Single dish radio telescopes!

Transcription

1 NRO-ALMA Science/Development Workshop 2015 Nobeyama, July 2025 Single dish radio telescopes Is there a need to grow? Is there a limit to growth? Development of (sub)-millimeter telescopes Jacob W.M. Baars Max-Planck-Institute for Radio Astronomy Bonn, Germany and Ideas for future large reflectors Hans J. Kärcher MT-Mechatronics Mainz, Germany 1

2 Beginning of Radio Astronomy - Instrument and Observation Jansky s Bruce Antenna at Holmdel Sep signal trace of 9 revolutions of the antenna Reber s home-made dish (10 m) Maps of the Galactic plane with a few strong discrete sources (1940) 2

3 Detection of the 21-cm neutral hydrogen line WWII radar antenna in Holland and the first published line spectra by Muller and Oort Horn antenna at Harvard with published observation by Ewen and Purcell 3

4 Serendipitous discoveries in radio astronomy < <Lunar occultation 3C273, Hazard, Schmidt 1963 CO-line at 115 GHz, >> Wilson, Jefferts, Penzias1970 CMB, Penzias, Wilson, 1965 V V V Pulsar, Bell, Hewish,

5 Statements in Cosmic Discovery by Martin Harwit in The most important observational discoveries result from substantial technological innovation. Still true 2. Once a powerful new technique is applied, the most profound discoveries follow with little delay. Probably still true This results in a new instrument soon exhausting its capacity for discovery. Doubtful. It better not apply to ALMA 3. Observational discoveries of new phenomena frequently occur by chance. Still true, but more rare About half the cosmic phenomena now known (1980) were chance discoveries. Many of these were discovered by individuals who designed and built ( owned ) the equipment. Few originated at national observatories. This has changed significantly with the big synthesis telescopes. VLA, IRAM, ALMA, SKA and Space Telescopes are beyond the capacity of individual, university-type institutes. They are now multinational collaborations. 5

6 Major telescope parameters Science - sensitivity (S, Tb) and angular resolution Engineering - Surface precision Pointing accuracy and stability Beam shape (sidelobe level) Environment - Gravity effects Temperature and wind influences Atmospheric conditions - water vapour, clouds 6

7 The basic requirements of a radio telescope Sensitivity - Collecting area, reflector surface precision Angular resolution - Linear size and axes control Surface accuracy better than λ/20 (for 75% efficiency) Pointing accuracy and stability better than 0.1 HPBW Examples with required (black) and achieved (red) surface Diam. (m) Min. λ (mm) HPBW ( ) Surf. (µm) Example ALMA ? CCAT future? LMT future? Eff./GBT 7

8 Natural limits for steerable radio telescopes Sebastian von Hoerner, Design of large steerable antennas, 1967 Natural limits: Gravity - λg 70 ( D/100) 2, Thermal λt 6 ΔT (D/100) (D in meter and wavelength in millimeter, temperature in Kelvin) 8

9 To go beyond the natural limits requires new, non-classical design methods (homology) and/or additional actions like thermal insulation, new materials (CFRP), active optics 9

10 Homology design - Effelsberg 100-m telescope (1971) Homologous behaviour transfers the shape of the reflector to equally accurate paraboloids as function of elevation angle with a computable change in focus position (axis direction and focal length). The feed/ subreflector must be adjusted to this moving focus during observation. A homology telescope has more than an order of magnitude better surface accuracy than a classical design, while weighing much less. Only points B of the blue umbrella reflector structure are connected to the red octahedron elevation structure at B. The octahedron is supported by the black alidade at the elevation bearings A. The quadripod is part of the octahedron and does not touch the reflector at all. 10

11 Progress in Design and Fabrication since Continuous improvements in the precision of structural design optimisation (FEA) 2. Optimum choice of structural materials (CFRP) 3. Improved manufacturing methods, specifically surface panels, bearings. 4. Sophisticated servo-control methods and hardware, algorithms, encoders, drives 5. Application of active optics, motor-controlled surface actuators, using FEA data 5. Improvements in the measurement and setting of the reflector surface, holography 6. Active thermal control of structural elements 7. Flexible Body Control(FBC) for improved pointing with sensors (tilt meter, pressure gauge, temperature sensor, displacement sensor) 11

