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Long Wavelength Array. Joseph Lazio Naval Research Laboratory. High Angular Resolution, Long-Wavelength Radio Astronomy. An Historical Overview Why now? The Long Wavelength Array Science Technology. Early Days: Telescopes. Jansky first detected celestial radio emission at 20 MHz.

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long wavelength array

Long Wavelength Array

Joseph Lazio

Naval Research Laboratory

high angular resolution long wavelength radio astronomy
High Angular Resolution, Long-Wavelength Radio Astronomy
  • An Historical Overview

Why now?

  • The Long Wavelength Array
    • Science
    • Technology
early days telescopes
Early Days: Telescopes
  • Jansky first detected celestial radio emission at 20 MHz.
  • Long wavelength astronomy stimulated much of modern astronomy.

Non-thermal emission, Pulsars, Quasars, …

  • Large telescopes built.


Clark Lake TPT


early days science
Early Days: Science
  • Ultra-high Energy Cosmic Rays: 45 MHz (~ 1965)
  • Pulsars: 80 MHz (1967)
  • VLBI: (1967)

What happened?


Clark Lake TPT


ionospheric phase effects

> 5 km

<5 km





Ionospheric Phase Effects


  • If antennas are close together, Df << 1 radian
    • Imaging possible
  • If antennas are far apart, Df > 1 radian
    • Imaging possible only if phase effects can be corrected

Df = rel DNe

ionosphere refraction
Ionosphere Refraction
  • Both global and differential refraction seen.
  • Time scales of 1 min. or less
  • Equivalent length scales in the ionosphere of 10 km or less

q = l/D

  • ~ 1´

rms ~ 3 mJy/beam

  • ~ 10´

rms ~ 30 mJy/beam

nrl nrao 74 mhz very large array

> 5 km

<5 km

NRL-NRAO 74 MHz Very Large Array
  • Early 1980s: development of self-calibration
    • Data driven
    • Solve for N antenna phases using N(N-1)/2 observed interferometric phase differences
  • Early 1990s: 8-antenna prototype
  • 1998: All 27 antennas outfitted
nrl nrao 74 mhz very large array1
NRL-NRAO 74 MHz Very Large Array

74-MHz VLA is the world’s most powerful long-wavelength interferometer.

first sub arcminute imaging 74 mhz vla
First Sub-arcminute Imaging74 MHz VLA



Cas A

(Kassim et al. 1995)


(Beitenholz et al. 1996)



Hydra A

(Lane et al. 2004)


(Kassim et al. 1995)

approaching arcsecond imaging vla pt


Approaching Arcsecond ImagingVLA+PT

Cygnus A: A Long-Wavelength Resolution of the Hot Spots (Lazio et al.)

Highest angular resolution imaging at wavelengths longward of 3 m ( < 100 MHz)

~ 10" angular resolution

PT antenna, 70 km distant 

vla low frequency sky survey
VLA Low-frequency Sky Survey


  • Image 3π sr north of d = 30°

95% complete

  • Frequency = 74 MHz (l4 m)
  • Resolution = 80" (FWHM)

VLA B configuration

  • Noise level ≈ 0.1 Jy beam-1
  • Point-source detection limit  0.7 Jy
  • Nearly 70,000 source catalog


Survey region covered by 523 individual pointings

TOS: 75 minutes per pointing

Each pointing is separated into five, 15-min. observations spread out over several hours

Data reduced by completely automated pipeline

Once reduced and verified, all data posted to the Web



Field-Based Calibration

Correcting the Ionosphere

Field-Based Calibration

Take snapshot images of bright sources in the field and compare to NVSS positions.

Fit to a 2nd order Zernike polynomial phase delay screen for each time interval.

Apply time variable phase delay screens

Field-Based Calibration

developed by

J. Condon & W. Cotton



VLSS Image Gallery

Imaging Parameters:

RMS noise level: ~0.1 Jy/beam

Resolution: 80 ''

Gallery of

unusually large objects



Long Wavelength Array

Long Wavelength Array

Long Wavelength Array

A New Window on the Universe

Angular resolution


Current Capabilities


lwa science case
LWA Science Case
  • Acceleration of Relativistic Particles
    • Supernova remnants (SNRs) in normal galaxies (E < 1015 eV)
    • Radio galaxies & clusters at energies (E < 1019 eV)
    • Ultra-high energyc cosmic rays (E ~ 1021 eV?)
  • Cosmic Evolution & the High-z Universe
    • Evolution of Dark Matter & Energy by differentiating relaxed and merging clusters
    • Study of the 1st black holes
    • H I during the Dark Ages?
  • Plasma Astrophysics & Space Science
    • Ionospheric waves & turbulence
    • Acceleration, Turbulence, & Propagation in the interstellar medium (ISM) of Milky Way & normal galaxies
    • Solar, Planetary, & Space Weather Science
  • Radio Transient Sky
pulsars at long wavelengths

PSR B0809+74

Pulsars at Long Wavelengths
  • 4C 21.53W recognized as steep spectrum source.
  • Later identified as PSR B1937+21.
  • A high dynamic range, long-wavelength instrument may find interesting pulsars.
    • PSR B0809+74 is steepest spectrum source in pilot VLSS observations.
    • Viz. PSR J0737-3039 (S1400 ≈ 5 mJy).
long wavelength array1

400 km

Long Wavelength Array
  • 20–80 MHz
  • Dipole-based array stations
  • 50 stations across New Mexico
  • 400-km baselines  arcsecond resolution
long wavelength demonstrator array
Long Wavelength Demonstrator Array
  • 60–80 MHz
  • 16-element dipole station + 1 outlier
  • At VLA site in NM
long wavelength demonstrator array1
Long Wavelength Demonstrator Array
  • Dual-polarization dipole + active balun
  • Cable to (shielded!) electronics hut
  • Receiver (reconfigurable FPGA) selects frequency, digitizes, time-delays, filters to 1.6 MHz bandwidth
  • Beamforming or all-sky imaging
lwda first light movie
LWDA First Light Movie

Cas A

Cyg A

Galactic plane

lwda first light movie2
LWDA First Light Movie

Cas A

Cyg A = 17 kJy @ 74 MHz

cf. STARE program found no transients above 27 kJy at 610 MHz

Cyg A

Galactic plane

rfi environment1
RFI Environment


TV audio and video carriers

FM radio

Frequency (MHz)

lwa progress
LWA Progress
  • Several candidate antennas being field tested
  • Site testing around New Mexico
  • Program office at the U. New Mexico
  • Southwest Consortium
    • U.Iowa
  • Multi-year funding through Office of Naval Research
  • Target is first, full LWA station, LWA-1, in 12–18 mon.
  • LWA Science and Operations Center in New Mexico in ~ 3 yr
  • LWA will open a new, high-resolution window below 100 MHz  one of the most poorly explored regions of the spectrum
  • Key science drivers:
    • Particle Acceleration
    • Cosmic Evolution & the High-z Universe
    • Plasma Astrophysics & Space Weather
    • Radio Transient Sky
  • Long Wavelength Demonstrator Array (LWDA) already demonstrating potential for transient surveys.
  • Rapid progress being made toward Long Wavelength Array deployment