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Advanced MWA tile beam models Randall Wayth ICRAR/Curtin University

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MWA Primary beam Background: Beam amplitude bootstrapping in MWA GLEAM Survey False Q seem by Emil in high freq, large zenith angle obs Q/I = (XX-YY)/(XX+YY) Cal solutions transferred from calibrator close to zenith. Current model of tile is analytic: = array factor * dipole pattern. No mutual coupling. Team: Adrian Sutinjo, John OSullivan, Emil Lenc, Shantanu Padhi, Tim Colegate, Budi Juswardy, RW

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GLEAM background Meridian drift scans Night-time observing 8-10 hours of RA per campaign Week-long campaigns 7 DECS, 1 DEC per night, all freqs 80-230 MHz Year 1: GLEAM 1.1:Aug 2013(RA 19 3 ) GLEAM 1.2:Nov 2013(RA 0 8 ) GLEAM 1.3,1.4: 2014-A(6-16, 12-22)

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GLEAM 1: 2013/14 +90 0 h 6 h 12 h18 h12 h GLEAM 1.1 Aug 2013 GLEAM 1.2 Nov 2013 GLEAM 1.3 Feb 2014 GLEAM 1.4 May 2014 0 -90

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GLEAM 1: 2013/14 +90 0 h 6 h 12 h18 h GLEAM 1.1 Aug 2013 GLEAM 1.2 Nov 2013 GLEAM 1.3 Mar 2014 GLEAM 1.4 May 2014 As at Dec 2013 0 -90

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MWA Primary beams Background: when calibration soln from 3C444 (DEC-17) are transferred to the DEC -27, -14, and 1.6 scans at 216MHz Clearly the magnitude of XX and YY are off. (phase OK)

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Fitting a simple primary beam model Based on work @189MHz in Bernardi et al, 2013.

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Inter-port (mutual) coupling model Known (=delays) Unknown (=dipole complex gain) Known (=LNA impedance, diagonal) Known (=impedance matrix, via sims)

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MWA LNA impedance

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Example Z_tot matrix, 216MHz NS-NS interactions EW-EW interactions NS-EW interactions

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Example Z_tot matrix, 155MHz

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Example Z_tot matrix, 118MHz

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216 MHz: zenith vs ZA=14 degs

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How about other freqs? 186 MHz No gradient across tileModest gradient across tile

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How about other freqs? 155 MHz No gradient across tile

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216 MHz cuts through az=0 beams

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Whats going on? Dipole is 74cm across = wavelength at ~200 MHz Below this freq, short dipole approximation is increasingly valid Also, magnitude of coupling decreases with freq

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Whats going on? The phase delay gradient forces side-to-side interactions on the X bow- ties as opposed to end-to- end interactions on the Y bow-ties. Such asymmetry does not occur when pointing the telescope in the diagonal plane as the interactions are symmetric with respect to the X and Y bow-ties. end-to-end vs side-to-side asymmetry is reduced with increasing wavelength. Direction of increasing delay for az=0, za > 0

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Predicted False Q for simple model

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The way forward Three tier model: 1.Analytic dipole model with impedance matrix from simulations or measurements (basically what has been presented in this talk) 2.Average dipole response based on simulations with impedance matrix. One lookup pattern per freq. Relatively straightforward 3.Individual dipole pattern for each dipole per pointing per frequency. Expensive

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How well can we do with 2 nd tier? 216 MHz, ZA=14 deg Blue line: predicted false Q for simple model using incorrect (old) model for calibration Red line: expected false Q for 2 nd tier model using 3 rd tier model as truth. (zero is good)

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How well can we do with 2 nd tier? 155 MHz, ZA=14 deg Blue line: predicted false Q for simple model using incorrect (old) model for calibration Red line: expected false Q for 2 nd tier model using 3 rd tier model as truth. (zero is good)

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Tile: bottom line Mutual coupling does affect the tile beam, especially at higher freqs (>= 200 MHz) At high freqs, far from zenith along cardinal axes (0,90 degs az) mag and phase is quite different Cal solutions for amplitude are only valid at that pointing (this is not new, but model is) False Q due to transferring near-zenith amp cal to off- zenith meridian data. Mutual coupling also affects the relative response of dipoles, even at the zenith. This can taper the tile response, hence affect beam width and sidelobes.

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Questions?

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Optical H-alpha image: Luigi Fontana. http://www.astrobin.com/27523/B/

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Rosette Nebula Orion Nebula Barnards loop

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Impedance matrix: 32x32 Labels: 1-16 N-S Y dipoles 17-32 E-W X dipoles

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200 MHz az=45 o el=59.8 o tile