OPTICS OF SINGLE BEAM, DUAL BEAM & ARRAY RECEIVERS ON LARGE TELESCOPES J A M E S W L A M B, C A L T E C H

Transcription

1 OPTICS OF SINGLE BEAM, DUAL BEAM & ARRAY RECEIVERS ON LARGE TELESCOPES J A M E S W L A M B, C A L T E C H

2 OUTLINE Antenna optics Aberrations Diffraction Single feeds Types of feed Bandwidth Imaging feeds Dual feeds Beam switching Nutating secondary Focal plane arrays Aperture plane arrays Conclusion 19-Sep-2010 MMIC & Array Rx Workshop, Bonn 2

3 ANTENNA ABERRATIONS 19-Sep-2010 MMIC & Array Rx Workshop 3

4 COMA VS. ASTIGMATISM ALMA antenna D = 12 m 10 f = 4.8 m d = 750 mm M = 20 F = Mf = 96 m N = F/D = 8 RMS path error, mm Astigmatism Coma Radius of secondary 0.01 surface error: 20 μm Note Ritchey Chrétien telescopes designed to reduce coma, but they degrade astigmatism Feed lateral offset, mm 19-Sep-2010 MMIC & Array Rx Workshop 4

5 ANTENNA PATTERN SZA Antenna Wide-Field Pattern Secondary mirror shadow boundary main beam Primary mirror shadow boundary Amplitude, dbi 20 0 first sidelobe feed spillover E-plane H-plane Poisson (Arago) spot secondary mirror edge diffraction Boresight angle, deg 19-Sep-2010 MMIC & Array Rx Workshop 5

6 OTHER ANTENNA PERFORMANCE ISSUES Focal surface Petzval surface Half sum of optical element curvatures Surface has radius of curvature ~2 x secondary radius of curvature Single offset pixel refocus secondary (e.g., ALMA) Pixel array place on Petzval surface Polarization Offset feeds have beam separation between LCP and RCP 19-Sep-2010 MMIC & Array Rx Workshop 6

7 PLANAR FEEDS J. Zmuidzinas, Caltech 19-Sep-2010 MMIC & Array Rx Workshop 7

8 WAVEGUIDE FEEDS Rectangular Conical Diagonal Potter (dual-mode) Dielectric loaded Quad-ridge Corrugated ( scalar ) Smooth-wall Potter horn (picture J.F. Johansson) CSIRO, Aust 19-Sep-2010 MMIC & Array Rx Workshop 8

9 CORRUGATED HORN Gold Standard Properties Circularly symmetric pattern Low sidelobes Low cross-pol ~40 % bandwidth Low VSWR Variants Diffraction limited Wideband Profiled Dual-band Ring-loaded slots 19-Sep-2010 MMIC & Array Rx Workshop 9

10 GAUSSIAN BEAM PARAMETERS 19-Sep-2010 MMIC & Array Rx Workshop 10

11 CORRUGATED HORN AND GAUSSIAN BEAM Wavefront R a w Corrugated horn 1/e beam radius Aperture phase error -> diffraction limited or wideband Aperture close to waist, or in far-field of GB Aperture at confocal surface -> optimum gain horn 19-Sep-2010 MMIC & Array Rx Workshop 11

12 IMAGE FEED APERTURE ON TO SECONDARY T.-S. Chu, An imaging beam waveguide feed, IEEE Trans Antennas and Propagat., vol. AP-31, no. 4, pp , July Sep-2010 MMIC & Array Rx Workshop 12

13 FOCAL PLANE FIELDS: COUPLING Integrate over source and horn beams to obtain coupling Horn aperture field Diffraction limited horn Matches central Airy lobe well Normalised amplitude Fourier transform of horn aperture field Airy pattern Imaged horn Matches central and first sidelobe of Airy pattern well Normalised radius 19-Sep-2010 MMIC & Array Rx Workshop 13

