Advanced MWA tile beam models Randall Wayth ICRAR/Curtin
University
Slide 2
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
Slide 3
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)
Slide 4
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
Slide 5
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
Slide 6
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)
Slide 7
Fitting a simple primary beam model Based on work @189MHz in
Bernardi et al, 2013.
Slide 8
Inter-port (mutual) coupling model Known (=delays) Unknown
(=dipole complex gain) Known (=LNA impedance, diagonal) Known
(=impedance matrix, via sims)
Slide 9
MWA LNA impedance
Slide 10
Example Z_tot matrix, 216MHz NS-NS interactions EW-EW
interactions NS-EW interactions
Slide 11
Example Z_tot matrix, 155MHz
Slide 12
Example Z_tot matrix, 118MHz
Slide 13
216 MHz: zenith vs ZA=14 degs
Slide 14
How about other freqs? 186 MHz No gradient across tileModest
gradient across tile
Slide 15
How about other freqs? 155 MHz No gradient across tile
Slide 16
216 MHz cuts through az=0 beams
Slide 17
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
Slide 18
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
Slide 19
Predicted False Q for simple model
Slide 20
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
Slide 21
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)
Slide 22
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)
Slide 23
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.
Slide 24
Questions?
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Slide 28
Optical H-alpha image: Luigi Fontana.
http://www.astrobin.com/27523/B/
Slide 29
Rosette Nebula Orion Nebula Barnards loop
Slide 30
Impedance matrix: 32x32 Labels: 1-16 N-S Y dipoles 17-32 E-W X
dipoles