CASS Radio Astronomy School R D Ekers 25 Sep 2017 25 Sep 2017 R D Ekers 2 WHY National Facilities Easy for nonexperts to use dont know what you are doing Cross fertilization Doing the best science ID: 1044561
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1. Fundamentals of Radio AstronomyCASS Radio Astronomy SchoolR. D. Ekers25 Sep 2017
2. 25 Sep 2017R D Ekers2WHY?National FacilitiesEasy for non-experts to usedon’t know what you are doingCross fertilizationDoing the best scienceValue of radio astronomy
3. 27 Sep 2015R D Ekers3Indirect Imaging ApplicationsInterferometryradio, optical, IR, space...Fourier synthesisEarth rotation, SAR, X-ray crystallographyAxial tomography (CAT)NMR, Ultrasound, PET, X-ray tomographySeismologyFourier filtering, pattern recognitionAdaptive optics, speckleAntikythera
4. 29 Sep 2008R D Ekers4Doing the best scienceThe telescope as an analytic toolhow to use itintegrity of resultsMaking discoveriesMost discoveries are driven by instrumental developments recognising the unexpected phenomenondiscriminate against errorsInstrumental or Astronomical specialization?
5. 29 Sep 2008R D Ekers5Why Radio Astronomy?Provides unique information about the Universenon-thermal processes: quasars, pulsars, ...highest angular resolution: VLBIlow opacity: Galactic nuclei
6. Synchrotron Radiation25 Sep 2017R D Ekers6- David Wilner, ANITA lectures, Swinburne, 2015
7. Bremsstrahlung (braking radiation)25 Sep 2017R D Ekers7- David Wilner, ANITA lectures, Swinburne, 2015Radio Image ofIonised Hydrogen in Cyg XCGPS (Penticton)
8. Dust Emission25 Sep 2017R D Ekers8- David Wilner, ANITA lectures, Swinburne, 2015
9. Spectral line25 Sep 2017R D Ekers9- David Wilner, ANITA lectures, Swinburne, 2015
10. 29 Sep 2008R D Ekers10HOW ?Don’t Panic!Many entrance levels
11. 29 Sep 2008R D Ekers11Basic conceptsImportance of analogies for physical insightDifferent ways to look at a synthesis telescopeEngineers modelTelescope beam patterns…Physicist electromagnetic wave modelSampling the spatial coherence functionBarry Clark Synthesis Imaging chapter1Born & Wolf Physical OpticsQuantum modelRadhakrishnan Synthesis Imaging last chapter
12. References29 Sep 2008R D Ekers12 ─ David Wilner, ANITA lectures, Swinburne, 20153rd2017,
13. 25 Sep 2017R D Ekers13Detecting Signals from Radio Telescopes
14. 25 Sep 2017R D Ekers14Planck’s LawRayleigh-Jeans approximation
15. 25 Sep 2017R D Ekers15Noise in Radio and OpticalRadhakrishnan ”noise” ASPC 180 671, 1999
16. 29 Sep 2008R D Ekers16Resolving PowerAngular resolution = wavelength/apertureLightRadioWavelength0.00005cm10cmAperture10cm100mResolution0.00005/10 rad = 1” arc10/1000 rad = 200” arc
17. 29 Sep 2008R D Ekers17Imaging at Radio WavelengthsBad newsRadio waves are bigNeed large aperture or an interferometerGood newsRadio frequencies are lowInterferometers are easy to build
18. 29 Sep 2008R D Ekers18Greenbank 300' Radio Telescope
19. 29 Sep 2008R D Ekers19Greenbank 300’ Radio Telescope
20. 23 Sep 2012R D Ekers20Analogy with single dishBig mirror decomposition
21. 21
22. 22
23. 23( Vi )2Free spaceGuided
24. 24( Vi )2Free spaceGuided
25. 25( Vi )2Phased arrayFree spaceGuidedDelay
26. 26( Vi )2Free spaceGuidedPhased arrayDelay
27. 27( Vi )2 = (Vi )2 + (Vi Vj )Free spaceGuidedPhased arrayDelay
