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Radio Astronomy Overview 9 May 2005 F.Briggs, RSAA/ATNF Radio `source’ Goals of telescope: maximize collection of energy (sensitivity or gain) isolate source emission from other sources… (directional gain… dynamic range) Collecting area
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Login: obstech passwd: [see me or Peter]
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* *** (biblical status in field)
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More references: Synthesis Imaging in Radio Astronomy, 1998, ASP Conf. Series, Vol 180, eds. Taylor, Carilli & Perley Single-Dish Radio Astronomy, 2002, ASP Conf. Series, Vol 278, eds. Stanimirovic, Altschuler, Goldsmith & Salter AIPS Cookbook, http://www.aoc.nrao.edu/aips/
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ground based radio techniques 10 MHz 350 VLAALMA ATCA GMRT LOFAR + MWA-LFD
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Parkes “The Dish”
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VLA, Very Large Array New Mexico
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Arecibo Telescope… Arecibo Telescope… in Puerto Rico 300 metres
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GBT Green Bank West Virginia … the newest… … and last (perhaps)… … big dish Unblocked Aperture 91 metres
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LOFAR “elements”
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Very Large Array USA Westerbork Telescope Netherlands A tacama L M illimeter A rray
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VLBA Very Long Baseline Array for: Very Long Baseline Interferometry
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EVN: European VLBI Network (more and bigger dishes than VLBA) LBA: Long Baseline Array in AU
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ESO Paranal, Chile
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Radio “Sources” Spectra: 1) Thermal 2) Non-Thermal 1)Thermal emission mechanism related to Planck BB… electrons have ~Maxwellian distribution 2) Non-Thermal emission typically from relativistic electrons in magnetic field… electrons have ~power law energy distribution Frequency MHz Flux Density Distinctive Radio Spectra !
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Nonthermal
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M81 Group of Galaxies Visible Light Radio map of cold hydrogen gas Thermal
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(from P. McGregor notes)
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I where I ~ S / can assign “brightness temperature” to objects where “Temp” really has no meaning…
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Frequency MHz Flux Density [Jansky] (from Kraus, Radio Astronomy) Brightest Sources in Sky
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Radio `source’ Goals of telescope: maximize collection of energy (sensitivity or gain) isolate source emission from other sources… (directional gain… dynamic range) Collecting area
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Radio telescopes are Diffraction Limited Incident waves
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Radio telescopes are Diffraction Limited Incident waves Waves arriving from slightly different direction have Phase gradient across aperture… When /2, get cancellation: Resolution = ~ /D
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Celestial Radio Waves?
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Actually……. Noise …. time series Fourier transform Frequency Time
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Narrow band filter B Hz Envelope of time series varies on scale t ~ 1/B sec F.T. of noise time series Frequency Time
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thought experiment… 2 wires out (antennas are “reciprocal” devices… can receive or broadcast) Cartoon antenna
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Black Body oven at temperature = T thought experiment …
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Black Body oven at temperature = T thought experiment … R
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Black Body oven at temperature = T thought experiment … R … wait a while… reach equilibrium… at T warm resistor delivers power P = kT B (B = frequency bandwidth; k = Boltzmann Const)
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R real definition… warm resistor produces P = kT B = P a = kT a B temp = T Measure Antenna output Power as “T a ” = antenna temperature TaTa
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Radio `source’ Collecting area Reception Pattern or Power Pattern
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Radio `source’ Collecting area If source with brightness temperature T b fills the beam (reception pattern), then T a = T b (!! No dependence on telescope if emission fills beam !!)
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receiver “temperature” … quantify Receiver internal noise Power as “T r ” = “receiver temperature” TaTa Ampl, etc T r +T a Ampl, etc Real electronics adds noise …treat as ideal, noise-free amp with added power from warm R
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“ system temperature” … quantify total receiver System noise power as “T sys ” T sys +T a Ampl, etc RMS fluctuations = T T = (fac) T sys /(B t int ) 1/2 Fac ~ 1 – 2 B = Bandwidth, Hz t int = integration time, seconds [include spillover, scattering, etc]
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Radio `point source’ Collecting area Power collected = S A eff B/2 S = flux density (watts/sq-m/Hz) [ 1 Jansky = 1 Jy = 10 -26 w/sq-m/Hz ] A eff = effective area (sq-m) B = frequency bandwith (Hz) T a = S A eff /2k
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If source fills the beam T a = T b S = flux density A eff = effective area (sq-m) T a = S A eff /2k “Resolved” “Unresolved” RMS = T = (fac) T sys /(B t int ) 1/2 fac ~ 1 – 2 B = Bandwidth t int = integration time
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Example 1 High Velocity HI Cloud: N HI = 1 x 10 19 cm -2 N HI = 1.8 x 10 18 T b V km/s = 1.8 x 10 18 T b (10) T b = 0.6 K 5 rms = 5 T = T b = 0.6 K rms = T = (fac) T sys /(B t int ) 1/2 = (1) ( 30)/(B t int ) 1/2 t int = ( 30/0.12) 2 /(50x10 3 ) = 1.2 seconds B = (10/3x10 5 )x1420x10 6 = 50 KHz (To reach N HI = 1 x 10 17 cm -2 need 10,000 times longer ~ 3 hours)
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Example 2 High redshift quasar with continuum flux density S = 100 mJy 5 rms = 5 T = T b = 0.07 K rms = T = (fac) T sys /(B t int ) 1/2 = (1) ( 30)/(B t int ) 1/2 t int = ( 30/0.014) 2 /(64x10 6 ) ~ 0.1 sec K a = T a / S = A eff /2k [K/Jy] = 0.7 K/Jy Parkes = 10 K/Jy Arecibo = 2.7 K/Jy VLA = 300 K/Jy SKA (T a = S A eff /2k) Parkes: 100 mJy yields T a = 70 mK 64 MHz continuum bandwidth for receiver
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Example 3: Array High redshift quasar with continuum flux density S = 1 mJy rms = S = (fac)( T sys /K a )/(B t int ) 1/2 = ( 1.4 ) ( 30/0.6)/(B t int ) 1/2 t int = ( 70/0.0002) 2 /(128x10 6 ) ~ 16 min K a = T a / S = A eff /2k [K/Jy] = 0.7 K/Jy Parkes = 6 x 0.1 = 0.6 K/Jy ACTA (T a = S A eff /2k) ATCA (B=128 MHz) : 1 mJy = 5 rms means S = 0.2 mJy rms = S = (fac)( T sys /K a )/(B t int ) 1/2
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