Transcript Slide 1
Calibration and Imaging Requirements Keith Grainge (plus teams at ASTRON, Oxford, Cambridge) AAVP 2010 Calibration requirements Keith Grainge Limits to Imaging • What limits the final map sensitivity? – Thermal noise (A/T; integration time) – Confusion noise (source counts; maximum baseline) – Inadequate dynamic range • Related question of image fidelity – configuration and uv-plane coverage • Key question: how accurate must calibration be such that it is not limiting performance? AAVP 2010 Calibration requirements Keith Grainge AA Calibration • Aim is overall model for calibration of entire AA system – Investigate calibration requirements at all sub-system levels – Provide feedback and comment on specs • Eventually will be limited by non-linear effects of systematics – Need to consider interaction of various errors • Scope: Phase I is intial priority • Also take forward Phase II considerations • Approach: – Use existing experience, results from pathfinders – Combination of simulation and theory tested with measurement – Once simulations validated, extrapolate to SKA AAVP 2010 Calibration requirements Keith Grainge Antenna Placement • Accuracy of placement affects beam-forming quality • Also determines extent can exploit features such as regularity / redundancy • LOFAR uses λ/40 – Based on experience and seems reasonable... – Should attempt to justify this • Equivalent to 10 degrees – too much? • Can we relax this - installation issues AAVP 2010 Calibration requirements Keith Grainge Antenna alignment • Usual argument is that alignment error cause negligable loss in beamformer – Depends on cos(Δθ) – LOFAR uses 1o • However, polarisation errors depend on sin(Δθ) • Also, standard polarimetric performance (co-, cross- pol) is not meaningful for AAs – Reconstructability of polarimetric properties of incoming field • Are element level polarimetric corrections required? – Apply during beamforming – Only one point in FoV correct – A LOFAR study exists AAVP 2010 Calibration requirements Keith Grainge “Estimation noise” • Winjholds and van der Veen (2008) • Self-calibration will be required to reach desired dynamic range • Estimation of calibration parameters extracts information from the data – Effective noise in the image increases with number of calibration parameters – Imposes a stability requirement • Do the astronomical sources provide sufficient SNR to estimate the parameters? – May need dedicated calibration hardware AAVP 2010 Calibration requirements Keith Grainge Other Calibration Issues • What is allowable gain/phase error in analogue chain – Encompasses all electronic and temperature effects • Minimum system level for various calibration issues – Element? Tile? Station? – Purely astronomical calibration or artificial source required? • Beam formation – Element coupling effects – Bandpass changes; filter performance (aliasing) – Sample bit count through processing chain • Further issues in documents from: Faulkner; Wijnboer and Nijboer AAVP 2010 Calibration requirements Keith Grainge Simulation Outline AAVP 2010 Calibration requirements Keith Grainge Sky and atmosphere • Initially concentrate on total intensity • Construct sky model through linear superposition of: – Poisson sources from source count within inner area of PB – Spherical Harmonic representation of galaxy – Bright sources in far sidelobes – Consider effects of sources coming above horizon • Simple ionospheric model – Function of time, geometry and frequency – Antenna based phase screen AAVP 2010 Calibration requirements Keith Grainge Effects of Coupling • AA elements very strongly coupled; use macro basis fns • Issues: sparceness; configuration; channel bandwidth • See Nima’s talk AAVP 2010 Calibration requirements Keith Grainge Beam Forming - Array Factor • Station beam from Array factor and mean embedded element pattern (Razavi-Ghods, Lera Acedo) • Array factor: configuration; convolutional gridding; FFT • Interpolation for different directions and frequencies AAVP 2010 Calibration requirements Keith Grainge OSKAR • Oxford OeRC: Salvini, Mort and Dulwich • A tool to investigate: – Hierarchical beam forming issues – Processing design structure – Digitisation issues – Weights during beam formation • Has capability of simulating station time stream data • Adapted to import embedded element patterns AAVP 2010 Calibration requirements Keith Grainge Station beam formation: OSKAR AAVP 2010 Calibration requirements Keith Grainge Interferometry • Input from sky model, atmosphere and station beam work here. • Initially concentrate on Phase I: 50 stations AAVP 2010 Calibration requirements Keith Grainge Interferometry • Wide field considerations important – Formulate problem in wavelets basis (McEwen and Scaife 2009) • Primary beam issues – Each station has different randomised element configuration – Station beams change as track source across sky – Every baseline has its own PB at each time step – Apply atmospheric effects to station beam • Add random noise to visibility (model for Tsys(ν)) • Many “blocks” of code exist; link with Python wrapper AAVP 2010 Calibration requirements Keith Grainge Summary • Calibration critical to realise potential sensitivity • Guide AA design decisions at station and system level • Will specific calibration hardware be required? • Address issues with pathfinders and simulation tested against measurements • Is overall calibration problem computationally tractable? AAVP 2010 Calibration requirements Keith Grainge