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Low Frequency Arrays S Low Frequency Arrays S. Bhatnagar Socorro, 5-6 March 2013 NRAO CASA and CASA based Pipelines Review CASA (Common Astronomy Software Applications) by NRAO CASA Group in collaboration with ALMA Integrated Computing Team staff at ESO and NAOJ Atacama Large Millimeter/submillimeter Atacama Large Millimeter/submillimeter Array Array Karl G. Jansky Very Large Array Karl G. Jansky Very Large Array Robert C. Byrd Green Bank Telescope Robert C. Byrd Green Bank Telescope Very Long Baseline Array Very Long Baseline Array Introduction ● “Low frequencies”: Frequencies less than a few GHz ● From a post-processing point of view, Low Frequency Arrays can be classified into two categories ● Antenna Arrays: EVLA, LOBO, GMRT, MeerKAT, ASKAP ● Aperture Arrays: LOFAR, MWA, LWA, PAPER,... – Tracking (LOFAR) vs. Transit (MWA, PAPER, LWA?) ● Common issues ● Relatively wider fields of view (FoV) – Relatively higher data rates, particularly with high resolution – Wide field effects: W-term, PB effects, wide-band effects ● Higher dynamic range imaging ● Presences of extended emission/emission filling the FoV ● Ionospheric corruption ● RFI/Bad data flagging Low frequency sky Deep single pointing @ 150MHz GMRT@150MHz, Sirothia et al. Shallow all-sky survey @150MHz WSRT@150MHz, Bernardi et al. Status ● Already covered in CASA for low-frequency work ● W-term: W-Projection, Faceting, hybrid ● Wide-band imaging to account for sky spectral effects ● Multi-Scale deconvolution, combination of MS- and wide-band imaging ● Data flagging: rflag, tfcrop, various combinations ● Outlier fields, mosaicking with W-term ● All optimized for run-time efficiency with large data volumes ● Most of this is in use for EVLA, LWA, MWA, LOFAR (PAPER?) ● LOFAR Imager: Uses CASA imager with LOFAR specific extensions ● LOFAR EoR Group: Uses CASA for imaging as distributed ● LWA, PAPER: Some groups use CASA (or parts) as distributed Real-time all Sky imaging: LWA J. Hartman/LWA Team LOFAR Imager: LOFAR Transform with CASA imager Tasse et al, A&A, 2013 Issues ● Direction Dependent Effects (DDE) are more severe at low frequencies ● Time varying PB effects amplified due to stronger and FoV-filling emission ● Frequency and polarization dependent PB effects ● Frequency dependence of the emission more complex ● Non-isoplanatic ionospheric effects ● Non co-planar effects (The W-Term) ● EVLA L-band imaging is affected by DDEs. ● EVLA P-band/LOBO imaging is limited by DDEs. ● Most SKA scientific requirements require accounting for all these DDEs simultaneously! ● Very hard, unsolved problem ● Need algorithm development (System Integration of component algorithms – see slides for “Algorithms Prioritization” talk) – Type of work significantly different from our current focus Issues ● Aperture Arrays ● Field of view either covers the whole sky or has strongly varying PB due to projection effects. All or many of the strongest (many Jy) sources always within the FoV ● In-beam PB polarization in the 10s of percent level: Need full Mueller Matrix treatment ● PB time variability the strongest DDE effect ● Need simultaneous correction for W-, A-, and Pol-terms with wide bandwidths using wide-band multi-scale deconvolution! ● Antenna Arrays ● FoV limited by finite-size, well behaved PB (typically 10s of arcmin) ● In-beam polarization in the few percent range: Can ignore the off- diagonal Mueller Matrix elements (higher order leakage terms) ● PB time-variability relatively lower and easier to model (rotation with PA) Possible path forward ● Solutions independently for most of these problems exist ● No robust integrated solution demonstrated to work at the scientific requirements level ● Critical for next-gen low frequency, high sensitivity telescopes ● We focus on developing integrated solutions for Low Frequency Antenna Arrays? ● Benefit from significant in-house expertise ● Benefit from “controlled” parameter space of Antenna Arrays ● Focus on imaging with antenna arrays ● Develop pipeline processing solutions Conclusions ● Participation in low frequency Antenna-Array work more aligned with in-house strengths, telescope needs, resources ● Develop post-processing cost models/equations? ● A critical input to SKA (even Phase-I) ● Observing strategies: E.g., All-configuration data vs. wide-band uv-coverage ● Cost of post-processing for key science drivers ● Significant overlap with other Antenna Arrays: MeerKAT, ASKAP, GMRT ● Collaborate to develop capabilities of common interest ● Collaborate with groups like ASTRON for HPC ● Collaborations with e-Merlin? ● Wide-band, long baselines, highly heterogeneous array The National Radio Astronomy Observatory is a facility of the National Science Foundation operated under cooperative agreement by Associated Universities, Inc. www.nrao.edu • science.nrao.edu.
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