Thermal Design and Thermal Behaviour of Radio Telescopes and their Enclosures (eBook)

Foreword 6
1 Radio Astronomy and Radio Telescopes 25
1.1 Radio Wavelengths and Radio Telescopes 25
1.2 Deep Space Network and Large Communication Antennas 33
1.3 Radio Telescopes in Space 34
1.4 Overview of Telescope Constructions 34
2 Radio Telescope Constructions in View of Thermal Aspects 37
2.1 Optical Configurations 38
2.1.1 The Parabolic Reflector 38
2.1.2 The Cassegrain and Gregory System 39
2.2 Basic Telescope Constructions 43
2.2.1 The Telescope with Alidade Support 44
2.2.2 The Telescope with Pedestal–Yoke Support 46
2.2.3 The Telescope with Pedestal–Fork Support 49
2.3 Telescope Components 53
2.3.1 Reflector Backup Structure (BUS) 53
2.3.2 The Homologous BUS 58
2.3.3 The Open and Closed, Ventilated and Climatised BUS 59
2.3.4 Reflector Panels 62
2.3.5 Active Reflector Surface 66
2.3.6 The Quadripod and Subreflector 67
2.3.7 The Secondary Focus Cabin 67
2.4 The Thermal Design of Radio Telescopes 68
3 Telescope Enclosures 73
4 The Variable Thermal Environment 79
4.1 The Environment and Observatory Sites 79
4.2 The Ambient Air 81
4.3 The Ambient Air Temperature 83
4.3.1 Measured Ambient Air Temperatures 83
4.3.2 Approximation of the Daily Ambient Air Temperature 86
4.3.3 Rate of Ambient Air Temperature Change 88
4.4 Wind Speed and Wind Direction 89
4.4.1 Measured Wind Speeds 89
4.4.2 Wind Speed and Convective Heat Transfer 92
4.5 Ground Temperature 93
4.5.1 Measured Ground Temperatures 93
4.5.2 Approximation of the Ground Temperature 94
4.6 Humidity, Condensation, Icing 95
4.6.1 Humidity and Sky Radiation 97
4.7 Sky Temperature 98
4.7.1 Measured Sky Temperature 99
4.7.2 Calculation of the Sky Temperature 99
4.8 Cloud Coverage 103
4.9 Solar Radiation 103
4.9.1 Measurements of Solar Radiation 105
4.10 Meteorological Design Specifications 106
4.11 Space Environment 107
5 Calculation of Solar Illumination 108
5.1 The Apparent Motion of the Sun 108
5.2 The Plane Surface 111
5.3 The Tube Surface 114
5.4 The Parabolic Reflector Surface 115
5.4.1 Borderline between Shadow and Sunshine 115
5.4.2 Solar Illumination of the Reflector Surface 117
5.5 The Open and Closed BUS 119
5.6 The Box–type Enclosure (Astrodome) 120
5.7 The Cylindrical Enclosure (Astrodome) 121
5.8 The Over–Hemispherical Radome 122
5.9 Manipulation of Insolation through Surface Finish 125
5.10 Sun on Subreflector and Quadripod 127
6 Temperature Measurements and FEM Thermal Deformation Calculations 130
6.1 Electric Resistance Temperature Sensors 131
6.2 Temperature Measurements by Infrared Radiation 133
6.3 Temperature Sampling Rate 133
6.4 Location of Temperature Sensors (Interpolation Matrix and Influence Matrix) 134
6.5 Precision of Temperature Measurements 141
6.6 Prediction of Temperature induced Deformations 141
6.7 Empirical Relations 143
6.8 Temperature related Measurements 143
7 Heat Transfer 145
7.1 Laws of Thermodynamics and Modes of Heat Transfer 145
7.2 Amount of Heat transferred 146
7.3 Heat Content 148
7.4 Heat Transfer by Conduction 149
7.5 Heat Transfer by Convection 152
7.6 Heat Transfer by Conduction and Convection 154
7.7 The Convective Heat Transfer Coefficient (h) 155
7.7.1 Forced Convection 156
7.7.2 Natural Convection 160
7.7.3 Forced Convection and Ventilation 161
7.7.4 Natural Convection and Ventilation 162
7.8 Radiative Heat Transfer 163
7.9 Heat Transfer by Conduction, Convection and Radiation 167
7.10 Radiative Nodes and View Factors 168
7.10.1 Radiative Nodes 168
7.10.2 View Factor Relations 170
7.10.3 Internal View Factors 171
7.11 Energy/Heat Conservation 174
7.12 The Steady and Non–Steady Thermal State 177
7.12.1 External Thermal Time Constant ext 177
7.12.1.1 ext(h) for Convective Heat Transfer 177
7.12.1.2 ext(rad) for Radiative Heat Transfer 179
7.12.2 Internal Thermal Time Constant int (Conduction) 181
8 Radiative Coupling towards Sky and Ground (External View Factor) 182
8.1 Vertical and Horizontal Walls 184
8.2 The Radome and Astrodome 186
8.3 The Parabolic Reflector 187
8.4 The Open and Closed BUS 191
9 Measured Thermal Behaviour of Radio Telescopes 193
