Transcript Document

Deep wide field VLBI imaging
“The need for speed”
Harro Verkouter
Joint Institute for VLBI in Europe
Information content
VLBI datasets contain much more
information than is extracted
Aug. 23rd 2002
need for speed [URSI XXVII GA - Maastricht]
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Information content
Usually a field of 50x50 synthesized
beams is imaged
Still only 0.1% of information!
Typically corresponds with a field-ofview of 1” x 1”
Primary beam of Effelsberg
is  6’ (@18cm)
Aug. 23rd 2002
need for speed [URSI XXVII GA - Maastricht]
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Field of view limitations
1. Pre-correlation
a) Sensitivity
Aug. 23rd 2002
need for speed [URSI XXVII GA - Maastricht]
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Field of view limitations
1. Pre-correlation
a) Sensitivity
2. Post-correlation
b) Time smearing
c) Spectral resolution
Aug. 23rd 2002
need for speed [URSI XXVII GA - Maastricht]
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Aug. 23rd 2002
need for speed [URSI XXVII GA - Maastricht]
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Field of view limitations(2)
VLBI datasets >> connected
element array datasets
Almost obsessive need for averaging to
make datasets manageable
Time smearing/spectral smearing
scale with baseline length!
Aug. 23rd 2002
need for speed [URSI XXVII GA - Maastricht]
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What can we do about it?
1. Pre-correlation:
a) EVN: actively support development for
more sensitivy, e.g. PCEVN, MarkV.
b) EVN: 12hrs @512Mbit  10Jy/beam
2. Post-correlation: a lot more!
a) Decrease correlator integration time
b) Increase spectral resolution
Need enhanced output data
rate!
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need for speed [URSI XXVII GA - Maastricht]
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And there’s more…
Once the data is correlated, is that
it? ‘re-correlate’ by mathematics;
produce multiple phasecentres
Experiment with the data: apply
algorithms at will
Helps with near real time space VLBI
Apply filters before exporting to user
Aug. 23rd 2002
need for speed [URSI XXVII GA - Maastricht]
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And there’s more (2)
Applicability of the algorithms is limited
by the aforementioned time- and
bandwidth smearing
Need enhanced output
datarate!
Aug. 23rd 2002
need for speed [URSI XXVII GA - Maastricht]
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Requirements
Want to map out considerable fraction
of Ef primary beam (6’ @18cm):
Field-of-view of 1’x1’ needs
Integration time 0.25s
62kHz channelwidth  2k channels for
64MHz bandwidth
Integration time ~ 1/ 
Channelwidth of 62kHz remains
Aug. 23rd 2002
need for speed [URSI XXVII GA - Maastricht]
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need for speed [URSI XXVII GA - Maastricht]
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need for speed [URSI XXVII GA - Maastricht]
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EVN MkIV Correlator limits
Integration time
Cycle time of 0.015s (actually, 1/64th of a
second)
Spectral resolution
131072 complex lags per readout =
65536 spectral points per readout
Divided over 32 products leads to 2048
spectral channels per product
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need for speed [URSI XXVII GA - Maastricht]
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Datarate
Each lag occupies 5 bytes of
data (4 data+1 overhead)
Hence, ¼ million lags at 64
readouts/second equals
16Mlags/second
Which works out to be
160MByte/s
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need for speed [URSI XXVII GA - Maastricht]
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The EVN MkIV Correlator
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The PCInt project
Short for Post Correlator
Integrator
We want to capture the full output
of the EVN MkIV correlator to disk
Need to replace output datapath
Steve Parsley
Marcel Loose
Harro Verkouter
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need for speed [URSI XXVII GA - Maastricht]
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The PCInt project (2)
High speed readout of the correlator
was already prepared
Via DSP powered serial port
Need hardware and software to
enable this
Receiving end of serial port
Gbit ethernet for transfer from correlator rack to
datacollection host
Fast disk subsystem in order to support
160MByte/s (parallel RAID arrays in a Storage
Area Network)
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need for speed [URSI XXVII GA - Maastricht]
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Correlator Board (x8)
VME
High Speed Serial
C40 COMM
PCI
Ethernet Card
SBC (x2)
Correlator rack (x4)
100TX
1000FX
(x8)
Switch
CCC
DDD (xn)
Fibre Channel
EEE (xk)
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FC Switch
Raid
Array (xm)
need for speed [URSI XXVII GA - Maastricht]
Current situation
RT System
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RT System
Correlator Board (x8)
VME
High Speed Serial
PCI
Ethernet Card
SBC (x2)
Correlator rack (x4)
100TX
1000FX
(x8)
Switch
CCC
DDD (xn)
Phase 0
C40 COMM
Fibre Channel
EEE (xk)
Aug. 23rd 2002
FC Switch
Raid
Array (xm)
need for speed [URSI XXVII GA - Maastricht]
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RT System
Correlator Board (x8)
VME
High Speed Serial
PCI
Ethernet Card
SBC (x2)
Correlator rack (x4)
100TX
1000FX
(x8)
Switch
CCC
DDD (xn)
Phase 1
C40 COMM
Fibre Channel
EEE (xk)
Aug. 23rd 2002
FC Switch
Raid
Array (xm)
need for speed [URSI XXVII GA - Maastricht]
22
RT System
Correlator Board (x8)
VME
High Speed Serial
PCI
Ethernet Card
SBC (x2)
Correlator rack (x4)
100TX
1000FX
(x8)
Switch
CCC
DDD (xn)
Phase 2
C40 COMM
Fibre Channel
EEE (xk)
Aug. 23rd 2002
FC Switch
Raid
Array (xm)
need for speed [URSI XXVII GA - Maastricht]
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Aug. 23rd 2002
need for speed [URSI XXVII GA - Maastricht]
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Post-processing issues
Data volume = huge
1hour @160MByte/s equals
560GBytes of data
Use a cluster of nodes
Need automated processing
Use a processing pipeline
Aug. 23rd 2002
need for speed [URSI XXVII GA - Maastricht]
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Processing pipeline
Correlated data is saved to disk
Apply automatically
Normalization correction
Flagging
Produce dataset with requested
phasecentre(s) for user
Can also apply filtering
Export to FITS
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Processing pipeline (2)
Use AIPS++
JIVE data quality assessment is
already done using AIPS++
AIPS++ supports parallel
processing
Export of data to user is also
already done using AIPS++
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need for speed [URSI XXVII GA - Maastricht]
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