Advanced MWA tile beam models Randall Wayth – ICRAR/Curtin University

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 O’Sullivan, Emil Lenc, Shantanu Padhi, Tim Colegate, Budi Juswardy, RW

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 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)

GLEAM 1: 2013/ 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

GLEAM 1: 2013/ 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

MWA Primary beams Background: when calibration sol’n 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)

Fitting a simple primary beam model Based on in Bernardi et al, 2013.

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

MWA LNA impedance

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

Example Z_tot matrix, 155MHz

Example Z_tot matrix, 118MHz

216 MHz: zenith vs ZA=14 degs

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

How about other freqs? 155 MHz No gradient across tile

216 MHz cuts through az=0 beams

What’s going on? Dipole is 74cm across = ½ wavelength at ~200 MHz Below this freq, short dipole approximation is increasingly valid Also, magnitude of coupling decreases with freq

What’s 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

Predicted “False Q” for simple model

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

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)

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)

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.

Questions?

Optical H-alpha image: Luigi Fontana.

Rosette Nebula Orion Nebula Barnard’s loop

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

200 MHz az=45 o el=59.8 o tile