Advanced Technology for Modern Radio Telescopes and some history Gie Han Tan 5th EIROforum School on Instrumentation The radio spectrum Radio astronomy operates between ~10 MHz (30 m) up to >1,5 THz (0,2 mm) 5th EIROforum School on Instrumentation A condensed history of radio telescopes Radio astronomy is a relatively young discipline in experimental astronomy but has seen a very rapid development …. Karl Jansky – 1932 Grote Reber – 1937 Lovell Telescope – 1957 Westerbork Synthesis Radio detected radiation at 9 m parabolicantenna 76 m parabolicantenna Telescope (WSRT) – 1970 20.5 MHz from the Milky Way Operating at 3.3 GHz Operating up to ~5 GHz 14 X 25 m parabolic antenna / 900 MHz / 160 MHz Operating up to ~8.4 GHz 5th EIROforum School on Instrumentation A condensed history of radio telescopes … and continues to make technological progress. Green Bank Telescope – 2000 Low Frequency Array (LOFAR) 100 m off-set parabolic antenna 2012 Phased array Operating up to ~116 GHz Operating range 10-250 MHz Atacama Large Millimeter/submillimeter FAST Telescope – 2016 Array (ALMA) - 2013 500 m spherical antenna 54 x 12 m / 12 x 7 m Operating up to ~3 GHz Operating range 30-950 GHz 5th EIROforum School on Instrumentation Outline Key performance drivers for radio telescopes Sensitivity Angular resolution Frequency coverage Design examples of modern radio telescopes Atacama Large Millimeter/submillimeter Array (ALMA) Square Kilometre Array (SKA) Some new receiver technologies Phased array feeds Aperture arrays Radio Frequency Interference 5th EIROforum School on Instrumentation Sensitivity Sensitivity equation expressed in flux density S Ssys 2 k Tsys S Ssys B t Aeff Increase collecting area Aeff (larger dishes / combining multiple dishes) Reduction of system noise sources Tsys (e.g. amplifiers, antenna spill over) Increase observing bandwidth B (only works for continuum sources, e.g. thermal radiation) 5th EIROforum School on Instrumentation A sensitivity comparison LOFAR: > 1 km2 (@ 30 MHz) SKA1-Low:465.500 m2 (@ 100 MHz) 2 SKA1-Mid: 26.555 m (ηap = 80%) JVLA: 13.250 m2 5th EIROforum School on Instrumentation Sky noise Below < 100 MHz sky noise is becoming a dominant contributor to Tsys 1E+9 1E+8 1E+7 Cosmic noise towards 1E+6 Galactic pole Man made noise (quiet rural) 1E+5 Atmospheric noise (day) 1E+4 Atmospheric noise (night) 1E+3 Total antenna noise (day) 1E+2 Equivalent noise temperature [Kelvin] 1E+1 1E+0 1 10 100 1000 10000 Frequency [MHz] 5th EIROforum School on Instrumentation Angular resolution • Antenna Half-Power Beam Width (HPBW) as a measure for angular resolution 휆 휃 = 푘. 퐻푃퐵푊 퐷 Increase aperture size D Increase frequency → smaller λ • Practical limits to increasing D • Aperture synthesis • WSRT (3 km), VLA (27 km), SKA Phase 1 (150 km) • Very Long Baseline Interferometry • < 100 – > 300.000 km 5th EIROforum School on Instrumentation Angular resolution • Very Long Baseline Interferometry • Groundbased VLBI • Event Horizon Telescope • Baseline • 230 / 450 GHz • Resolution: 10 μas (goal) • Space VLBI • HALCA - VSOP • 1997 - 2005 • 8 m diameter reflector • Baseline 56 – 21.400 km • 1.6 