Radio Telescope Antennas:

Transcription

1 Radio Telescope Antennas: from Single Dish to Multielement Interferometer Carla Fanti IRA-INAF Bologna MCCT - SKADS 1

2 What is Radio Astronomy? Astronomy using Radio Waves (cm to 10 m) need a Radiotelescope radio waves MCCT - SKADS 2

3 What is Radio Astronomy? Astronomy using Radio Waves (cm to 10 m) need a Radiotelescope Ionosphere Atmospheric Transmission MCCT - SKADS 3

4 Radio Sky MCCT - SKADS 4

5 Short History of Radio Astronomy Radio signal from the sky at λ ~ 14.5 m ( ( a faint steady hiss, Karl Jansky,, 1932) Jansky s merry-go-round MCCT - SKADS 5

6 May 5, New 1933York times news MCCT - SKADS 6

7 Short History of Radio Astronomy Grote Reber ( ) 1943) (first) radio amateur Galaxy (1949) CassA Cyg First radio map of the Milky Way 9m MCCT - SKADS 7

8 The Milky Way at 408 MHz MCCT - SKADS 8

9 Main goals for Astrophysics at Radio Wavelengths High sensitivity for weak obiects the main goal - large collecting area) obiects (originally High resolution for good imaging (subsequent main goal - high frequency / large size) Wide frequency coverage for spectral studies (frequency agility accurate reflecting surface) MCCT - SKADS 9

10 Technical requirements Antenna properties / performance parameters (Receivers see E.Natale, G. Comoretto ) How large an antenna can be?? (Confusion problem) Need for sparse arrays MCCT - SKADS 10

11 (cont.) How are these goals currently achieved What can be done (can you do) next MCCT - SKADS 11

12 How is a Radiotelescope made????? MCCT - SKADS 12

13 A Radio Telescope is not made in this way (G.Sinigaglia ~1960) but almost MCCT - SKADS 13

14 Scheme of a Radiotelescope In principle not different from an Optical Telescope Mirror collects radio waves selects and detects them receiver (see E.Natale, G. MCCT - SKADS 14

15 Scheme of a Radiotelescope In principle not different from an Optical Telescope mirror (antenna) MCCT - SKADS 15

16 Antenna Parabolic Reflector (Mirror) Feed MCCT - SKADS 16

17 Antenna Types (simplest) (λ>1 m) Wire Antennas (usually arrays of) dipole Helix Yagi MCCT - SKADS 17

18 Cambridge Low Frequency (<150 MHz) Telescope (1980) MCCT - SKADS 18

19 Antenna types (λ<1 m) Mostly Paraboloids (single dish) GBT Effelsberg MCCT - SKADS 19

20 or more complex configurations (multi dish interferometer) VLA 36 km 27 antennas distributed as a Y MCCT - SKADS 20

21 a survey of Historical professional Radiotelescopes (<1965) Mostly paraboloids & Parabolic cylinders (Single dishes) or combinations of (usually not or only partially steerable) MCCT - SKADS 21

22 Historical Radiotelescopes Mills Cross m x 450m (dipole arrays, Electric steering in NS) 85 MHz (49 arcmin) (2000 radio sources) MCCT - SKADS 22

23 Historical Radiotelescopes Dwingeloo Stop 2000 HI line (Oort) (21 cm) MCCT - SKADS 23

24 Historical Radiotelescopes Jodrell Bank 76 m Mark I (1957) (18, 21, 75 cm) Lovell (resurfaced to work at 6 cm) MCCT - SKADS 24

25 90 corner 3300ft EW Historical Radiotelescopes (+ 100ft movable in NS Aperture Synthesis See later) 35 miles of wires Cambridge (GB) MHz MCCT - SKADS 25

26 Historical Radiotelescopes Cambridge 3C & 4C interferometer (see below) elements (early 60s) 3Cλ= 2 m main (see below) main parabolic cylinder, 450 m 4C λ= 1.7 m MCCT - SKADS 26