12 Examples of large radio telescopes with passive surface MPIfR 100m Effelsberg Bonn Germany IRAM 30m MRT Pico Veleta Spain IGN 40m Yebes Spain First light m main reflector diameter Centrally suspended homologous reflector Perforated outer reflector rim 400 µm rms overall surface accuracy (passive, solid surface) Temperature control; white painting Active subreflector 2005 First light m main reflector diameter Yoke type support system 70 µm rms surface accuracy Outside insulated cladding Active temperature control Passive optics 12 First light m main reflector diameter Yoke type support system 220 µm rms surface accuracy Outside cladding Temperature control: forced ventilation Provisions for active optics (open loop, not yet installed)

13 Examples of large radio telescopes with active surface LMT/GMT 50 m Cerro la Negra Mexico SRT 64 m INAF San Basilio Sardinia m GBT Green Bank USA First light m main reflector diameter Yoke type support system 75 µm rms overall surface accuracy (goal) Outside cladding Temperature control via forced ventilation Active optics (open loop) First light m main reflector diameter First light 2000 Centrally suspended, semihomologous reflector m off-set main reflector Centrally suspended 200 µm rms overall surface non-homologous reflector accuracy Off-set subreflector Provisions for outside cladding 200 µm rms surface accuracy goal Provisions for forced ventilation Active optics (open loop) Active optics (open loop) Temperature control: white paint Closed loop surface control goal 13

14 Weight comparison per collecting area MRT IGN Effelsb LMT SRT GBT/d GBT/b Diameter (m) Precision (µm) Active surface no no no yes yes yes yes Total weight (ton) Surface weight /m Bus weight /m Elev. Str./m Alidade /m Overall weight/m Last row: note high numbers for MRT and LMT and very low value for Effelsberg, less than half of GBT 14

15 Design and Technological Progress Overcoming Natural Limits Design: Homology, FEA, Thermal analysis - Effelsberg, MRT, ALMA Materials: CFRP - Backup Structure - IRAM, HHT, ALMA Panels - IRAM, NRO, HHT Fabrication: Surface panels - machined aluminium - small, 4 µm; APEX, ALMA replica from mold - aluminium composite - large, µm, MRT, JCMT - CFRP/Alu composite - large, 5 µm. HHT - electro-formed nickel/alu composite - medium to large, 5-10 µm, ALMA, LMT Active Optics: gravitational surface correction by actuators based on FEA - LMT, GBT - thermal surface correction by temperature measurement and FEA - MRT, LMT, GBT Thermal control: active temperature equalisation system - MRT Flexible Body Control: pointing/tracking - tilt meter, displacement sensor Reflector measurement/setting: photogrammetry (30 µm), holography (10 µm) - ALMA, LMT ALL 15

16 Temperature effects the second natural limit - Asymmetric temperature distribution in structure - Time dependent difference between structural sections with different thermal time constant - Temperature gradients, possibly time-variable, in structure and reflector panels Possible Actions - Insulation of structural members - Forced air circulation in structure - Active temperature control with heating/cooling system - Measure temperature distribution and correct via FEM - Use of material with low thermal expansion (CFRP) - Restrict observations to night-time 16

17 Temperature effects and their Control in the IRAM 30-m mm-telescope Temperature gradient in BUS of 1K will cause 10 µm surface error. 5 K or more can happen during daytime from the Sun. In the 30-m telescope the BUS and Quadripod are actively controlled to equal the temperature of the Yoke Focus changes by 1 mm when Yoke and BUS temperature differ by 1 K; corrected in real time 17

18 New Technologies and Materials Carbon fiber reinforced plastic (CFRP) HHT on Mt.Graham, Arizona - 10 m diameter (1993). BUS truss of CFRP tubes, invar steel nodes. Thermally essentially inert. High stiffness to weight ratio. Gravitational deformation <3 µm. Overall surface precision 12 µm. IRAM on Pl. de Bure, France 15 m diameter (1988). BUS mixture of steel and CFRP. Overall precision 50 µm. 18

19 Parameters of some submillimeter radio telescopes CSO IRAM HHT ALMA- NA ALMA- EU CCATspec. Diameter (m) Precision (µm) Panels alu cfrp cfrp alu ni/alu alu BUS steel cfrp/steel cfrp/invar cfrp/alum cfrp cfrp Elevation steel steel steel steel cfrp cfrp Thermal control some no no some no no Active surface yes no no no no yes FBC/pointing no no no yes yes yes Year design study 19

