An Amateur Radio Astronomy Observatory David Morgan

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Radio Astronomy. An Amateur Radio Astronomy Observatory David Morgan. Part 2 Interferometers & Aperture Synthesis From amateur equipment to future global systems. Radio Astronomy. Radio Astronomy. A. B. Total Power Receiver systems - Part 1 Interferometers - Part 2 (A&B) - PowerPoint PPT Presentation

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  • An Amateur Radio Astronomy Observatory

    David MorganRadio AstronomyPart 2 Interferometers & Aperture Synthesis

    From amateur equipment to future global systems

    Website - dmradas.co.uk

  • Radio Astronomy

    Website - dmradas.co.uk

  • Radio Astronomy

    Total Power Receiver systems - Part 1

    Interferometers - Part 2 (A&B)Basic conceptObserving point sourcesSpatial resolution and sensitivityMultiple baselines & aperture synthesisFringe visibility functionsTodays best instrumentsThe future global radio telescope - The SKA

    Radio WindowVLA New MexicoMk1 at Jodrell BankSKA 2020Cosmic hydrogen distribution< 300,000years after big bangOrigin of Galaxies

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  • Radio Astronomy - Interferometer Basicssame signalfrom sourceoutputwave crests sometimes instepsometimes out of stepdepending on arrival angle Adding two waves

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  • Radio Astronomy Adding signals together - Phasing of two wavesSignal 1Signal 2

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  • Radio AstronomyTimeBEAT AMPLITUDEThe BEAT or Fringe frequency depends on Earths rotation & antenna baseline20This is the signalthat gets recordedMHz

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  • Radio AstronomyCombined signal - omni directional antennasequally sensitivein all directionsbeat frequency signalposition / time

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  • Radio Astronomy Radio Interferometry - enables detection of small sourcesSignals drift in and out of phase as the angle to the source line of sight from the baseline changes over time (Right Ascension)

    As q changes the signalsgo in and out of phase so thatsignal strength varies with angleand therefore timeResponse of single antennaResponse of two antennaeAngular resolutionExample of Interferometer fringesTimesidelobesidelobe

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  • Radio AstronomyWhy use an Interferometer ?Higher spatial resolution than a Total Power system using only one antenna Pick out small diameter sources against a general bright radio backgroundThe hardware is cheaper (small antennae spaced apart v single very large antenna)Interferometer more gain stable than a total power systemBut more processing is required to recover the source brightness pictureMost modern professional Radio Telescopes are Interferometers

    My Amateur Radio Telescope Interferometer30m East West BaselineFrequency = 408MHzl= 0.735m & baseline =30mDq = 0.735/ 30 = 0.0245 radsor 1.4 degreesEarth rotation angular velocity= 150 / hr

    Fringe frequency = 1.4 /15 hrs = 5.6mins

    This time is > signal averaging TC

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  • Radio AstronomyMy twin 15 element Quagi antenna 30m E-W Interferometer

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  • Radio AstronomyTwin Quagi Antenna responsesEach twin Yagi unit has a response shown belowAngle from antenna bore sight00+900-900Antenna outputCartesian Antenna responsePolar Antenna response14dB Gain & 170 Beamwidth

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  • Radio AstronomyMoving through interferometer beamsposition / timebeat frequency signalAntennapattern

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  • Radio AstronomyObservable discrete Point sources (northern hemisphere)VirgoVirgo ACass A 3C461 RA 23:23:21, DEC +58:49:59Cygnus A 3C405RA 19:59:28, DEC +40:44:00Taurus A 3C144 (crab)RA 05:34:30, DEC +22:00:57Virgo A 3C274 (M87)RA 12:30:48, DEC +12:22:59

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  • Radio AstronomyTaurus signal embedded in galaxy backgroundSystem Data

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  • Radio Astronomy30m separationTaurus ACrab NebulaNGC 19526,300LySignal fringesSNRAD 1054 Extracted Signal from Taurus A - The Crab

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  • Radio AstronomySource strength = 1200Jy1Jy = 10-26W/m2/HzSo we receive from the Crab about1.2x10-23W/m2/Hz

    This produces about 0.01mV in the antennaTransit was bang on scheduleThis fringe amplitude plotwas derived by cross - correlation of the signal on the previous graphwith the theoretical interferometerfringe frequencyPlot of Fringe amplitudeFringe frequency 1/ 5.6mins

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  • Radio AstronomyCross Correlation (are you like me ?)Calculated fringe signalperiod =5.6minSignal from Taurus A 16:00 01:00GMT 25/1/08

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  • Radio AstronomyLooking toward Galactic N PoleLooking toward centre of galactic planeVirgo A is a compact Radio SourceGood example of how an interferometer can distinguish compact from diffuse Radio sourcesM87 Virgo A NGC4486Giant Elliptical Galaxy withintense relativistic jet~ 60 million LY distantRadio Galaxy Virgo A M87

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  • Radio AstronomyVirgo A RadioSpectrumThis is close to the limit of whatcan be measured with my 2 antenna interferometerAnother view of theenergetic jet in M87Virgo AFlux =klx (x=spectral index)

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  • Radio AstronomyThis fringe visibility plot was derived by cross - correlation of the signal with the theoretical interferometer fringe frequencyVirgo A M87There are no more sources visible from the northern hemisphere at this levelPulsars are < 100Jy and would require a very costly 10m dia dishFringe Amplitude

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  • Radio AstronomyEstimating the size of a radio sourceIf the source produces fringes then we know that its angulardiameter is less than l/b (l= wavelength, b = baseline)The longer the baseline the smaller the source diameter that can be measuredLarge distributed sources dont produce fringes

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  • Radio AstronomyExample of distributed & point sources

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  • Radio AstronomyComparison of strengths of point radio sourcesCygnus A

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  • Radio AstronomyAmateur capabilityPossible to detect point sources within the Milky Way - Taurus A Possible to detect other Galaxies - Virgo A (60MLy)Easily possible to detect powerful Radio Galaxies - Cygnus A 700MLy

    Limiting sensitivity ~ 100Jy or 10-24 W/m2/HzPulsar detection requires 100x increase in sensitivityThis would need a larger antenna arrayCygnus A Radio Galaxy

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  • Radio Astronomy Part 2 B Aperture SynthesisObtaining radio picturesBy using multiple antennas with variable baselines it is possible to synthesise the performance of a very large single dishRadio Telescopes use Aperture Synthesis to give Radio Pictures

    The ultimate system is the Square Kilometre Array SKA

    Partly operational in 2015Fully on line in 2020

    This is subject of Part 2 B

    SKA

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