27 Two famous radioastronomers (Sir.. M. Ryle and dr. G. Smith) ) at work MCCT - SKADS 27

28 Historical Radiotelescopes Northern Cross ( ) parabolic cyliders (feed lines) 408 MHz 600m x 300m MCCT - SKADS 28

29 Northern cross present days (1976) (can you see the mirror wires?) feed line MCCT - SKADS 29

30 Northern Cross iced mirror MCCT - SKADS 30

31 Historical Radiotelescopes Parkes 210 ft Australia 75 & 11 cm (1962) resurfaced in to 1.3 cm MCCT - SKADS 31

32 (1964) 21, 18, 9 cm Nançay ay (Paris) radio waves tiltable flat mirror (200 m x 40 m) feed movable on rail spherical mirror (300 m x 35 m) MCCT - SKADS 32

33 Technical requirements Antenna properties / performance parameters (Receivers see E.Natale & G. Comoretto) How large can an antenna be?? (Confusion problem) Need for sparse arrays MCCT - SKADS 33

34 Antenna properties (In principle not different from an Optical Mirror) (CCD in modern optical Telescopes many pixels at a time) (1 pixel at a time) no photographic plate (CCD in modern optical feed in the focal plane (1 pixel at a time) (modest) resolution ( λ / D ; arcmin ) short focal length ( F / D ) MCCT - SKADS 34

35 Antenna Parabolic Reflector (Mirror) 1 pixel at a time Feed MCCT - SKADS 35

36 Feed: converts radio waves into electric voltages Dipole feed (most simple) Horn feed (often half wave) MCCT - SKADS 36

37 Antenna properties no photographic plate feed in the focal plane (modest) resolution ( Half Power Beam Width HPBW λ / D; arcmin ) short focal length ( F / D ) MCCT - SKADS 37

38 Power Pattern or Beam (Response of the radiotelescope to a point source) for a paraboloid same as diffraction pattern from a hole (Airy Disk) (polar coordinates) g radio source HPBW MCCT - SKADS 38

39 Antenna properties no photographic plate feed in the focal plane (modest) resolution ( λ / D ; arcmin ) short focal length ( F / D ~ 0.4) (field of view free from coma is small) MCCT - SKADS 39

40 Mount Palomar 5 m Telescope Long focal length MCCT - SKADS 40

41 NRAO 300 ft ( 1962)( short focal length (obvious reasons) MCCT - SKADS 41

42 Antenna properties many different pointings to get an image nowadays multibeam capability always a few pixels at a time MCCT - SKADS 42

43 MCCT - SKADS 43

44 def: Flux Density (S) Flux Density = power power received on Earth by an ideal antenna in 1 m² per Hz ª 1 Jansky (Jy) = watt/ m² /Hz (10-4 Jy S 10 4 Jy) The total monocromatic power received by an antenna of geometric area A g is P = S A g watt / Hz The larger A g the better ª (received signal is not monocromatic - v) MCCT - SKADS 44

45 Antenna performance main parameters I Aperture efficiency II Power Pattern (beam) shape MCCT - SKADS 45

46 Antenna performance main parameters I (Aperture efficiency) A g D² (geometric area) (ideal antenna) A e = η A g (effective area) (real antenna) η : Aperture efficiency (<1) frequency dependent (see below) typically 0.5 <η< < 0.7 (but also 0.2 close to mm wavelength) MCCT - SKADS 46

47 Antenna performance main parameters I (Aperture efficiency) The total monocromatic power received by an antenna of geometric area A g efficiencyη and from a source of flux density S is P = S A e watt / Hz MCCT - SKADS 47

48 Antenna performance main parameters I (Aperture efficiency) Aperture efficiency: depends on surface quality - deviations from theoretical shape blockage spillover MCCT - SKADS 48

49 Antenna performance main parameters I (Aperture efficiency) surface accuracy r.m.s.. ( σ ) gaps between panels deviation from geometry Minimum usable wavelegth λ σx 20 e.g. Northern Cross: distance between wires 2 cm λ min 40 cm λ obs = 75 cm MCCT - SKADS 49

50 Antenna Performance main Parameters I Surface quality (mesh at long λ) GBT Effelsberg MCCT - SKADS 50

51 Antenna performance main parameters I (Aperture efficiency) Aperture efficiency: depends on surface quality - deviations from theoretical shape blockage spillover MCCT - SKADS 51

52 Antenna performance main parameters I minimize blockage Effelsberg GBT MCCT - SKADS 52

53 Antenna performance main parameters I (Aperture efficiency) Aperture efficiency: depends on surface quality - deviations from theoretical shape blockage spillover (not all reflected radiation is collected by the feed power pattern spurious radiation received) MCCT - SKADS 53

