SKA Long Baseline Configurations Comments

1
SKA Long Baseline ConfigurationsComments

• Ron Ekers
• CSIRO
• 27 Feb 2009

2
Distribution of collecting area

• Since 2004 we have converged on the distribution
  of area in SKA mid (phase 2)
• Area stations
• core lt1km 20 20
• inner lt5km 50 30
• mid lt180km 75 25
• outer 3000km 100 25
• The stations is based on the usual to
  assumption of 100 stations of 102 m-2 each (eg
  site RFP)
• This needs an infrastructure cost - benefit
  trade-off
• Maybe low by x2?

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Effect of Central Core

• 50 stations within 5km
• Central 1km is close packed so log distribution
  doesnt apply
• This is 50x the weight of individual outer
  stations
• For this analysis it doesnt matter if you
  consider it a single phased array or 50 nearbye
  stations
• But either the number or weight have to be
  correct
• It does effect the correlator requirements and
  FoV shaping

4
Effects at the long baselines

• Mutifrequency Synthesis
• Frequency synthesis has a huge impact for VLBI
• 1 band at 3000km 30km of UV coverage
• with the log spiral bandwidth synthesis will give
  full coverage with about 10 bandwidth
• High angular resolution spectral line has very
  modest UV coverage requirements
• Processed FoV has to be restricted
• Multiple phased array beams
• Data compression eliminate empy parts of the FoV

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Consider the two extremes

• Natural weighting gives full SKA sensitivity but
  modest UV coverage
• 20-50 baselines depending on core definition
• Uniform weighting will give good UV coverage
  but only 25 max sensitivity
• 300-1200 baselines depending on core definition
• This can be modelled with Mattiea de Villiers
  algorithm
• Some science requires max sensitivity, some max
  UV coverage
• There will be no unique best configuration

6
Sensitivity Limited Science

• HI absorption VLBI in AGN
• Eg Arthur Wolfe, Nature 2008
• 3C286 HI Zeeman, 84uG, z0.69
• No high UV coverage requirement
• extragalactic OH masers in starformining galaxies
• Robishaw and Heiles, 2007
• No high UV coverage requirement
• Super Massive Binary Black Holes
• Eg NGC6240 Roger Brisenden last week
• Must go fainter to detect all BH

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Conclusion

• The optimum configuration for max sensitivity and
  max UV coverage will be different in a core
  dominated array
• Apart from general requirements for good coverage
  the scientific constraints will be minor compared
  to the logistical constraints
• Infrastructure cost
• Access
• RFI
• Weather
• Use the tools to fine tune sensible configurations