Download - 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)
VLBAThe DiFX software correlator
VLBAThe DiFX software correlator
• 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
VLBAUnique DiFX capabilities
• Much more flexible pulsar processing (dynamic allocation of resources); allows pulse-phase dependant studies (binning) and “matched filtering” forrecovery ofoptimal S/N fromcomplexprofiles
VLBAUnique DiFX capabilities
• 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
VLBAUnique DiFX capabilities
freq
uenc
y
time
raw filterbank data
bandpass, tcal corrected data
VLBAUnique DiFX capabilities
• 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’
Smearing-limitedfield of view
2’
phase centre
Time resolution:200 ms
Freq. resolution:50 kHz
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’
Smearing-limitedfield of view
30’
phase centre
Time resolution:20 ms
Freq. resolution:4 kHz
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
Smearing-limitedfield of view
Correlateat high
resolutionfor ~10ms
phase centre
Apply uv shift
primary beam
Smearing-limitedfield of view
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)
VLBAMulti-field imaging
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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
VLBAMulti-field imaging
• 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)
VLBAMulti-field imaging
• 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