54 Antenna performance main parameters I Feed (spillover if its sidelobes out of mirror i.e. if feed smaller than antenna Airy Disk) MCCT - SKADS 54

55 Antenna Parabolic Reflector (Mirror) Airy Disk Feed MCCT - SKADS 55

56 Antenna performance main parameters II (beam shape) Antenna Power Pattern (e.g. paraboloid) P ~ sin²(x)/x² xd / λ sidelobe level Antenna resolution: (HPBW) Half Power Beam Width: λ / D & directivity Pointing Accuracy ( 10 % HPBW ) MCCT - SKADS 56

57 Response of the radiotelescope to a point source (beam) diffraction pattern from a hole radio source (polar coordinates) MCCT - SKADS 57

58 Power pattern (beam) same as before but in log scale (usual coordinates) = 0.5 θ (15%) tricks to reduce sidelobe level (reduce alsoη!) MCCT - SKADS 58

59 Antenna performance main parameters II (beam shape) Antenna Power Pattern (e.g. paraboloid) P ~ sin²(x)/x² sidelobe level x D / λ Antenna resolution: (HPBW) Half Power Beam Width: λ / D & directivity Pointing Accuracy ( 10 % HPBW ) MCCT - SKADS 59

60 Antenna performance main parameters II (beam shape) aim at high resolution i.e. large dishes or / and short λ (not always feasible or appropriate) MCCT - SKADS 60

61 Technical requirements Antenna properties / performance parameters (Receivers see E.Natale, G. Comoretto) How large can an antenna be?? (Confusion problem) Need for sparse arrays MCCT - SKADS 61

62 (receiver) Any electronic device produces some power The Receiver produces spurious power (P R ±σ R ) MCCT - SKADS 62

63 (receiver) Any electronic device produces some power The Receiver produces spurious power (P R ±σ R ) The Astronomical signal produces power (S A ±σ A ) MCCT - SKADS 63

64 (receiver) Any electronic device produces some power The Receiver produces spurious power (P R ±σ R ) The Astronomical signal produces power (S A ±σ A ) both fluctuate at random about an average value σ A S A σ R P R (receiver noise - N) MCCT - SKADS 64

65 (receiver) Any electronic device produces some power The Receiver produces spurious power (P R ±σ R ) The Astronomical signal produces power (S A ±σ A ) both fluctuate at random about an average value σ A S A σ R P R (receiver noise - N) for detection S A σ R (high S/N) MCCT - SKADS 65

66 (receiver) Any electronic device produces some power The Receiver produces spurious power (P R ±σ R ) The Astronomical signal produces power (S A ±σ A ) both fluctuate at random about an average value σ A S A σ R P R (receiver noise - N) for detection S A σ R (high S/N) For a given receiver the noise decreases with (integration time) (the longer the observation the better) MCCT - SKADS 66

67 Antenna performance The Signal (S A ) received by the Antenna has to overcome receiver noise (N) (good S/N) Antennas have to have large collecting areas & high efficiency (S A A e ) Receivers have to be low noise & integration time has to be long (steerable antennas) MCCT - SKADS 67

68 Antenna performance in addition Antenna resolving power has to be high Antenna pointing has to be accurate Antennas have to be large (HPBWλ/D) MCCT - SKADS 68

69 Early solutions for Large Antennas Transit instruments (i.e. technically easier) i.e. not mechanically steerable Impossible to track a source for a long time to reduce the noise Limited surface accuracy e.g. wire mesh (to make the mirror light for mechanical problems) relative long λ ( mt ) modest HPBW MCCT - SKADS 69

70 Some Early Huge Antennas 1965: Mills Cross: Molonglo (Canberra) - Australia Northern Cross: Medicina Italy Arecibo: Puertorico USA Ooty: Ootacamund India Ratan-600: Pulkovo (ex-urss) MCCT - SKADS 70 Aggiungere HPW nelle fig.