20 Wind Effects Time- and Direction-Dependent - Wind causes Surface deformation and Pointing variations, varying quickly in time - Wind has constant and gusty component and its angle of attack is variable - Major influence on Pointing stability/accuracy - Can cause pointing errors not detected by angle encoders - Correct with sensors on structure via FEA (finite element analysis) and FBC (flexible body control) - Sensors include inclinometer, linear displacement sensor, accelerometer, pressure gauge - Surface deterioration normally of secondary importance; difficult to measure and correct 20

21 Active controlled Optics Open loop active optics for radio telescopes FBC - Flexible Body Control FEA - Finite Element Analysis Position Commands Desired Pos. - Position Controller Main Axes Drives Pointing correction FEA - focus, surface Deformation status Image Quality Controller Deform. Status Subreflector Positioner Surface Adjusters Temperature Sensors Structure & Mechanics Reflectors / Receivers Actual Pos. FEA FBC Model Alidade Inclin. & others Main Axes Encoders 21

22 Large single Dish or Interferometric Array? IRAM NRO and IRAM have both s ALMA, plus ACA NRO ALMA 22

23 Timeline and growth of Millimeter Telescopes (pictures about to scale) 80m VLMT MTM NRAO NRAO m IRAM m NRO m LMT m VLMT -?? - 80 m? IRAM NRO LMT 23

24 wavelength 1 mm NAOJ IRAM-PV LMT CCAT NOEMA ALMA ACA SMA Comparison of (sub)mm telescopes size (m) number surface area (m 1600 ( - ) 700 (350) 1960 (800) 490 (470) 2100 (52) (1400, 42) 5700 (85) (5400, 83) 910 (34) (880, 33) 225 (17) (220, 16.5) 24 resolution (arcsec) sensitivity (rel. point s.) Blue numbers refer to 1 mm wavelength. Array area equivalent diameter in brackets - ALMA is vastly superior in sensitivity and angular resolution - ACA is equivalent to 33 m single dish with full sensitivity in submm, almost twice that of CCAT

25 1000m D Ideas for future large radio telescopes New concepts based on rough extrapolation by Hans Kärcher design concepts MT Mechatronics GmbH SKA. Arecibo FAST VLTHT Very Large Tera Hertz Telescope Reflector 40 m Surface accuracy 10 µm Frequency range <2 THz 100m. ALMA. VLMT GBT Effelsberg OWLRT 300-ft VLMT Very Large Millimeter Telescope NOEMA VLTHT CCAT LMT SRT IGN MRT MERLIN Parkes GMRT Reflector 80 m Surface accuracy 60 µm Frequency range <300 GHz 10m 1µm ALMA IRAM SKA 1mm Low cost Standard passive surface 1m σ OWLRT Extremely Large Radio Telescope Reflector 160 m Surface accuracy 5 mm Frequency range < 4 GHz Open loop flexible body control Closed loop shape control 25

26 VLTHT Very Large Tera Hertz Telescope Reflector 40 m Surface accuracy 10 µm rms Frequency range <2 THz MT Mechatronics Basic Features - BUS and Elevation structure in CFRP - Homologous structure - Closed-loop Active Surface (Keck-type?) - FBC with advanced sensors - Seeing (anomalous refraction) correction system operational - Dome for survival and wind protection 40m 40m 90m 26 80m

27 Basic Features - BUS and Elevation structure in CFRP - Homologous Effelsberg-type structure - Closed-loop Active Surface (tip-tilt sensor/actuator) - FBC with advanced sensors - Seeing (anomalous refraction) correction system operational VLMT MT Mechatronics Very Large Millimeter Telescope 80m Reflector 80 m Surface accuracy 60 µm rms overall 20 µm rms inner 40m Frequency range <300 GHz overall ( )? <1000 GHz inner 40m unlikely 27

28 OWLRT Overwhelming Large Radio Telescope (a desk exercise ) MT Mechatronics Transition to rocking chair concept Reflector 160 m Wire mesh surface possible Surface accuracy 5 mm Frequency range < 4 GHz 39m E-ELT 2010 MT Mechatronics design and concept validation study for ESO 180m Sugar Grove Not a realistic alternative to Square Kilometre Telescope (SKA)

29 Single dish millimeter radio telescopes Is there a need to grow? I am not convinced there is Is there a limit to growth? There is, and we are approaching that limit both in technology and funding 29