Widefield Imaging at Long Wavelengths

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Wide-field Imaging at Long Wavelengths

• Joseph Lazio
• Namir Kassim, Aaron Cohen, Wendy Lane (Naval
  Research Laboratory)

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Scientific Motivations

• VLA, GMRT, LWA, LOFAR,
• Surveys for high-z AGN
• Searches for radio halos and relics
• Acceleration processes
• Are clusters relaxed?
• Dark energy?
• Extrasolar planets
• Transients?
• e.g., GCRT J1745-3009

• SKA Key Science Projects
• Galaxy Evolution, Cosmology, and Dark Energy
• Galaxy power spectrum
• Weak lensing
• Probing the Dark Ages
• Finding the first AGN
• H I absorption toward AGN during the Epoch of
  Reionization
• The Origin of Cosmic Magnetism?

Often it is not that the source of interest is
large, but that the field of view is large and
filled with nuisance sources.
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Long Wavelength Array (LWA)

• Returning to roots of radio astronomy
• Several technological issues solved from
  previous generation of instruments
• Frequency range 2080 MHz
• Initial operation in 2008
• Southwest Consortium (NRL, UNM, UTARL, LANL)
• http//lwa.nrl.navy.mil/

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Wide-Field Imaging Requirements

• 74 MHz VLA
• 11º field of view
• 10 km baselines (B config.)
• 35 km baselines (A config.)
• 70 km baselines (VLAPT)
• LWA
• 3º5 º field of view over 2080 MHz
• 400 km baselines
• SKA
• 50100 deg.2 field of view near 500 MHz
• lt 150 km baselines?
• goal is for SKA to operate down to 60 MHz(!)

74 MHz VLA field of view 11º
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Current Wide-field Imaging

• Self-calibration
• Strong sources
• Field-based calibration (Cotton Condon 2002)
• Limited baselines ( 10 km)
• VLA Low Frequency Sky Survey (VLSS)
• w-projection ?

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Self-calibration
(T. Delaney, L. Rudnick)
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Self-calibration II

• VLAPT Cgynus A
• 10 resolution
• still not high enough resolution
• like to be able to do more than just the
  brightest sources in the sky

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ChallengeIonospheric Remediation
Wedge Effects Faraday rotation, refraction Wave
Effects Rapid phase fluctuations These are the
easy parts!
50 km
Wedge characterized by TEC ?nedl
1017 m-2 Introduces extra electrical
path length ?L ? ?2 ? TEC Adds extra
phase ?? ?L?? ? Waves tiny (lt1)
fluctuations superimposed on the wedge
Wedge
VLA

• The wedge introduces thousands of turns of
  electrical phase at 74 MHz.
• A long wavelength interferometer is extremely
  sensitive to differences in phase and sees the
  much smaller superimposed waves very clearly
  even if it samples only a fraction of a wave.

We routinely derive DTEC maps of structures as
small as 100 m, varying on timescales shorter
than a minute, spread over a field of 10o, with
sub-milliTECU precision.
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Exquisite Sensitivity to Fine-Scale Ionospheric
Phenomena

• Phase variation on three 8-km VLA spacings at 3
  different azimuths
• Wide range of ionospheric phenomena seen
• Some of the ionospheric phase fluctuations arise
  from the sporadic E-layer in the ionosphere?

(Perley)
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ChallengeIonospheric Remediation II

• Self-calibration useful for correcting in a given
  direction (or over a small field).
• Field of view of antennas sees a large, and
  different, portion of the ionosphere.

(antennas, not stations)
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Differential Ionospheric Refraction

• These nine radio sources were selected from a
  deep 74 MHz image.
• The individual 30-second maps were compiled as
  animations of the nine hour measurement, running
  from nighttime through two hours past sunrise.
• The variations in position, peak intensity, and
  sidelobe structure show the effects of
  differential ionospheric refraction across the
  field.

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Field-based Calibration

• Take snapshot images of bright sources in the
  field and compare to known positions.
• Fit to a 2nd order Zernike polynomial phase
  delay screen for each time interval.
• Apply time variable phase delay screen to
  produce corrected image.

Self-Calibration
Field-Based Calibration
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Limits to Correction Method
Image Distortion
12 km Isoplanatic Patch (best we can do)
35 km Isoplanatic Patch
Sidelobe Confusion
15?
Striping due to sidelobe confusion from a far-off
source in a completely different IP
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Facetted Imaging

• 12 km baselines (B configuration) of 74 MHz VLA
• 300 facets
• 36 km baselines (A configuration) of 74 MHz VLA
  requires 10x more
• 3000 facets
• 108 pixels
• gt 100 km baselines of LWA means another order of
  magnitude

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Future?w-projection

• Project the 3-dimensional visibilities to the (u,
  v) plane.
• Suggested originally by Frater Doherty (1980)
• Implemented by Cornwell, Golap, Ghatnagar
  (2004).
• V(u, v, w) G(u, v, w) V(u, v, 0)
• Fresnel diffraction of radiation field from one
  antenna to another
• Convolutions inexpensive to implement with large
  memory systems.

Per A
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Long Wavelength Imaging
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Summary

• Limits to wide-field imaging at long-wavelengths
  being reached with current arrays.
• SKA, LWA, etc. will be even more challenging.
• Existing methods do have regimes of
  applicability.
• Self-calibration for strong sources.
• Field-based calibration for intermediate
  baselines.
• Future
• W-projection is a clear improvement for
  wide-field imaging.
• What about ionosphere?
• Basic research in radio astronomy at the NRL is
  supported by the Office of Naval Research.

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FINITO
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Long Wavelength Imaging
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Extending resolution and uv coverage Impact of
outlying elements
VLA alone
Spatial Frequency
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LWA Opening a New Window on the Universe
Current Capabilities
Proposed LWA
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Addressing Ionospheric Issue

• More aggressive empirical approaches
• Develop more robust, high-order corrections based
  on optical approaches.
• Woods and Greenaway method tested by NPOI group,
  enables use of Zernike components to order gt 80
  accounts for nonuniform distributions, variable
  intensities.
• Develop Kalman propagator to impose physicality
  on time variation.
• Physical model-based corrections
• Can we ingest empirical correction data, or even
  raw data into physical models within GAIM or
  IDA, and then use those models to provide better
  conditioned corrections?
• Opportunity to get improved understanding of
  ionospheric phenomenology to which we are
  uniquely sensitive.
• E.G., VLA data coupled with airglow imagery may
  distinguish phenomena occurring in E and F
  layers.

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Challenge Wide-Field Imaging

• NRL NRAO were the first to implement practical
  solutions to wide-field imaging with non-coplanar
  arrays.
• Problem Current approaches insufficient for
  future instruments.
• Approximation of 3D inversion with tessellated 2D
  facets is computationally unfeasible with much
  larger arrays.
• Even current VLA facet-based method introduces
  errors.
• New w-projection technique under development.
• Projection of intrinsically 3-D data to 2-D
  plane, with an appropriate Fresnel-like
  convolution.
• Suggested in the early 1980s, computational power
  just now becoming capable of implementing it.
• Removes reliance on tessellation generates one
  contiguous distortion-free image.
• Order of magnitude faster than facet-based
  methods.
• We think this will work for the emerging large
  arrays.