71 Early Huge Antennas: Molonglo Cross (1966) (parabolic cylinder 1Mile 408 MHz) MCCT - SKADS 71

72 Early Huge Antennas: Molonglo Cross Bernard (Bernie) Mills (Grote Reber Medal 2006) MCCT - SKADS 72

73 Early Huge Antennas: Northern Cross Medicina (1976) 600m x 600m 408 MHz (steerable in NS) Radio sources MCCT - SKADS 73

74 Early Huge Antennas Arecibo (Puertorico - USA) m (A e = m 2 ) - totally fixed 1m to 3 cm; HI MCCT - SKADS 74

75 Arecibo Feed MCCT - SKADS 75

76 Early Huge Antennas Ooty antenna (Ootacamund India) (1970) 530 m, 327MHz 5.5 / cos(δ) in NS N Earth axis Hill slope 11 dg (~ Ooty latitude) Equatorial mount S Lunar occultations mechanical steering in EW, electric steering in NS MCCT - SKADS 76

77 Ratan 600 Early Huge Antennas 600 ( North Caucasus ~ 150 km east of Black Sea 600 m diameter λ = 1.35 to 30 cm) MCCT - SKADS 77

78 Technical requirements Antenna properties / performance parameters (Receivers see E.Natale) How large can an antenna be?? (Confusion problem) Need for sparse arrays MCCT - SKADS 78

79 How large can an antenna be?? 1 arsec rad D λ e.g. λ = 0.1 cm D = 200 m 1 cm 2 km 20 cm 40 km is this at all feasible???? even at very shortλ( small D) it would be very expensive D (2-3) MCCT - SKADS 79

80 NRAO 300 ft (transit) MCCT - SKADS 80

81 NRAO 300 ft (transit) One night. (15 Nov. 1988) MCCT - SKADS 81

82 NRAO 300 ft (transit) GBT project started MCCT - SKADS 82

83 finally Confusion problem many unresolved sources within a beam MCCT - SKADS 83

84 finally Confusion problem many unresolved sources within a beam increase A g decrease beam size MCCT - SKADS 84

85 finally Confusion problem many unresolved sources within a beam increase A g decrease beam size increase sensitivity observe many weaker sources with same S/N MCCT - SKADS 85

86 finally Confusion problem many unresolved sources within a beam increase A g decrease beam size increase sensitivity observe many weaker sources with same S/N steepness of source counts conspires to give more sources within a beam MCCT - SKADS 86

87 finally Confusion problem finally many unresolved sources within a beam increase A g decrease beam size increase sensitivity observe many weaker sources with same S/N steepness of source counts conspires to give more sources within a beam (cannot make good use of the increased sensitivity) Necessary to balance sensitivity & resolution (e.g. <1 source every beams) MCCT - SKADS 87

88 Technical requirements Antenna properties / performance parameters (Receivers see E.Natale) How large an antenna can be?? (Confusion problem) Need for sparse arrays MCCT - SKADS 88

89 To summarize: Wishlist for a good Radiotelescope a) many sparse antennas (large collecting area) b) distributed over large distances (high resolution) a) + b) = no confusion c) fully steerable (long integration time) d) excellent receivers (low noise) c) + d) = high S / N MCCT - SKADS 89

90 You need an Interferometer MCCT - SKADS 90

91 CygA (Development of high resolution) Intensity interferometer (1953) (no phases used, see below) 1.5 arcmin Cambridge 1-Mile 20 cm (1965) Cambridge 5-km 6 cm (1974) (Sorry for the low quality) MCCT - SKADS 91

92 How does an interferometer work? Simple two element adding interferometer : two holes in a mask (see your optics textbook) A B cables + cables detector MCCT - SKADS 92

93 (cont.) How is this currently achieved What can be done (can you do) next MCCT - SKADS 93

94 Sea Interferometry (Bolton Australia how to make a (Bolton &Stanley 1951, how to make a 2 element interferometer with 1 antenna) λ = 3.5 m Yagi antenna image MCCT - SKADS 94

95 Dover Heights (east of Sidney 1943) MCCT - SKADS 95

96 How does an interferometer work? Simple two element adding interferometer P =(E A +E B )² = E² A + E² B + 2 E A E B A B (E A,B electric field on A,B) + detector MCCT - SKADS 96

97 Adding interferometer E² A and E² B UNWANTED!!! they are total power terms and include all the signals collected by the two antennas (also spurious ones) MCCT - SKADS 97

98 E A = E 0 sin (2πνt) ; E B = E 0 sin [2 ν (t + τ g )] E A E B cos (2 ντ g ) cos [2π D/λ sin θ(t)] (other terms in t disappear with int. time) θ τ g wave front interference B D A fringe pattern MCCT - SKADS 98