14 FOCAL PLANE FIELDS: TRUNCATION Effect of truncation depends on location effect worst near (image of) focal plane effect least near (image of) aperture plane Clear diameter of 5 beam radii is conservative Relative efficiency, % r/w 19-Sep-2010 MMIC & Array Rx Workshop 14

15 EXAMPLE OF FREQUENCY-INDEPENDENT OPTICS SEST frequency-independent optics Beam radius, mm Amplitude, db Beam Phase, radius, deg mm Phase, deg Angle from boresight, deg Distance from feedhorn, m Phase Slippage, radians Phase Slippage, radians 19-Sep-2010 MMIC & Array Rx Workshop 15

16 Efficiency, % IMAGED VS. NON-IMAGED HORN % 90 f min f max % 83.7% At Secondary Focus At Primary Focus Relative Frequency 40% BW 19-Sep-2010 MMIC & Array Rx Workshop 16

17 DUAL-BEAM SYSTEMS Principle Rapidly switch between two close positions on sky Difference removes atmospheric fluctuations beams overlap in atmosphere (near field) v. important for (sub-)millimeter Rx gain fluctuations Dicke switching Considerations Frequency atmosphere: 1-10 Hz receiver: 1 Hz 10 khz Beam throw: < 1 deg Single dish not required for interferometry 19-Sep-2010 MMIC & Array Rx Workshop 17

18 IMPLEMENTATION: WITH FEEDS Move feed mechanically Switch between feeds (mechanical or electrical) Optical chopper Correlation radiometer 19-Sep-2010 MMIC & Array Rx Workshop 18

19 IMPLEMENTATION: USING SECONDARY 100 Vertex of secondary Effective rms surface error, m arcmin Center of mass 3 arcmin 1.5 arcmin Distance of rotation center from prime focus, mm 19-Sep-2010 MMIC & Array Rx Workshop 19

20 DIFFRACTION AT SECONDARY Adds ground spillover noise Increases with feed offset secondary motion Cancels for symmetrical beam switching Nutating secondary and focal plane array combine offset and switching effects large imbalance reduce with shield primary secondary p [ o ] 2.0 I/I Sep-2010 MMIC & Array Rx Workshop 20

21 FOCAL PLANE ARRAY CONSIDERATIONS Number of pixels Cost Backends Focal plane size Aberrations Beam spacing ~2.5 beams minimum horn spacing Heterogeneous interferometer arrays? Diffraction limited or imaged feeds? Image de-rotation (Az/El antennas) Rotate in optics Rotate receiver Rotate in software 19-Sep-2010 MMIC & Array Rx Workshop 21

22 NRAO 8-BEAM 1-MM RX Originally Schottky Retrofitted with SIS 19-Sep-2010 MMIC & Array Rx Workshop 22

23 IRAM 9-BEAM SIS RECEIVER 19-Sep-2010 MMIC & Array Rx Workshop 23

24 PHASED ARRAY FEEDS xw -3 xw -2 xw -1 xw 0 xw 1 xw 2 xw Sep-2010 MMIC & Array Rx Workshop 24

25 PHASED ARRAY FEEDS Need to add gain to maintain S/N xw -2 xw -1 xw 0 xw 1 xw 2 xw -2 xw -1 xw 0 xw 1 xw Sep-2010 MMIC & Array Rx Workshop 25

26 PHASED ARRAY FEEDS Sep-2010 MMIC & Array Rx Workshop 26

27 APERTURE PLANE FEEDS Phased array feeds sample complex field Can be placed anywhere along beam In aperture plane amplitude uniform, phase varied Can place individual feeds in aperture plane Use: e.g., CARMA correlate subapertures on 10-m antennas with 3.5-m antennas 19-Sep-2010 MMIC & Array Rx Workshop 27

28 CONCLUSIONS Radio antennas capable of wide field imaging Imaging often limited by focal plane size Many feed designs to choose from corrugated horn is probably still the best Large arrays may compromise single pixel performance Synthesized feed arrays may become practical large digital back-ends will help 19-Sep-2010 MMIC & Array Rx Workshop 28