28. 28( Vi )2 = (Vi )2 + (Vi Vj )Free spaceGuidedPhased arrayDelayCorrelation arrayRyle & Vonberg (1946) phase switch
29. 29
30. 30IPhasedArrayx2x2x2x2x2x2Split signalno S/N losstPhased array ( Vi )2I() ( Vi )2
31. 31IPhasedArrayx2x2x2x2x2x2Split signalno S/N losstPhased array ( Vi )2I() ( Vi )2Tied arrayBeam former
32. 32<Vi Vj>t= t/correlatorFourier TransformI(r)van Cittert-Zernike theoremSynthesis Imaging
33. Fourier Transform33
34. 23 Sep 2012R D Ekers34Analogy with single dishBig mirror decompositionReverse the process to understand imaging with a mirrorEg understanding non-redundant masksAdaptive opticsSingle dishes and correlation interferometersDarrel Emerson, NRAOhttp://www.gb.nrao.edu/sd03/talks/whysd_r1.pdf
35. 29 Sep 2008R D Ekers35Filling the apertureAperture synthesismeasure correlations with multiple dishesmoving dishes sequentiallyearth rotation synthesisstore all correlations for later usePartially unfilled aperturesome spacings missingRedundant spacingssome interferometer spacings occur twiceNon-redundant aperture
36. 1 2 3 4 5 61unit 5x (source same atmosphere different)2units 4x3units 3x4units 2x5units 1x 15n(n-1)/2 = Redundancy
37. 1 2 3 4 5 6 7 81unit 1x2units 1x3units 1x4units 1x5units 0x6units 1x7units 1xetcNon Redundant
38. 29 Sep 2008R D Ekers38Basic Interferometer
39. 27 Sep 2015R D Ekers39Storing visibilitiesStorage A powerful tool to manipulate the coherence function and re-image. Not possible in most other domainsBut will this be part of our future?
40. 29 Sep 2008R D Ekers40Aperture Array or Focal Plane Array?Why have a dish at all?Sample the whole wavefrontn elements needed: n Area/( λ/2)2For 100m aperture and λ = 20cm, n=104Electronics costs too high!Computer
41. 29 Sep 2008R D Ekers41Radio Telescope Imagingimage v aperture planeComputerComputerDishes act as concentratorsReduces FoV Reduces active elementsCooling possibleλIncrease FoV Increases active elementsActive elements ~ A/λ2
42. 29 Sep 2008R D Ekers42Fourier Transform and ResolutionLarge spacingshigh resolutionSmall spacingslow resolution
43. Fourier Transform Propertiesfrom Kevin Cowtan's Book of Fourierhttp://www.ysbl.york.ac.uk/~cowtan/fourier/fourier.htmlFTFT
44. Fourier Transform PropertiesFT10% data omitted in rings
45. Fourier Transform PropertiesAmplitude of duckPhase of catFT
46. Fourier Transform PropertiesAmplitude of catPhase of duckFT
47. 29 Sep 2008R D Ekers47Analogies RADIOgrating responsesprimary beam directionUV (visibility) planebandwidth smearinglocal oscillator OPTICAL aliased orders grating blaze angle hologram chromatic aberration reference beam
48. 25 Sep 2017R D Ekers48Terminology RADIOAntenna, dish SidelobesNear sidelobesFeed legsAperture blockageDirty beamPrimary beam(single pixel receivers) OPTICAL Telescope, element Diffraction pattern Airy rings Spider Vignetting Point Spread Function (PSF) Field of View
49. 22 Sep 2012R D Ekers49Terminology RADIOMap SourceImage planeAperture planeUV planeApertureUV coverage OPTICAL Image Object Image plane Pupil plane Fourier plan Entrance pupil Modulation transfer function
50. 22 Sep 2012R D Ekers50Terminology RADIODynamic rangePhased arrayCorrelatorno analogReceiverTaperSelf calibration OPTICAL Contrast Beam combiner no analog Correlator Detector Apodise Wavefront sensing (Adaptive optics)