99 1 definition Baseline [u u = D/λ cos θ] (vector) : apparent separation between the two antennas of an interferometer and orientation (in the plane of the sky); length is measured in units of λ. (e.g. u = 1000 rad -1 at 20 cm, D cos θ = 200 m) Fringe period depends on D/λ cos θ (i.e. on baseline length projected perpendicular to the source line of sight, not on D/λ) MCCT - SKADS 99

100 E A =E 0 sin (2πνt) ; E B = E 0 sin [2πν(t + τ g )] E A E B cos (2πντ g ) cos [2π D/λ sin θ (t)] (terms in t disappear with int. time) θ τ g B u = D/λ cos θ D A interference fringe pattern (amplitude & phase) MCCT - SKADS 100

101 1 definition Baseline [u u = D/λ cos θ] (vector) : apparent separation between the two antennas of an interferometer and orientation (in the plane of the sky); length is measured in units of λ. (e.g. u = 1000 rad -1 at 20 cm, D cos θ = 200 m) Fringe period depends on D/λ cos θ (i.e. on baseline length projected perpendicular to the source line of sight, not on D/λ) MCCT - SKADS 101

102 Amplitude and Phase of fringe pattern carry information on structure and position of the radio source. Fringes is what needed We get these data in a simple way with a Correlation interferometer MCCT - SKADS 102

103 Correlation interferometer fundamental equation P(t) = E= (t) E (t-τ ) dt A B g i.e. electric fields are multiplied (not added) and the result is integrated in time MCCT - SKADS 103

104 Interferometer technique also applied to large Cross-type radiotelescopes MCCT - SKADS 104

105 to ALMA(Atacama Large Mm/ m/submm Array) start antennas within 15 km 0.3 to 9.6 mm arcsec Chile 5000 m above see level MCCT - SKADS 105

106 What is Radio Astronomy? Astronomy using Radio Waves (cm to 10 m) need a Radiotelescope Ionosphere Atmospheric Transmission MCCT - SKADS 106

107 to VLTI (Very( Large Telescope Interferometer) Cerro Paranal (Atacama desert) 2635 m above see level 8.2 m (4 tel) 1.8 m (4 tel) 130 m baseline NIR - MIR MCCT - SKADS 107

108 2 definition Visibility Function (V): interferometer output (fringes), as a function of projected baseline u (discrete complex function) amplitude phase MCCT - SKADS u 108

109 Relation between Sky brightness and Interferometer signal The Visibility Function (V) is a sampling of the Fourier Transform of the source brightness distribution e.g.:.: point source P(x) = A δ(x-x ) (1D) 0 FT: V = Ae -2 iu x 0 (i = -1 ) [ u = D / λ cos θ ] = Ae iφ(u) [A = const, φ(u) u x 0 ] MCCT - SKADS 109

110 one baseline provides one Fourier Component at the (spatial( spatial) frequency u = D/λ cos θ A simple two element interferometer is not enough to derive a radio source brightness distribution / position MCCT - SKADS 110

111 1 baseline MCCT - SKADS 111

112 one baseline provides one Fourier Component at the (spatial( spatial) frequency u = D/λ cos θ A simple two element interferometer is not enough to derive a radio source brightness distribution. Many different baselines are required MCCT - SKADS 112

113 2 baselines MCCT - SKADS 113

114 3 baselines MCCT - SKADS 114

115 4 baselines MCCT - SKADS 115

116 5 baselines MCCT - SKADS 116

117 6 baselines artifacts due to baseline truncation MCCT - SKADS 117

118 Symmetric double source Visibility Function Amplitude Phase 2 equal Point sources 1 arcmin separation 1π jump 1 jump u (kλ) u =D/λ cos θ = 30 kλ D = 6 λ= 20 cm MCCT - SKADS 118

119 Amplitude Baseline sampling Phase 1 jump 1π jump kλ u (kλ) MCCT - SKADS 119

120 arcmin 2 sources MCCT - SKADS 120

121 Amplitude Baseline sampling Phase 1 jump 1π jump kλ u (kλ) MCCT - SKADS 121

122 arcmin 2 sources MCCT - SKADS 122

123 Amplitude Baseline sampling Phase 1 jump 1π jump u (kλ) kλ MCCT - SKADS 123

124 arcmin 2 sources Artifacts Image reconstruction Problems (Dallacasa) MCCT - SKADS 124

