Radio Astronomy An Introduction

Transcription

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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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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