GHz / 5 GHz / 22 GHz • Resolution: ~120 μas • Radioastron – Spektr-R • 2011 • 10 m diameter reflector • Baseline 10.000 – 339.000 km • 327 MHz / 1.6 GHz / 5 GHz / 22 GHz • Resolution: ~8 μas 5th EIROforum School on Instrumentation Angular resolution comparison 5th EIROforum School on Instrumentation What is ALMA? The Atacama Large Millimeter/submillimeter Array - a global endeavor to build an interferometric array of radio telescopes operating at wavelengths between 10 and 0.3 mm (30 to 950 GHz) 7000 m2 collecting area (54 12-m antennas & 12 7-m antennas) 5000-m altitude site in Chile Reconfigurable antennas allowing baselines up to 14 km The ALMA Partners Europe (ESO on behalf of its 16 member states) North America (US/NSF and Canada/NRC ) East Asia (Japan/NAOJ, Korea and Taiwan/ASIAA) 5th EIROforum School on Instrumentation Geographical location of ALMA Northern Chili Atacama desert 5th EIROforum School on Instrumentation San Pedro de Atacama (2400m), AOS (5000m) OSF (2900m) December 2007 5th EIROforum School on InstrumentationChile Observatories Earthquake Preparedness Workshop 14 Llano de Chajnantor Chajnantor plateau, elevation 5000+ m 5th EIROforum School on Instrumentation ALMA baseline instrument Key technical parameters: 54 & 12 antennas: • 12 m / 7 m diameter, Cassegrain configuration • Surface accuracy: < 25 μm rms / 12 m - < 15 μm rms / 7 m • Pointing accuracy: 0.6 arcsec (offset, 2 deg in position and 15 min time), 2.0 arcsec (absolute) • Nutating sub-reflector (4 antennas) Zoom configuration: • Baseline length up to 14 km • 192 antenna stations • All antennas fully reconfigurable over all antenna pads Front ends: • Dual polarisation • IF bandwidth: 8 GHz / polarisation Correlator: • Number of baseband inputs: 8 / antenna • Maximum sampling rate per baseband input: 4 GHz • Digitising format: 3 bit - 8 level • Correlation format: 2 bit - 4 level 5th EIROforum School on Instrumentation ALMA Front End key specifications Receiver noise temperature ALMA Frequency Mixing Receiver Band Range TRx over 80% of TRx at any RF scheme technology the RF band * frequency * 1 31.3 – 45 GHz 17 K 28 K USB HEMT 2 67 – 90 GHz 30 K 50 K LSB HEMT 3 84 – 116 GHz 37 K (40K) 62 K (50K) 2SB SIS 4 125 – 169 GHz 51 K (45K) 85 K (55K) 2SB SIS 5 163 - 211 GHz 65 K 108 K 2SB SIS 6 211 – 275 GHz 83 K (45K) 138 K (60K) 2SB SIS 7 275 – 373 GHz 147 K (80K) 221 K (90K) 2SB SIS 8 385 – 500 GHz 196 K (160K) 294 K (270K) 2SB SIS 9 602 – 720 GHz 175 K (100K) 263 K (150K) DSB SIS 10 787 – 950 GHz 230 K 345 K DSB SIS * In brackets achieved noise temperature 5th EIROforum School on Instrumentation Simplified ALMA block diagram Receiver systems: Super heterodyne Antenna based electronics Central back end architecture R I ADC Dual frequency conversion L Digital correlator (only single conversion shown) 1 Optical transmission systems for digitised IF (120 Gb/sec Data storage each antenna) and LO reference distribution The depicted system is Antenna based electronics located at the Array R I ADC Operations Site at 5000+ m. L Central frequency reference Remotely controlled from the Operations Support Facility 64 at ≈2800 m. 5th EIROforum School on Instrumentation ALMA Antenna based electronics General specs: IF sub-system Bandwidth: Front end RF R I ADC • 8 GHz / pol 31 - 950 GHz L IF1 4 - 12 GHz T Dual orthogonal R I o L o Band p t polarisation LO1 selector R I ADC i c 27 - 938 10:1 L a l GHz T channels Frequenc X y s multiplier u b N = 1 .. 