Advanced Topic in Astrophysics Lecture 1 Radio Astronomy - Antennas & Imaging

Transcription

1 Advanced Topic in Astrophysics Lecture 1 Radio Astronomy - Antennas & Imaging

2 Course Structure Modules Module 1, lectures 1-6 (Lister Staveley-Smith, Richard Dodson, Maria Rioja) Mon Wed Fri 1pm weeks 8 & 9 Radio astronomy: antennas, interferometry and VLBI Module 2, lectures 7-12 (Simon Driver) Mon-Wed-Fri 1pm weeks 10 & 11 Galaxies and cosmology Module 3, lectures (Ravi Subrahmanyan & Lakshmi Saripalli) Mon-Wed-Fri 1pm weeks 12 & 13 AGN and radiation mechanisms Module 4, lectures (Ken Freeman) Semester 2 Dynamics of the Milky Way

3 Assessment Continuous (30%) 1 question sheet after every 2-3 lectures Due by the end of the following week Examination (70%) 1 paper per module, approx 40min each.

4 Resources ATNF Synthesis Imaging workshops 2003, Texts Kraus: Radio Astronomy Rohlfs & Wilson: Tools of Radio Astronomy p.4

5 Outline Imaging Resolution Direct and indirect imaging Telescopes Parabolic and hyperbolic The Antenna Horns and receivers Radio astronomy fundamentals Brightness temperature Flux density Radiometer equation p.5

6 Direct imaging onto a focal plane Resolution Δθ p.6

7 Diffraction limits Δθ=1.22 λ/d Δθ=1 λ D Optical 500 nm 125 mm Radio 20 cm 50 km p.7

8 Direct and indirect imaging Direct imaging Normal imaging method whereby an image is projected onto a detector. Examples: camera, telescope, single-dish radio telescope Indirect imaging Used where we cannot form a direct map of the object on the focal plane. We infer the properties of the object from certain characteristics of the received electromagnetic field. Examples: interferometry, NMR, ultrasound, PET, speckle. p.8

9 p.9 Direct Imaging: Single dish radio telescope

10 Direct Imaging - Angular Resolution Angular Resolution (radians) D θ = λ D Wavelength, λ=0.21m, D=64m θ=0.003 rad, or 11 arcmin Such a radio telescope only matches the resolution of the human eye at its shortest wavelength 1-2cm. p.10

11 Parabolic reflector: A parabolic reflector adds all the fields from a surface (aperture) at a single focal point. p.11 R. Subrahmanyan

12 p.12 A radio telescope at Parkes, NSW (prime focus)

13 Hyperbolic reflector B.. A Light from the point A reflecting off the hyperbola appears to come from point B p.13

14 Parabolic reflector Prime Focus p.14

15 Hyperbolic reflector Parabolic reflector Classical Cassegrain p.15

16 Hyperbolic reflector Hyperbolic reflector p.16 Ritchey-Chrétien or shaped-reflector

17 AT antenna: Cassegrain configuration 22-m diameter primary main reflector 2.75-m secondary reflector p.17

18 Signal path: p.18

19 The antenna collects the E-field over the aperture at the focus The focus is a fixed spot at all frequencies. The reflector antenna is achromatic. p.19 the feed horn at the focus adds the fields and gives the sum to the receiver as a voltage.

20 Feeds are compact and corrugated horns The inner profile is curved The inner surface has grooves Cross-section of a horn p.20

21 Brightness Temperature Solid angle of source, Ω 1 Jansky (Jy) = W m -2 Hz -1 In radio astronomy T is known as the brightness temperature, even where the object is not a black body. Examples of non-thermal sources at 1 GHz: Radio galaxy core: size = 10-3 arcsec, S=1 Jy T=10 12 K Crab giant pulses: 100m at 1.9 kpc, S=1000 Jy T=10 34 K p.21

22 Antenna Temperature and Gain N.B. 2kT=SA expected from thermodynamics Solid angle of antenna pattern, Ω A Antenna temperature is same as brightness temperature only if the source fills the antenna beam. Examples Parkes 64-m antenna (η=60%): G=0.7 K Jy -1 SKA 1km 2 : G=362 K Jy -1 p.22

23 System Temperature, T sys System Temperature is the equivalent noise power produced in the antenna receiver from sources other than the object being observed. It is the same noise that would be produced by a perfect resistor in a heat bath of temperature T sys. T sys =T rx +T spill +T atm +T cmb +T gal + T rx, receiver: internal noise in electronics (4-20K at low frequency) T spill, spillover: reflected ground emission (few % of 300K) T atm, atmosphere: frequency dependent emission (2-300K) T cmb, cosmic microwave background (3K) T gal, Galactic background (10K at 1 GHz, T gal ν -2.7 ) p.23

24 Temperature sensitivity Radiometer equation ΔT = T τδν Temperature sensitivity (K) System temperature (K) Integration time (s) Bandwidth (Hz) T=20K, τ=7200 sec, Δν=200 MHz ΔT=50µK! System temperature describes the signal a telescope sees if looking at a blackbody of temperature T which fills the field of view. NOT physical temperature. p.24

25 Fluctuations in the Microwave Sky -200µK to 200µK NASA/WMAP Science Team p.25 Structure of Universe at z=1100

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