125 The longer the baseline the better you describe the structure??? Amplitude Phase 1 jump 1π jump kλ u (kλ) MCCT - SKADS 125

126 arcmin 2 sources MCCT - SKADS 126

127 a drawback: Missing short baselines A extended structures give Fourier components at short baselines (observableθ max 1/ u min ) Good baseline coverage at short spacings important u= D/λ MCCT - SKADS 127

128 Cen A at many λ and resolutions ( ) Single dish MCCT - SKADS 128

129 Multi-element interferometer: a Fourier Transformer N antennas provide N (N-1)/2 (possibly different) baselines to sample the Visibility Function (i.e. the FT of brightness) The inverse FT of the Visibility Function provides a first approximation of the sky brightness (see Dallacasa) MCCT - SKADS 129

130 Multi-element interferometer brightness distribution is bidimensional: bidimensional antenna distributions are required to derive bidimensional Visibility Function and compute FT. MCCT - SKADS 130

131 3 definition u-v plane: : the plane perpendicular to the source line of sight containing all the projected baselines (vectors) of a multiple interferometer MCCT - SKADS 131

132 Aperture Synthesis when we add the complex response of many baselines (FT) we synthesize an aperture as large as the interferometer length (for HPBW only) MCCT - SKADS 132

133 Early times: Caltech 2 element variable spacing Interferometer ( 1960) E W == ============= a few days (4 different baselines, 1 at a time) MCCT - SKADS 133

134 Caltech 2 element Interferometer E W ============ ===== a few days MCCT - SKADS 134

135 Caltech 2 element Interferometer E ============= Then a few days S (3 different baselines) MCCT - SKADS 135

136 for each configuration: fringes in the sky Dec No info on source structure/position baseline orientation R.A. MCCT - SKADS 136

137 Fringes in the sky rough idea on source position * baseline orientation MCCT - SKADS 137

138 baseline 2 Fringes in the sky * baseline 1 MCCT - SKADS 138

139 Fringes in the sky baseline 2 baseline 3 * baseline 1 MCCT - SKADS 139

140 baseline 2 True Position baseline 3 * baseline 4 baseline 1 MCCT - SKADS 140

141 Amplitude (modelfitting) A double source almost point source MCCT - SKADS 141 u

142 Caltech 2 element Interferometer a few baselines in EW + a few baselines in NS = 7 Visibility points. This allowed A. Moffet to modelfit 3C radio sources proving that in the majority of cases powerful radio sources have a double structure MCCT - SKADS 142

143 nowadays it is possible to build arbitrarily complex Interferometers MCCT - SKADS 143

144 WSRT (~1970) WSRT 3 km EW 10 fixed + 4 movable antennas 40 (91) pairs (baselines) MCCT - SKADS 144

145 VLA (1980) ~36 km - 27 antennas 351 pairs (baselines) MCCT - SKADS 145

146 Earth Rotation Aperture Synthesis In 1965 Sir M. Ryle (nobel prize for radio astronomy) realized that an array of antennas changes continuosly its position with respect to a radio source while the Earth carries it around. MCCT - SKADS 146

147 Earth Rotation Aperture Synthesys MCCT - SKADS 147

148 Earth Rotation Aperture Synthesis The baselines change position (and length) continuosly providing a very large number of baselines. On the u-v plane the baselines track incomplete Ellipses (time length the source is above horizon) MCCT - SKADS 148

149 Fourier plane ( u-v v ) sampling VLBI MCCT - SKADS 149

150 baseline (u-v) coverage not continuous grating lobes appear Dirty beam (WSRT) secondary lobes MCCT - SKADS 150

151 Dirty Maps (WSRT) (Image reconstruction see Dallacasa) MCCT - SKADS 151

152 How large can an interferometer be??? Short local scale (< 30 km, cable linked ) Long regional scale ( km, radio-linked) Very long global scale (>1000 km, not linked) Extremely long - space scale (> Earth radius) MCCT - SKADS 152

153 How does an interferometer work? Simple two element multiplying interferometer A B cables x cables detector MCCT - SKADS 153

154 up to 30 km (cable linked) MCCT - SKADS 154

155 How large can an interferometer be??? Short local scale (< 30 km, cable linked ) Long regional scale ( km, radio-linked) Very long global scale (>1000 km, not linked) Extremely long - space scale (> Earth radius) MCCT - SKADS 155

