SKA Specifications Tiger Team Dish with Focal Plane Array

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SKA Specifications Tiger TeamDish with Focal
Plane Array

• Ron Ekers Peter Dewdney
• Manchester, Sep 28 2007

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Radio Telescope Imagingimage v aperture plane
?
Active elements A/?2
Dishes act as concentrators Reduces FoV Reduces
active elements Cooling possible
Increase FoV Increases active elements
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Redundancy (1)
1unit 5x 2units 4x 3units 3x 4units 2x 5units
1x 15
n(n-1)/2
1 2 3 4 5
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Redundancy (2)
1unit 1x 2units 1x 3units 1x 4units 1x 5units
0x 6units 1x 7units 1x etc
1 2 3 4 5
6 7 8
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Dynamic Rangesparse or filled arrays

• Aperture plane arrays can be made sparse
• They measure redundant Fourier components
• These are not needed to generate the image
• Redundant Fourier components improve dynamic
  range
• Optical telescopes have worse aperture errors
  than Radio
• So why do they have better dynamic range
• Its all in the redundant Fourier components
• Image plane arrays cannot be sparse
• You miss information
• Over sampling (many elements/beam) will improve
  dynamic range
• Compact core of SKA has a lot of redundancy and
  should have good dynamic range

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Multibeam Dishes or Multiple Dish Arrays
Structural ? signal processing cost
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Phased Array Feeds

• Focal plane arrays
• Phased Array Feeds (PAF)
• ASTRON Vivaldi
• ATNF Checkerboard
• Cluster feeds
• Parkes Multibeam

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Some PAF designs
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Phased Array Feed Dish Solutions

• Provides mechanism for science-based optimization
  of Ae/Tsys SS independently.
• A risk mitigator for expensive antennas?
• Substitutes FoV for Ae/Tsys in survey speed
  equation (Ae/Tsys)2 ?FOV.
• Success depends on cost of PAFs versus antenna
  cost.
• Moores Law cost component to beamformer gt
  reduces over time.
• In the long run may be the only way to get high
  survey speed.
• Development time could be an issue.
• Possibly need to future-proof antennas and
  other things to permit substitution, especially
  if Pathfinders develop directly into SKA.
• More control over feed pattern
• potentially be able to reduce polarization
  systematics and increase efficiency gt lower
  system costs.
• Can optimise beam for science (sidelobes v G/T v
  G)
• Tsys potentially dominant technical problem.
• Cyro-cooling may be expensive (capital
  operating).
• At lt1.4 GHz promising uncooled solutions are
  being developed.

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SPF v PAF Optimised for Survey Speed

• PAF
• Tsys35K
• 3GHz PAF
• 25GHz dish
• SPF (18K)
• Tsys 18K
• Single Rx
• Max 25GHz

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LNSD v PAF Optimised for Survey Speed

• PAF
• Tsys35K
• 3GHz PAF
• 25GHz dish
• SPF (18K)
• Tsys 18K
• Single Rx
• Max 25GHz
• SPF (26K)
• Tsys 26K

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PAF cost break down Optimised for Survey Speed

• SSFOM3.2.108
• Antenna to 25GHz
• 3GHz PAF

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LNSD cost break down Optimised for Survey Speed
SSFOM3.2.108 Antenna to 25GHz
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Survey Speed or Sensitivity?
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Sensitivity v Survey Speed for const cost

• FoV set by
• 15m dish PAF size
• SPF dish size
• Tsys 35K
• ( for both )
• SSFoM
• FoV(A/T)2
• assumes same bandwidth
• Calc at 1.4GHz
• Bandwidth
• 375 MHz for both
• Computing
• 5km imaging

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Sensitivity v Survey Speed for const cost

• FoV set by
• 15m dish PAF size
• SPF dish size
• Tsys 35K
• ( for both )
• SSFoM
• FoV(A/T)2
• assumes same bandwidth
• Calc at 1.4GHz
• Bandwidth
• 375 MHz for both
• Computing
• 5km imaging

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Sensitivity v Survey Speed for const cost

• FoV set by
• 15m dish PAF size
• SPF dish size
• Tsys 35K, 25K
• SSFoM
• FoV(A/T)2
• assumes same bandwidth
• Bandwidth
• 375 MHz for both
• Computing
• 5km imaging

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Challenges for PAFs
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Dual reflector

• Inspired by
• Evidence that FPA wanted F/Dgt0.4
• Weight for possible cooling
• Desire for wide fielding to support multiple
  feeds/FPAs
• Possible large reduction in feed support blockage
  dynamic range
• Options considered
• Cassegrain, Schwarzchild, shaped to optimise FOV
• Problems
• Sub-reflector blockage - 7
• Sub-reflector spillover - 7
• Cf perhaps 10 loss due to FPA with F/D0.4 prime
  focus
• Multiple reflections are a potential problem but
  seem OK - work proceeding

John Kot, Christophe Granet
But note that for a given size PAF the FoV 1/F
(MK)