9.1 Telescope Supports 194
9.1.1 The Pedestal 194
9.1.2 The Alidade 198
9.1.3 The Fork Support 206
9.1.4 Path Length Variations of Telescope Supports (for radio interferometer and VLBI arrays) 212
9.2 BUS Supports 216
9.2.1 The (climatised) Yoke 217
9.2.2 The Central Hub 219
9.3 Backup Structures (BUS) 223
9.3.1 The Open BUS 226
9.3.2 The Closed BUS 229
9.3.3 The Ventilated BUS 236
9.3.4 Radial and Circular Ventilation 240
9.3.5 The Climatised BUS (IRAM 30–m Telescope) 242
9.3.6 Thermal Deformations and Active Reflector Surfaces 245
9.3.7 The Radome/Astrodome enclosed BUS 246
9.3.7.1 Radome enclosed Telescopes 246
9.3.7.2 Astrodome enclosed Telescopes with open Slit 249
9.3.7.3 Astrodome enclosed Telescopes with Membrane covered Slit 249
9.3.8 Temperature Changes of the BUS and Focus Changes 251
9.4 Panels 252
9.4.1 Experimental Data of Panel Temperatures 253
9.4.2 Panel Temperatures measured on Telescopes 254
9.4.3 Thermal Panel Buckling 254
9.4.4 Heated Panels, De–icing of Panels 259
9.5 Quadripod, Subreflector, Focus Cabins 262
9.5.1 Quadripod 262
9.5.2 Subreflector 266
9.5.3 Focus Cabins 267
9.6 Observations in the Direction of the Sun 268
9.7 De–icing of a Telescope 270
9.8 Measured Temperature Uniformity of Radio Telescopes 271
9.9 The measured Thermal Behaviour of Telescopes 275
9.10 Temperature Monitoring and Trouble–Shooting 278
9.11 Other Thermal Effects 281
10 Measured Thermal Behaviour of Enclosures 282
11 Thermal Model Calculations 286
11.1 Mechanical Models of Structural Components 287
11.2 Static Thermal Calculations (The Use of Known Temperature Distributions) 290
11.3 Dynamic Thermal Model Calculations (The Derivation of Temperature Distributions) 290
11.4 Node Structure of Thermal Models 292
11.5 The Thermal Environment as Input Data 293
11.6 Models of Panels 294
11.7 Model of Telescope Supports 298
11.7.1 Model of an Alidade 298
11.7.2 Model of a Yoke 300
11.7.3 Model of a Fork Support 301
11.8 Model of a Backup Structure 307
11.8.1 Model of an Open BUS 308
11.8.2 Model of a Closed BUS with Natural Ventilation 309
11.8.3 Model of a Closed BUS with Forced Ventilation 310
11.9 Model of a Radome and Closed Astrodome 316
11.10 Servo–loop Controlled Ventilation/Heating/Cooling 318
11.11 Model Calculations and Energy Balance 319
11.12 Model Calculations for Operational Purposes 321
11.13 Precision of Model Calculations 322
11.14 Programmes for Model Calculations 323
12 Beam Formation and Beam Degradation 324
12.1 Wave Propagation and Beam Formation 324
12.2 The Perfect Radio Telescope 327
12.3 The Beam Pattern of the Perfect Radio Telescope 330
12.4 The Real Radio Telescope 332
12.4.1 Systematic Wavefront Deformations 332
12.4.2 Repetitive Wavefront Deformations (Thermal Panel Buckling) 335
12.4.3 Random Wavefront Deformations 337
12.5 Superposition of Random and Systematic (Thermal) Deformations 340
12.6 Beam Deformations and Thermal Tolerances 341
13 Thermal Tolerances 344
13.1 Global Estimates of Temperature Influences 345
13.1.1 Reflector Diameter Estimates 346
13.1.2 Estimated Thermal Behaviour of a BUS 347
13.1.3 Estimated Thermal Behaviour of an Alidade/Fork Support 349
13.1.4 Estimated Thermal Behaviour of Panels 350
13.2 System Alignment Tolerances 350
13.2.1 Displacements and Tilts of Main Reflector and Subreflector 350
13.2.2 Temperature induced Pointing Errors 354
13.2.3 Alignment Errors: Temperature Tolerance Estimates 355
13.3 Random Reflector Errors 356
13.4 Design Specifications and Error Budget 359
14 Optical Telescopes and Enclosures 361
15 A Summary and Further Studies 365
15.1 A Summary of the Present State 365
15.2 Temperature Measurements and Dynamic Thermal Models 366
15.3 Further Studies 367
15.4 A Dynamic Thermal Error Budget for Design 370
15.5 A Fundamental Investigation of Radio Telescope/Antenna Constructions 371
15.6 Future Design 373
A Units and Fundamental Constants 374
B Average Value, Root–Mean–Square Value (rms) 377
C Pointing Model 378
D Zernike Polynomials 381
Picture and Graphic Credits 382
References 385
Indices 394
Historical Names Index 394
Author Index 394
Book Index 394
Subject Index 394