Huib Jan van Langevelde

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e-VLBIa real-time telescope larger than Europe

• Huib Jan van Langevelde
• Joint Institute for VLBI in Europe
• Sterrewacht Leiden

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Acronyms/Organizations involved

• VLBI Very Longs Baseline Interferometry
• A radio-astronomical technique to obtain high
  resolution
• EVN European VLBI Network
• Consortium of (European) Telescopes
• Arecibo, Puerto Rico, Cambridge (UK), Effelsberg
  (D) Jodrell Bank (UK), Medicina (I), Metsahovi
  (FI), Noto (I), Onsala (S), Shanghai (CN), Torun
  (PL), Urumqi (CN), Westerbork (NL), Yebes (ES)
• Joint Institute for VLBI in Europe
• Institute established in Dwingeloo, the
  Netherlands
• Funded by NWO (NL), ASTRON (NL), STFC (UK), INAF
  (I), ICN-IG (ES), OSO (S), CAS (CN), CNRS (F),
  MPG (D)
• EXPReS EXpress PRoduction e-VLBI Service
• EC project funded, started in 2006
• Partners most radio-telescopes in Europe, some
  outside
• DANTE and a number of NRENs, SURFNet, AARNET,
  PSNC

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Radio-astronomy

• Radio waves with ? of 0.7mm to 90cm
• Compared to optical light 400 700 nm
• Can be detected and amplified with antenna

• Radio emission from hot gas between the stars
• Super bright emission from vicinity of black holes

• The Galaxy at 320 MHz

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• Reber started pioneering radio-astronomy in 1937
• Unfunded research as an amateur

• Jansky discovered radiation from Galaxy in 1931
• Result largely ignored

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• Larger telescopes detect weaker signals
• Sensitivity ? D2

Lovell Telescope D 76 m, Manchester

• Larger telescopes resolve more details
• Resolution ? ?/D
• ? wavelength
• D Diameter

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• Connected radio-interferometry reaches higher
  resolution
• Based on distributed clock signal and central
  processor
• Westerbork Synthesis Radio Telescope

• JIVE ASTRON headquarters in Dwingeloo, the
  Netherlands

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G23.657-0.127, the methanol ring

• Based on Torun survey of Galactic plane
• Followed up with EVN
• November 2004 session
• First 8/9 antenna session

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Motion of the brightest spot

• Detect projected earth orbit wrt distant
  references 3.19 (0.41 -0.35) kpc (Bartkiewicz
  et al in press)

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Results in extraordinary high resolution (for
bright objects)
High enough to see things move at cosmological
distance
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Principle of VLBI

• Record same frequency band simultaneously at N
  telescopes
• Use best possible local clocks and frequency
  standard
• Sample and digitize and record (on magnetic
  medium)
• Find all ½N(N-1) correlation coefficients at
  correlator
• Compute back image from thousands of these
  measurements

weak radio source
correlator
recorder
maser clock
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Recap

• Obtain all correlation functions on all baselines
• Arrays of telescope sample aperture (u,v domain)
• Helped by the rotation of the earth
• Fourier relation with sky brightness
• Visibilities can be inverted to form dirty image
• Needs to be de-convolved to image source
• Effectively interpolating the gaps in the
  uv-domain
• Many ways to visualize this, here is one

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Optics holes in a mask
Double number of holes from frame to frame 2,
4, 8, 16, 32, 64, 128, 256, 512, 1024
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• Correlator also registers signals with different
  phase information
• Builds up Fourier components of sky image as the
  earth turns

• Electronic equipment brings signal to focus in
  phase
• Similar to properties of parabola
• Usually only room for single pixel detector

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Post processing (a diversion)

• Imaging is the task of the user
• Requires expert knowledge
• And strategy depends on objective
• Data size involved scale with
• Number of antennas (squared)
• Integration time
• Spectral resolution
• Time resolution
• Can be 100 Gbytes input data
• Still quite dependent on software from the 70s
• Established algorithms
• But requires parallel approach

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Large field of view

• Traditionally VLBI focuses on single target
• Faint sources require lots of resources
• Tape/disk media
• Telescope time
• Correlator time
• Astronomer time
• More efficient use of telescope time
• Study µJy sources
• As density goes up
• Starburst/AGNs
• Weak masers in star forming regions

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ParselTongue

• Python interface classic AIPS
• Publicly available
• Documentation (user and developer) available
• Repository for user contributed code
• Builds OK for most environments (eg my Mac)
• Widely used by PhD students for complex or large
  data processing
• Development focuses
• Infrastructure for distributed processing
• Easier builds
• Developed in context of ALBUS within RadioNet

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VLBI digital processing

• To reach the faint end of the universe
• Need many big telescopes
• And as much frequency space as you can get
• Bandwidth!
• Typical VLBI uses 64 MHz bandwidth in 2 pols
• Nyquist sampling
• 2 x 2pol x 32 MHz 128 M samples/s
• Noisy data 2 bits/sample recover 86 SNR
• 256 Mb/s, more is preferred
• Bits are not sacred
• Some losses are tolerable

radio sources in the Hubble deep field require
several days of integration (Garrett et al., 2000)
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Hardware Correlator

• Based on custom VSI chips
• Developed in global consortium
• 1024 chips
• Clock-speed of 32 MHz
• To deal with 16MHz band
• Can deliver 1024 spectral points
• On all baselines between 16 stations
• 262144 correlations
• 8 times a second
• Equivalent to a few hundred PCs
• Was 50000 when it was built!

