Radio Astronomy An Introduction
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1 Radio Astronomy An Introduction Felix James Jay Lockman NRAO Green Bank, WV References Thompson, Moran & Swenson Kraus (1966) Christiansen & Hogbom (1969) Condon & Ransom (nrao.edu) Single Dish School Proceedings (2002) ADS GOLDSMITH CAMPBELL LISZT
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3 Astronomy at Radio Wavelengths More than 130 interstellar molecules
4 Astronomy at Radio Wavelengths More than 130 interstellar molecules HCN from Comet Hale-Bopp -- Jewit & Saney
5 D. Balser GBT image
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8 Breton et al (2008)
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10 GASS HI Survey (McClure-Griffiths et al 2009)
11 GASS HI Survey (McClure-Griffiths et al 2009) GBT Image of Hydrogen in Smith s Cloud (Lockman et al 2008)
12 Astronomy at Radio Wavelengths -- Why is it Different? Atmosphere Low Energy Photons Diffraction Coherent Signal Processing Noise
13 What does every radio telescope shown so far have in common? Astronomy at Radio Wavelengths -- Why is it Different? Atmosphere Low Energy Photons Diffraction Coherent Signal Processing Noise
14 The Radio Window 0.01 GHz GHz
15 Transparency of the Atmosphere depends on altitude and H2O The Radio Window 0.01 GHz GHz
16 Radiative Transfer -- Specific Intensity Linear Absorption Coefficient + emissivity Optical Depth Relations through black-body radiation Absorption + emission
17 EMISSION: Blackbody Radiation
18 Blackbody radiation I ν (thermal) = 2hν3 c 2 1 exp(hν/kt ) 1 ν max (GHz) = 59 T (K) I ν (th) = 2kT λ 2 ν(ghz) << 22 T(K)
19 I ν = I 0 e τ ν
20 An aperture in the abstract Radio source! δa
21 An aperture in the abstract Radio source! δa Power Received W m = A e 2 4π I ν (θ, φ) P ν (θ, φ) dω Watts Hz 1 Power Pattern P (0, 0) = 1
22 When " << 22 T I ν (th) = 2kT λ 2 W m = ka e λ 2 4π T bν (θ, φ) P ν (θ, φ) dω Watts Hz 1
23 From Condon & Ransom
24 When " << 22 T I ν (th) = 2kT λ 2 W m = ka e λ 2 4π T bν (θ, φ) P ν (θ, φ) dω Watts Hz 1 Power from a resistor W=kT (watts Hz 1 ) Antenna Temperature T a = A e λ 2 4π T bν (θ, φ) P ν (θ, φ) dω (K Hz 1 )
25 T a = A e λ 2 4π T bν (θ, φ) P ν (θ, φ) dω (K Hz 1 ) Enclose the antenna in a blackbody of temperature Tb defining T a = T ba e λ 2 Ω a = 4π 4π P ν (θ, φ) dω P ν (θ, φ) dω
26 T a = A e λ 2 4π T bν (θ, φ) P ν (θ, φ) dω (K Hz 1 ) But we are looking through the atmosphere!
27 Specific Intensity (Brightness) I ν (θ, φ) (Watts m 2 Hz 1 str 1 ) Flux Density S ν = Ω s I ν (θ, φ) dω (Watts m 2 Hz 1 ) A flux density per unit area is actually a brightness!
28 Specific Intensity (Brightness) I ν (θ, φ) (Watts m 2 Hz 1 str 1 ) Flux Density S ν = Ω s I ν (θ, φ) dω (Watts m 2 Hz 1 ) A flux density per unit area is actually a brightness! What determines P?
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30 Uniform Illumination Boxcar Function FT Sinc Function Aperture Plane
31 For Uniform Illumination Airy rings
32 Main Beam and Sidelobes Main Beam Efficiency (from Kraus 1966)
33 In the far field, the electric-field pattern, f, of an aperture antenna is the Fourier transform of the electric field illuminating the aperture. And the power pattern, P, is the square of the modulus of f.
34 W m = A e 2 4π I ν (θ, φ) P ν (θ, φ) dω Watts Hz 1
35 W m = A e 2 4π I ν (θ, φ) P ν (θ, φ) dω Watts Hz 1 What is Ae?
36 Geometric Area Effective Area
37 Geometric Area Effective Area Reciprocity: f(t) = f(-t)
38 Reciprocity in action 1: Forward Spillover 2: Rear Spillover 3: Surface defect 4: Blockage
39 Reciprocity in action 1: Forward Spillover 2: Rear Spillover 3: Surface defect 4: Blockage Diffractive Optics: 4m at 5000Å = 8x10 6 # 100m at 21cm = 475 #
40 Spillover wastes power and can increase the noise, so taper the illumination
41 from S. Srikanth (1992)
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43
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45 November 15, 1988
46 November 16, 1988
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48 Effects of surface errors -- phase errors Scatter power out of main beam Create a sidelobe Reduce Ae Ruze equation for rms error $ Ae reduced by factor of 2 for $=#/16
49 Effects of surface errors -- phase errors Scatter power out of main beam Create a sidelobe Reduce Ae Ruze equation for rms error $ Ae reduced by factor of 2 for $=#/16 Where does it go? Atm phase errors
50 Radio Astronomical Signals must be distinguished from a background of naturally occurring noise Sources of Noise
51 Measurement error arising from noise Example: detect the HI line from a cloud with NH=2x10 18 and FWHM=20 km/s. Expected TL = 0.05 K
52 Blockage (from Goldsmith 2002)
53 Effects of blockage on dynamic range
54 Effects of blockage on dynamic range
55 Effects of blockage on dynamic range
56 The marvelous radio receivers/detectors From Condon & Ransom
57 Talkin about telescopes
58 Talkin about telescopes A radio telescope must: Survive Focus Point Track So it has a Mount Surface Optics Receiver
59 Parkes 210-ft
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