6 - s Front end: R I ADC y s L t e 31 GHz - 950 GHz m 10 bands R I ADC L IF2 IF: 2 - 4 GHz LO2 8 - 10 GHz / 4 x 2 GHz out / 12 - 14 GHz polarisation LO reference frequencies (27 - 122 GHz, 2 GHz, 25 MHz, 20 Hz) on optical fiber from central distribution 5th EIROforum School on Instrumentation ALMA / Front End Assembly 5th EIROforum School on Instrumentation ALMA Band 7 Cartridge Cold Cartridge Warm Cartridge 5th EIROforum School on Instrumentation ALMA Band 9 Cartridge 4 + 110 K Stage Assembly (NOVA/SRON) SIS Junction Lay Out (TUD) nd Partially 2 Optics Assembled Assembly 110K Stage Integrated on 4K Plate 1st Optics 4-12 GHz Block Isolators Warm Cartridge Assembly (NRAO) Cartridge Assembly (NOVA/SRON) YIG Oscillator (15-20 GHz) LO Phase Lock LO Bias Electronics Space for Cartridge WR8 Waveguide Bias Electronics Outputs (2) 5th EIROforum School on Instrumentation Cryogenic sub-mm Low Noise Amplifiers Replacing SIS-mixers by III-V transistor technology Increased RF bandwidth Reduced cooling requirements: ~20 K instead of < 4 K 1000 5000 ) 900 4500 × ) ♦ 800 4000 700 3500 600 3000 500 2500 400 2000 Low Noise Factory 300 1500 LNF-LNC65_115WA [K] for cryogenic LNAs ( [K] for cryogenic RECK et al - 2014 n 200 1000 T [K] for room temperature LNAs LNAs ( temperature[K] for room n RECK et al - 2014 100 500 T 0 0 DEAL et al - 2016 0 200 400 600 800 1000 Frequency [GHz] Ratio left / right y-axis is 1 / 5 Demonstrated reduction in Tn when cooling LNA from room temperature to ~ 25 K (see e.g. Reck et al) 5th EIROforum School on Instrumentation SKA Phase 1 / Phase 2 Phase 1: ~130,000 Phase 2: antennas across 80km 500,000 antennas across 200km (2025 – 2033) Phase 1: 197 dishes Phase 2: 2500 across 150km dishes across 3500km of southern Africa (2025 – 2033) 5th EIROforum School on Instrumentation SKA1-Low / Mid Main Characteristics SKA1-Low SKA1-Mid Frequency range 50 MHz – 350 MHz Band 1: 350 MHz – 1050 MHz Band 2: 950 MHz – 1760 MHz Band 5: 4.6 GHz - 13.8 GHz 2 2 Collecting area (effective) 465,500 m (@ 100 MHz) 26,555 m (ηap = 80%) Antenna type Log-Periodic Dipole Array Off-set Gregorian parabolic reflector Number of antennas ~ 130,000 197 (incl. 64 MeerKAT) Maximum baseline > 65 km 120 - 150 km Sub-arrays 4 4 Correlator channels 65536 65536 Instantaneous bandwidth 300 MHz Band 1: 700 MHz Band 2: 810 MHz Band 3: 2 x 2500 MHz Minimum channel width 256 Hz 256 Hz Brightness dynamic range > 50 dB > 60 dB 5th EIROforum School on Instrumentation SKA1-Low / AAVS1 system layout MRO Main Building Internet I/F Station box “TM” MWA TPMs Control Controller Power System 1 4 Data Switch Data Splice MCCS MCCS Box Servers Servers 3 Fibre cables Fibre 3 Data Switch Data Beams to 576-core …. splicing MWA 2 Correlator Power RF over fibre Signal data Patch Recorder (server) cables Internet 1 25 ….