new vlba capabilities with difx

Atacama Large Millimeter/submillimeter ArrayExpanded Very Large Array

Robert C. Byrd Green Bank TelescopeVery Long Baseline Array

New VLBA capabilities with DiFXWide-field imaging, multi-field imaging and more

Adam DellerNRAO / UC Berkeley

VLBAOutline

• The DiFX software correlator and its usage with the VLBA

• New capabilities offered by DiFX compared to the VLBA hardware correlator:– Broad compatibility – Spectral/temporal resolution– Pulsar analysis– Commensal science– Wide-field / multi-field capabilities

VLBAThe DiFX software correlator

• A C++ program running on commodity computer hardware (rack-mounted, multi-core servers)

• Development commenced in 2005, adopted by Australian Long Baseline Array in 2006, NRAO testing from 2008 and complete switch by December 2009

• Supported by numerous libraries and applications for job configuration, FITS file building etc; ~10 active developers (NRAO, MPIfR, ATNF/Curtin, Haystack)


• Performance is good; hardware capable of supporting 10 stations x 512 Mbps would cost ~$12,000 in 2011

• Low barriers to getting started has encouraged many adopters– Many contributors to code– This combined with ease of coding in C++ c.f.

FPGAs has contributed to the rapid development of new features like the ones focused on today

VLBAUnique DiFX capabilities

• Compatibility, expandability– Initial reason for adoption - needed something

capable of expansion to 4 Gbps system– incremental nature is extremely useful

(hardware purchased in 4 stages, minimizing overall cost through Moore’s Law)

– Handles all input/output VLBI formats

• Flexibility in parameter setting– Time, frequency resolution in particular


• Much more flexible pulsar processing (dynamic allocation of resources); allows pulse-phase dependant studies (binning) and “matched filtering” forrecovery ofoptimal S/N fromcomplexprofiles


• Ease of adding new features has allowed low-overhead commensal functionality

• One such feature produces ms time resolution spectrometer and spectral kurtosis data

• The V-FASTR project has been approved to search for fast transient events during all DiFX correlations of VLBA data

• Real-time pipeline captures, re-orders and flags data and searches for dispersed pulses


freq

uenc

y

time

raw filterbank data

bandpass, tcal corrected data


• V-FASTR has detected both normal and giant pulses from multiple (targeted) pulsars

• Running near full-time now• Exploring an unknown area of parameter

using a new technique at near-zero cost• Highly visible pathfinder for SKA transient

searches• Also produces valuable RFI information for

routine VLBA operations

VLBAWide-field imaging

• DiFX is the most capable VLBI correlator in the world for wide-field imaging, due to the attainable time and frequency resolution

primary beam: 30’

Smearing-limitedfield of view

15”

phase centre

Calculations for 1.6 GHz, total smearing = 10%

Time resolution:2000 ms

Freq. resolution:500 kHz

12hr VLBA dataset:2.4 GB

primary beam: 30’


2’

phase centre



12hr VLBA dataset:240 GB

VLBAWide-field imaging

• This ability has been widely used since the introduction of DiFX

• However, full-beam VLBA imaging is still a logistical impracticality

Calculations for 1.6 GHz, total smearing = 10%

primary beam: 30’


30’

phase centre



12hr VLBA dataset:30,000 GB

VLBAWide-field imaging• Generally, however, the sky is almost entirely

empty at VLBI resolution• Thus, usually do not want “full beam” imaging;

rather, many targeted small “fields”• This can be achieved by uv shifting after

correlation, but spectral/temporal resolution requirements are identical to imaging

• DiFX has moved the uv shift inside the correlator, allowing “multi-field” correlation and avoiding the logistical problem

VLBAMulti-field imaging

primary beam


Correlateat high

resolutionfor ~10ms

phase centre

Apply uv shift

primary beam


phase centre

phase shift

Averagein frequency

primary beam

Smearing-limited

field of view

phase centre

Repeat for many

phase centres

primary beam

THEN: Repeat for next ~10ms (average in time)


QuickTime™ and a decompressor

are needed to see this picture.

primarybeam

Image:Randomcutout, NRAO FIRSTsurvey

VLBI fields still not to scale!

Satisfactory “finder” catalogs already exist for most applications of this technique


• Some computational overhead (factor of ~2.5) due to higher upfront spectral resolution, but additional fields are almost free (factor of <1.01)

• Thus efficiency gain increases as number of targets per pointing increases

• VLBA is unparalleled for multi-field VLBI applications due to homogeneous, relatively small dishes (large antennas or phased arrays reduce useful field of view)


• For mJy-sensitivity secondary calibrator searches (me, later) with ~20 targets/pointing, net factor of 7 increase

• For sub-mJy sensitivity deep field AGN searches (e.g. Middelberg) with ~300 targets/pointing, net factor of ~100!

VLBAMulti-field imaging• Efficient VLBI surveys

of mJy and sub-mJy objects are feasible for the first time

• Middelberg et al. (2011) already published VLBA results on Chandra Deep Field South, more on the way covering variety of area and sensitivity ranges

From Middelberg et al., 2011

VLBAConclusions

• In addition to facilitating the ongoing sensitivity upgrade, DiFX has opened a number of new areas of parameter space for the VLBA– Advanced pulsar processing– Commensal transient observations– Wide-field and multi-field observations

• Of these, multi-field observations have the potential for opening up the most new applications - VLBI surveying is now practical

new vlba capabilities with difx

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