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Radio Astronomy – VLBI Technique Maria J Advanced Topic in Astrophysics, Lecture 4! Radio Astronomy – VLBI technique Maria J. Rioja, ICRAR, UWA Outline! What is VLBI?! From connected interferometry to VLBI! Spatial Resolution with VLBI ! Sensitivity with VLBI! Radio Telescopes: Resoluon • Resolving power (how small of a thing you can “see”) depends on the size of the telescope and the wavelength of the light For radio waves, this λ is large… size So this must also be large • “Size” = diameter of telescope for single dish; maximum distance between telescopes for arrays Radio Telescopes: Resoluon Green Bank Telescope, WV Very Large Array, NM Size Size Single Dish Connected Arrays European VLBI Network! Very Long Baseline Interferometry! p.5 VLBI array Beyond Earth limitaons: Space VLBI – VSOP What is VLBI? (Very Long Baseline Interferometry) • Radio interferometry with unlimited baselines – For high resoluon – milliarcsecond (mas) or beer – Baselines up to an Earth diameter for ground based VLBI – Can extend to space (VSOP) • Tradionally uses no IF or LO link between antennas – Atomic clocks for me and frequency– usually hydrogen masers – Disk based systems to record data – Delayed correlaon, aer media for data storage shipped – Real me over fiber is a reality now! • Can use antennas built for other reasons • Not fundamentally different from linked interferometry VLBI ‐‐ Craig Walker Synthesis Imaging Summer 7 School 2002 What is VLBI?! DELAY D Processing of VLBI observations! What is VLBI?! (Very Long Baseline Interferometry)! • Radio interferometry with unlimited baselines – For high resoluon – milliarcsecond (mas) or beer – Baselines up to an Earth diameter for ground based VLBI – Can extend to space (HALCA) • Tradionally uses no IF or LO link between antennas – Atomic clocks for me and frequency– usually hydrogen masers – Disk based systems to record data – Delayed correlaon, aer media for data storage shipped – Real me over fiber is a reality now! • Can use antennas built for other reasons • Not fundamentally different from linked interferometry VLBI ‐‐ Craig Walker Synthesis Imaging Summer 10 School 2002 DATA REDUCTION! VLBI vs LINKED INTERFEROMETRY! • VLBI is not fundamentally different from linked interferometry • Differences are a maer of degree – Separate clocks allow rapid changes in instrumental phase – Independent atmospheres give rapid phase variaons and large gradients • Different source elevaons exacerbate the effect – Sources bright enough to be both easily detectable and compact to VLBI are small, highly energec, and variable • There are no flux calibrators • There are no polarizaon posion angle calibrators • There are no good point source amplitude calibrators – Model uncertaines are can be large • Source posions, staon locaons, and the Earth orientaon are difficult to determine to a small fracon of a wavelength – Oen use antennas not designed for interferometry. Not very phase stable VLBI ‐‐ Craig Walker Synthesis Imaging Summer 11 School 2002 VLBI arrays like the EVN & VLBA have most of their telescopes located in the Northern hemisphere. The uv-coverage for sources in the southern sky (-ve declinations) is poor. The EVN can make use of telescopes located in the Southern hemisphere at European longitudes e.g. South africa: UV Coverages = Fourier sampling! At lower source declinationsEVN + (< VLBA +15 degrees)! the coverage of the EVN + VLBAEVN array (“Global+ SouthAfrica VLBI”) becomes! foreshortened (above left), resulting in an increasingly elongatedEVN beam and poorer uv-coverage. For the EVN this can be improved in the N-S direction by including Hartebeesthoek, in South Africa (above right). UV coverage for VSOP observations! Space Baseline length: 400 x 106 x 6 cm = 24000 km Resoluon = 1/ (400 e6) rad = = 0.5 mas VLBI provides….. HIGHER SPATIAL RESOLUTION: High fidelity imaging and high precision astrometry – What they look like and where they are TRADE‐OFF WITH SENSITIVITY: Targets: Very bright, compact e.g.: Acve Galacc Nuclei (AGNs), astrophysical masers THE QUEST FOR RESOLUTION Atmosphere gives 1" limit without corrections which are easiest in radio Jupiter and Io as seen from Earth 1 arcmin 1 arcsec 0.05 arcsec 0.001 arcsec Simulated with Galileo photo Radio jets & Black Hole physics Cygnus A VLA Jet = collimang ejecta that have opening angles =< 15o VLBI offers highest resolution! What is Sensivity & Why Should You Care? • Measure of weakest detectable radio emission • Important throughout research program – Technically sound observing proposal – Sensible error analysis in publicaon • Expressed in units involving Janskys – Unit for interferometer is Jansky (Jy) – Unit for synthesis image is Jy beam‐1 • 1 Jy = 10‐26 W m‐2 Hz‐1 = 10‐23 erg s‐1 cm‐2 Hz‐1 • Common to use milliJy or microJy J.M. Wrobel ‐ 19 June 2002 17 SENSITIVITY Sensivity – Antenna Performance • One antenna – System temperature Tsys – Gain G • Overall antenna performance is measured by “System Equivalent Flux Density” (SEFD): SEFD = Tsys /G – Units Jy €Credit: J.M. Wrobel Interferometer Sensivity • Antennas performance – SEFDi = Tsysi / Gi and SEFDj = Tsysj / Gj – Each antenna collects bandwidth • Interferometer built from these antennas has – Accumulaon me , system efficiency – Sensivity Credit: J.M. Wrobel Interferometer Sensivity • For SEFDi = SEFDj = SEFD drop subscripts [Units Jy] Image Sensivity . Interferometer sensivity • No. of interferometers • Sensivity of synthesis image: Units Jy beam‐1 How affects Image Sensivity? • Eg: VLBA connuum, 8.4 GHz – Observed ‐1 • T: Ipeak = 2 milliJy beam • B: Gaussian noise = 90 microJy beam‐1 – Posion error from sensivity? • Gaussian beam = 1.5 milliarcsec • Then = 34 microarcsec • Other posion errors dominate Credit: J.M. Wrobel Radio Telescopes: Sensivity • Sensivity (how faint of a thing you can “see”) depends on how much of the area of the telescope/array is actually collecng data – VLA B‐array: Total telescope collecng area is only 0.02% of land area • More spread‐out arrays can only image very bright, compact sources Brightness Temperature Sensivity • Tb sensivity = Tbs × Filling Factor – Tbs = Tb sensivity of equivalent area single dish Filling factor ∝1/ D2 so VLBI can only see very “Bright” sources Credit: C.Walker VERA : VLBI Exploraon of Radio Astrometry Construcon completed in 2002 Regular observaons from 2004 Mizusawa Iriki Ishigaki‐jima Ogasawara Target sources : Galacc masers (H2O@22GHz, SiO@43GHz) New aspect: dual‐beam for phase‐referencing Very Long Baseline Array (VLBA) • Larger arrays give you beer and beer resoluon • Trade‐off with sensivity (collecng area stays the same Size while diameter increases) END! Next lesssons! Monday next week:! - VLBI Science, by Phil Diamond (2pm - 3pm)! - VLBI Astrometry, by Maria Rioja (3pm – 4pm)! Location: ICRAR/UWA! March 29 ,Board room of the ICRAR/Curtin Brodie-Hall ! Building at 1 Turner Ave, Technology Park, Bentley!.
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