156 30 to km (radio linked) (Merlin) MCCT - SKADS 156

157 How large can an interferometer be??? Short local scale (< 30 km, cable linked ) Long regional scale ( km, radio-linked) Very long global scale (>1000 km, not linked) Extremely long - space scale (> Earth radius) MCCT - SKADS 157

158 1000 to km (no link) (VLBI) MCCT - SKADS 158

159 How large can an interferometer be??? Short local scale (< 30 km, cable linked ) Long regional scale ( km, radio-linked) Very long global scale (>1000 km, not linked) Extremely long - space scale (> Earth radius) MCCT - SKADS 159

160 Extremely Long Baseline Interferometers (end 2005) D = km MCCT - SKADS 160

161 VSOP (launched on Feb. 12, 1997) 8 m deploiable (umbrella-like) mesh dish 18, 6, (1.3) cm VSOP-2 launch MCCT - SKADS 161

162 (not linked interferometers) (τ g ) (correlator) MCCT - SKADS 162 in Europe at JIVE (Dwingeloo)

163 Optical fibers evlbi (2004) MCCT - SKADS 163

164 Summary need for large antennas (sensitivity/resolution) Antenna properties / performance parameters How large an antenna can be (costs, mechanical problems, confusion problem) MCCT - SKADS 164

165 Summary (cont.) Need for sparse arrays (interferometers) with good u-v coverage also at short baselines How is this currently achieved What can be done (can you do) next?? SKA!!!! MCCT - SKADS 165

166 GHz ( ) start 2010 complete 2020 MCCT - SKADS 166

167 The end MCCT - SKADS 167

168 TENERE! MCCT - SKADS 168

169 Radiotelescope performance Antenna collects radio-waves that are conveyed to the Receiver which transforms them into electric signals MCCT - SKADS 169

170 (receiver) Any electronic device produces some power The Receiver produces spurious power (P R ±σ R ) The Astronomical signal produces power (S A ±σ A ) both fluctuate at random about an average value σ A S A σ R P R (receiver noise - N) for detection S A σ R (high S/N) For a given receiver the noise decreases with (integration time) (the longer the observation the better) MCCT - SKADS 170

171 finally Confusion problem many unresolved sources within a beam given D: A e D², Ω b (HPBW) ² N radio sources/ster ster at limiting S L n = NΩ b sources in the beam with 2xD: A* e = 4 A e, Ω* b = Ω b / 4 increased sensitivity allows to reach S* L = S L / 4 with same S/ N MCCT - SKADS 171

172 Confusion problem source number counts N (S > S L ) ( S L )^(-δ) n* * = N*Ω* b = (4^δ N) ) (Ω( b / 4) = n 4 δ-1 n* * > n for δ > 1 Necessary to balance sensitivity & resolution (e.g. <1 source every beams) MCCT - SKADS 172

173 fringe period E A E B cos [2 D/λ sin θ(t)] 2 D/λ sin θ (t) - 2 D/λ sin θ (t) = 2 λ/d = sin θ - sin θ (θ θ ) cos< θ > (θ θ ) 1 / [ D/λ cos< θ >] MCCT - SKADS 173

174 recuperi?????? MCCT - SKADS 174

175 A Radio Telescope is not made in this way (G.Sinigaglia ~1960) MCCT - SKADS 175

176 Scheme of a Radiotelescope In principle not different from an Optical Telescope mirror receiver detector (see E.Natale) MCCT - SKADS 176

177 It was not always like that!! CygA (Development of high resolution) CygA 1.5 arcmin Intensity interferometer(1953) Cambridge 1-Mile 20cm (1965) Cambridge 5-km 6 cm (1974) (Sorry for the low quality) MCCT - SKADS 177

178 one of early designs of SKA Use small antennas to synthesize ~ 35km telescope. Can fill an area up to ~ 1km??? MCCT - SKADS 178

179 Types of Radiotelescopes Simplest: Horn Yagi antenna (dipoles) MCCT - SKADS 179

180 Ratan-600 Pulkovo (1977) MCCT - SKADS 180

181 Largest fully steerable antennas Collecting Area (~10000( m²) m GBT Effelsberg MCCT - SKADS 181

182 VLA A_g 27 x 25 ² m^2 Min. HPBW 0.04 arcsec at 43 GHz min flux density 0.01 mjy for 12 h int. time MCCT - SKADS 182