The current EVN Mk4 correlator is Based on 32x32
custom chips
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• Recording on Linux PC platform
• Harddisk recorder based system
• Parallel writing on 8 disk system

• Sending hard-drives around the world
• Typically have a few thousand going around

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Now turn to e-VLBI!

• PC based recording
• Also allows Internet transmission
• Upgrade EVN to e-EVN
• Started with a pilot in 2004
• And was boosted with EXPReS
• Retrofit correlator to work real-time
• Help solve last mile problem at telescopes
• Work closely with NRENs on robust connectivity
• Push to 1024 Mb/s limit
• Bring in the big telescopes
• And start the revolution in radio-astronomy
  culture

Express Production Real-time e-VLBI Service
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• Establish connectivity through Europe on GÉANT2
• Greatly catalyzed by having EC funded project

• All come together on the Dutch SURFnet6
• Large bundle from Amsterdam to Dwingeloo

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EXPReS network upgrade
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Remarkable progress

• 7 telescopes regularly on line, interesting for
  science
• Correlator operations optimal for direct results
  and feedback
• Connectivity reached impressive reliability
• Started with TCP, but obviously not the optimal
  protocol for e-VLBI
• Now dedicated light-paths to most telescopes
• UDP protocols implemented for optimal streaming

Size of balloon set by number of telescopes
participating, height by station sustained
bit-rate
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Connectivity progress Effelsberg

• On-line on 1/4/08
• Through GÉANT2 connection
• Thanks to local loop funded by MPG
• Reached almost 1 Gb/s in first test
• Also tried exercise new data format
• Extremely important for science impact
• Boosts the sensitivity of the e-VLBI array

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Connectivity progress towards 1Gbps

• Gbps connectivity
• Progress with data dropping
• First fringes on 27/12/07
• Channel selection coming soon
• Also longest user experiments so far
• 6 x 512 Mbps
• 2 user experiments
• gt 12h uninterrupted

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• Science runs now very reliable, competitive with
  recorded VLBI
• Run single-handed by post-doc or correltor
  operator

• Typical runs last 24hr
• Start-up usually during daytime (at least
  somewhere on the globe)

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TEIN2
Miyun
Urumqi
Seshan
Kunmin
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EXPReS connectivity progress

• Long-haul high-bandwidth data transport
• TCP on old Linux kernels clearly inadequate
• Circuit TCP performs well (TCP with UDP-like
  behaviour, without congestion control)
• UDP better, but needs modification of Mark5A
  control code
• And can be hostile to other users on open network
• Stability of original code a serious issue
• Has led to complete re-write of subset of Mark5
  control code

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• Arecibo, Puerto Rico
• TIGO Chile
• Hartebeesthoek, South Africa

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e-VLBI, an operational facility

• e-VLBI offered for Target-of-Opportunities
• From start of project in 2006 for brave
  astronomers

The X-ray binary CygX-3 was observed after a
major outburst in May 2006 using e-VLBI.
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Why is e-VLBI exciting for astronomy?

• Rapid response for rapid variability
• Fast response to requests
• Immediate analysis of data, adapt observing
  parameters
• Coordination with current and future
  observatories
• Immediate feedback
• More robust data
• Fewer consumables, logistics
• Constantly available VLBI network
• Monitoring for example astrometry
• Spacecraft tracking
• Growth path for more bandwidth
• More sensitive astronomy

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More spacecraft tracking

• LAPLACE and TANDEM
• accepted by ESA for study for 2015-2025
• Earlier projects may include Bepi-Colombo

• LAPLACE a mission to Jupiter and Europa
• VLBI experiments with Europa landers/orbiter
• Radio astronomy experiments Jovian orbiter

• TANDEM Titan and Enceladus mission
• VLBI experiments with Titan probes/balloons
• Radio astronomy exps Enceladus orbiter

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e-VLBI science

• Required a cultural change in observing policies
• Peer review process is careful and slow
• The sky is open to anybody
• The telescopes are involved in other programmes
  as well
• But e-VLBI is booming in 2008!
• Had several e-VLBI runs this year
• Some use trigger conditions
• Also 3 epoch ToO observation
• Open for spectral line observations (maser
  flares, astrometry)
• And big telescopes available

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What is next?

• Light paths, probably dynamically allocated
• To accommodate distributed correlation
• And around the globe in some uniform manner
• Must continue close collaboration with NREN
• Will use 10Gb/s to make fellow astronomers forget
  conventional VLBI...
• but not quite ready for that
• Considering some improvements in VLBI
  architecture
• Buffering data at telescopes and correlator for
  robustness
• Maybe involve more supercomputing/GRID in the
  future?
• Must address a number of things at astronomy side
• Decide on correlator architecture for next
  generation
• Keep a focus on Global e-VLBI, incl NRAO antennas
  in the US
• Develop common ground with E-LOFAR
• Continue to explore technological synergy with
  SKA
• Requires a big new correlator!

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Miyun 50m
Yebes 40m
Irbene 32m
Sardinia 64m
Kunming 40m
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• SKA will have simpler antennas
• But many more, more connectivity, more correlation

• e-VLBI is pioneering the development of signal
  transport for the SKA
• Can also be important in developing correlator
  solutions

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Relation to SKA

• EVN has a future in SKA era
• Shares a lot of technology interests correlators
• Overlap in science expertise and training
• Europe to have its own pathfinder scale
  facilities
• To maintain forefront facilities in the process
• And train generation of radio-astronomers for the
  SKA
• e-VLBI is a SKA pathfinder

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