Deep wide field VLBI imaging

-“The need for speed”

Harro Verkouter

Joint Institute for VLBI in Europe

Aug. 23rd 2002 need for speed [URSI XXVII GA - Maastricht] 2

Information contentVLBI datasets contain much more information than is extracted

Aug. 23rd 2002 need for speed [URSI XXVII GA - Maastricht] 3

Information contentUsually a field of 50x50 synthesized beams is imagedStill only 0.1% of information!

Typically corresponds with a field-of-view of 1” x 1”Primary beam of Effelsbergis 6’ (@18cm)

Aug. 23rd 2002 need for speed [URSI XXVII GA - Maastricht] 4

Field of view limitations

1. Pre-correlationa) Sensitivity

Aug. 23rd 2002 need for speed [URSI XXVII GA - Maastricht] 5

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Field of view limitations

1. Pre-correlationa) Sensitivity

2. Post-correlationb) Time smearingc) Spectral resolution

Aug. 23rd 2002 need for speed [URSI XXVII GA - Maastricht] 7

Aug. 23rd 2002 need for speed [URSI XXVII GA - Maastricht] 8

Field of view limitations(2)

VLBI datasets >> connected element array datasetsAlmost 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] 9

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 10Jy/beam

Need enhanced output data rate!

2. Post-correlation: a lot more!a) Decrease correlator integration timeb) Increase spectral resolution

Aug. 23rd 2002 need for speed [URSI XXVII GA - Maastricht] 10

And there’s more…

‘re-correlate’ by mathematics; produce multiple phasecentresExperiment with the data: apply algorithms at willHelps with near real time space VLBIApply filters before exporting to user

Once the data is correlated, is that it?

Aug. 23rd 2002 need for speed [URSI XXVII GA - Maastricht] 11

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] 12

Requirements

Field-of-view of 1’x1’ needsIntegration time 0.25s62kHz channelwidth 2k channels for 64MHz bandwidth

Integration time ~ 1/ Channelwidth of 62kHz remains

Want to map out considerable fraction of Ef primary beam (6’ @18cm):

Aug. 23rd 2002 need for speed [URSI XXVII GA - Maastricht] 13

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EVN MkIV Correlator limitsIntegration time

Cycle time of 0.015s (actually, 1/64th of a second)

Spectral resolution131072 complex lags per readout = 65536 spectral points per readoutDivided over 32 products leads to 2048 spectral channels per product

Aug. 23rd 2002 need for speed [URSI XXVII GA - Maastricht] 16

DatarateEach lag occupies 5 bytes of data (4 data+1 overhead)Hence, ¼ million lags at 64 readouts/second equals 16Mlags/secondWhich works out to be 160MByte/s

Aug. 23rd 2002 need for speed [URSI XXVII GA - Maastricht] 17

The EVN MkIV Correlator

Aug. 23rd 2002 need for speed [URSI XXVII GA - Maastricht] 18

The PCInt projectShort for Post Correlator IntegratorWe want to capture the full output of the EVN MkIV correlator to diskNeed to replace output datapath Steve Parsley

Marcel LooseHarro Verkouter

Aug. 23rd 2002 need for speed [URSI XXVII GA - Maastricht] 19

The PCInt project (2)High speed readout of the correlator was already prepared

Via DSP powered serial portNeed hardware and software to enable this

Receiving end of serial portGbit ethernet for transfer from correlator rack to datacollection hostFast disk subsystem in order to support 160MByte/s (parallel RAID arrays in a Storage Area Network)

Aug. 23rd 2002 need for speed [URSI XXVII GA - Maastricht] 20

Current situationCCC

EEE (xk)

DDD (xn)

Switch

FC Switch

Correlator Board (x8)

RaidArray (xm)

RT System

Ethernet Card

C40 COMM

VME High Speed Serial

SBC (x2)

Correlator rack (x4)

PCI

100TX1000FX(x8)

Fibre Channel

Aug. 23rd 2002 need for speed [URSI XXVII GA - Maastricht] 21

Phase 0

CCC

EEE (xk)

DDD (xn)

Switch

FC Switch

Correlator Board (x8)

RaidArray (xm)

RT System

VME High Speed Serial

Correlator rack (x4)

100TX1000FX(x8)

Fibre Channel

Ethernet Card

C40 COMM

SBC (x2)

PCI

Aug. 23rd 2002 need for speed [URSI XXVII GA - Maastricht] 22

Phase 1

CCC

EEE (xk)

DDD (xn)

Switch

FC Switch

Correlator Board (x8)

RaidArray (xm)

RT System

VME High Speed Serial

Correlator rack (x4)

100TX1000FX(x8)

Fibre Channel

Ethernet Card

C40 COMM

SBC (x2)

PCI

Aug. 23rd 2002 need for speed [URSI XXVII GA - Maastricht] 23

Phase 2

CCC

EEE (xk)

DDD (xn)

Switch

FC Switch

Correlator Board (x8)

RaidArray (xm)

RT System

VME High Speed Serial

Correlator rack (x4)

100TX1000FX(x8)

Fibre Channel

Ethernet Card

C40 COMM

SBC (x2)

PCI

Aug. 23rd 2002 need for speed [URSI XXVII GA - Maastricht] 24

Aug. 23rd 2002 need for speed [URSI XXVII GA - Maastricht] 25

Post-processing issues

Data volume = huge1hour @160MByte/s equals560GBytes of dataUse a cluster of nodes

Need automated processingUse a processing pipeline

Aug. 23rd 2002 need for speed [URSI XXVII GA - Maastricht] 26

Processing pipelineCorrelated data is saved to diskApply automaticallyNormalization correctionFlagging

Produce dataset with requested phasecentre(s) for userCan also apply filtering

Export to FITS

Aug. 23rd 2002 need for speed [URSI XXVII GA - Maastricht] 27

Processing pipeline (2)

JIVE data quality assessment is already done using AIPS++AIPS++ supports parallel processingExport of data to user is also already done using AIPS++

Use AIPS++