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)

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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

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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

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Galactic Building BlocksGalactic Building Blocks