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 (  =  / D) [ its usually BIG] Interferometry (c.f. the Very Large Array – VLA) Aperture synthesis

History of Radio Astronomy (the second window on the Universe) 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/NSF4

5 Optical and Radio can be done from the ground!

NRAO/AUI/NSF6 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

 f = 1850 Hz f trans F reflect = f trans + / -  f Mixing: Adding waves together

Mixers signal in LO local oscillator   signal out     and     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 signal LO original signal mixed signal 0 Hz

Mixers frequency signal LO original signal mixed signal 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

Local oscillator Downconverted signal Frequency mixer Single sideband mixer : f = 10 GHz F +  f = 10 GHz Hz 1850 Hz  f = f IF Band-pass of amplifier: Intermediate frequency = IF

A Simple Heterodyne Receiver low noise amplifier filter receiver horn LO tunable filter 1420 MHz 1570 MHz 1420 MHz tunable LO ~150 MHz Analog-to-Digital Converter Computer ++ outputs a power spectrum 150 MHz

Amplification Why is having a low noise first amp so important? – the noise in the first amp gets amplified by all subsequent amps – you want to amplify the signal before subsequent electronics add noise Amplification is in units of deciBells (dB) – logarithmic scale 3 dB = x2 5 dB = x3 10 dB = x10 20 dB = x dB = x1000

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 = GHz at 0.3 cm = 100 GHz

Observing in the Radio II i.e. The Arecibo Telescope (D ~ 300 m) at 21cm = GHz at 0.3 cm = 100 GHz

Observing in the Radio III: Brightness Temperature Flux: erg s -1 sr -1 cm -2 Hz -1 (10 23 Jy) B  (T): erg s -1 sr -1 cm -2 Hz -1 (10 23 Jy) We can use temperature as a proxy for flux (Jy) Conveniently, most radio signals have h  /kT << 1, so we can use the Raleigh-Jeans approximation B  (T) = 2kT/ 2 Thus, flux is linear with temperature

Antenna Temperature Brightness temperature (T B ) gives the surface temperature of the source (if it ’ s a thermal spectrum) Antenna Temperature (T A ): if the antenna beam is larger than the source, it will see the source and some sky background, in which case T A is less than T B Noise in the system is characterized by the system temperature (T sys ) – i.e. you want your system temperature (especially in the first amp) to be low T B = F  2 /2k T A ~ T B  s /  b

Radio Interferometry +  East positional phase delay to source 

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 Arrays The 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 Arrays ALMA: 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 Function The 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? NRAO/AUI/NSF37

NRAO/AUI/NSF38 Recipe for Radio Waves 1. Hot Gases

NRAO/AUI/NSF39 Electron accelerates as it passes near a proton. EM waves are released

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2. Atomic and molecular transitions (spectral lines) NRAO/AUI/NSF43 Recipe for Radio Waves

NRAO/AUI/NSF44 Gas Spectra Neon Sodium Hydrogen 656 nm 486 nm 434 nm

NRAO/AUI/NSF45 Electron accelerates to a lower energy state

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3. Electrons and magnetic fields NRAO/AUI/NSF48 Recipe for Radio Waves

NRAO/AUI/NSF49 Electrons accelerate around magnetic field lines

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NRAO/AUI/NSF56 Vela

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NRAO/AUI/NSF60 What do we get in future?

NRAO/AUI/NSF61 Pulsars 55 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 J A, is not only another neutron star, but is also a detectable pulsar. Powerful laboratory for GR! Ter5ad

NRAO/AUI/NSF62 Galactic Super Bubble

Black Holes Radio View of the Galactic Center

NRAO/AUI/NSF65 Organic Molecules; Seeds of Life Organic Molecules; Seeds of Life

NRAO/AUI/NSF66 Galactic Building Blocks