Applicability and limitations of redundancy cal. in
phased array stations
Parisa Noorishad, Stefan Wijnholds, Arnold van Ardenne & Thijs van der Hulst
YERAC2011, Manchester, 18th -20th July. 2011
A few words about new
radio telescopes
Introduction & Motivation
Introduction:
[1] J. E. Noordam & A. G. de Bruyn, Nature, 14 oct. 1982
[2] M. Wieringa, PhD thesis, Leiden Univ., 1991
Introduction:
Why redundancy?
•
Independent of a sky model,
•
Less sensitive to RFIs,
•
Computationally cheap and fast,
•
Simple to implement (important for SKA),
•
Could also be used as a diagnostic tool.
•
Regularity in a station configuration e.g. in MWA,
HBA, EMBRACE.
Redundancy in HBAs
Beam identicalness:
CAESAR: Computationally Advanced and Efficient Simulator for Arrays [Rob
Maaskant et al.]
When redundancy fails:
When redundancy holds:
Checking the influence of non-identical sidelobes
The systematic error due to non-identical sidelobes
Quantifying that systematic error:
The potential calibrator sources for RS208, on
24th of Nov. 2009 at 21:29:04 UTC and the
systematic errors in the amplitudes and the
phases of the redundant visibility due to the
contribution of other strong sources through
non-identical sidelobes.
The plots of the residuals for corrected redundant visibilities
Redundancy cal. Quality
CRB and the actual variance of the estimated parameters:
We tracked CasA on 24th November 2009 from 15:25:43 UTC until 22:12:19 UTC.
Redundancy cal. Limitations
Limitations of the redundancy calibration method:
•
Station configuration
•
Mutual coupling between the station elements which
leads to:
– Non-identical beams of the station elements
– Presence of baseline dependent noise
•
SNR of the observed sky
Station configuration and the condition of redundancy calibration
One more point
Model based cal. method:
•
The mutual coupling seems to be a non-dismissible effect
(for phased array stations).
•
We need beam model of each individual station elements
e.g. each tile at HBAs.
[1] Wijnholds & van der Veen, 2009
[2] Wijnholds & van der Veen, 2010
Summary & Conclusion
• Summary:
– Applicability and limitations of the redundancy cal. for phased
array station was investigated.
• Conclusions:
– Redundancy cal. is a stand-alone gain cal. method.
– Using redundancy requires taking into account the parameters
which affects its applicability and performance.
– Identical beam; Mutual coupling seems to be a non-dismissible
effect, no matter which method we use.
• Next Step:
– To do more rigorous CRB and Monte-Carlo analysis for two
existing implementations of the redundancy method.
– To evaluate the merit of the redundancy in UVW-space at the
whole array level using SKA configuration.