Radio Waves
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Gas Spectra
Neon
Sodium
Hydrogen
656 nm486 nm434 nm
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Electron accelerates to a lower energy state
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3. Electrons and magnetic fields
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Recipe for Radio Waves
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Electrons accelerate around magnetic field lines
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Vela
0329+54
0531+21
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What do we get in future?
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Pulsars55 discovered in globular clusters (Ransom et al).
Image Credit: Michael Kramer (Jodrell Bank Observatory, University of Manchester)
• Compact object orbiting the 23-millisecond pulsar PSR J0737-3039A, is not only another neutron star, but is also a detectable pulsar.
• Powerful laboratory for GR!
Ter5ad
NRAO/AUI/NSF 62
Galactic Super Bubble
Black Holes Radio View of the Galactic Center
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Organic Molecules;
Seeds of Life
Organic Molecules;
Seeds of Life
NRAO/AUI/NSF 66
Galactic Building BlocksGalactic Building Blocks