New VLBA capabilities with Di FX Widefield imaging
New VLBA capabilities with Di. FX Wide-field imaging, multi-field imaging and more Adam Deller NRAO / UC Berkeley Atacama Large Millimeter/submillimeter Array Expanded Very Large Array Robert C. Byrd Green Bank Telescope Very Long Baseline Array
Outline VLBA • The Di. FX software correlator and its usage with the VLBA • New capabilities offered by Di. FX compared to the VLBA hardware correlator: – – – Broad compatibility Spectral/temporal resolution Pulsar analysis Commensal science Wide-field / multi-field capabilities
The Di. FX software correlator VLBA • 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, MPIf. R, ATNF/Curtin, Haystack)
The Di. FX software correlator VLBA
The Di. FX software correlator VLBA • 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
Unique Di. FX capabilities VLBA • 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
Unique Di. FX capabilities VLBA • Much more flexible pulsar processing (dynamic allocation of resources); allows pulse-phase dependant studies (binning) and “matched filtering” for recovery of optimal S/N from complex profiles
Unique Di. FX capabilities VLBA • Ease of adding new features has allowed lowoverhead 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 Di. FX correlations of VLBA data • Real-time pipeline captures, re-orders and flags data and searches for dispersed pulses
Unique Di. FX capabilities frequency raw filterbank data bandpass, tcal corrected data time VLBA
Unique Di. FX capabilities VLBA • 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
VLBA Wide-field imaging • Di. FX is the most capable VLBI correlator in the world for wide-field imaging, due to the attainable time and frequency resolution primary beam: 30’ Time resolution: 200 ms Time resolution: 2000 ms phase centre Smearing-limited field of view 15” Freq. resolution: 500 k. Hz 12 hr VLBA dataset: 2. 4 GB phase centre Smearing-limited field of view 2’ Freq. resolution: 50 k. Hz 12 hr VLBA dataset: 240 GB Calculations for 1. 6 GHz, total smearing = 10%
VLBA Wide-field imaging • This ability has been widely used since the introduction of Di. FX • However, full-beam primary beam: 30’ Time resolution: VLBA imaging is still 20 ms a logistical phase centre Freq. resolution: impracticality 4 k. Hz Smearing-limited field of view 30’ 12 hr VLBA dataset: 30, 000 GB Calculations for 1. 6 GHz, total smearing = 10%
Wide-field imaging VLBA • 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 • Di. FX has moved the uv shift inside the correlator, allowing “multi-field” correlation and avoiding the logistical problem
VLBA Multi-field imaging primary beam phase centre Smearing-limited field of view Correlate at high resolution for ~10 ms primary beam phase centre Smearinglimited field of view phase shift primary beam Smearing-limited field of view Apply uv shift Average in frequency Repeat for many phase centres THEN: Repeat for next ~10 ms (average in time)
Multi-field imaging Satisfactory “finder” catalogs already exist for most applications of this technique VLBA primary beam VLBI fields still not to scale! Image: Random cutout, NRAO FIRST survey
Multi-field imaging VLBA • 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)
Multi-field imaging VLBA • For m. Jy-sensitivity secondary calibrator searches (me, later) with ~20 targets/pointing, net factor of 7 increase • For sub-m. Jy sensitivity deep field AGN searches (e. g. Middelberg) with ~300 targets/pointing, net factor of ~100!
Multi-field imaging • Efficient VLBI surveys of m. Jy and sub-m. Jy 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 VLBA From Middelberg et al. , 2011
Conclusions VLBA • In addition to facilitating the ongoing sensitivity upgrade, Di. FX 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
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