Radio Interferometry

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Radio Interferometry

• Jeff Kenney

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Outline of talk

• Differences between optical radio
  interferometry
• Basics of radio interferometry
• Connected interferometers VLBI
• How radio interferometers are used

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Differences between radio and optical
interferometry

• Wavelength larger in radio by factors of
  103-106
• Resolution poorer than optical for given D, but
  very large Ds (earth!) are used in radio VLBI,
  so best resolution very good 0.1-1 mas
• Effect of atmosphere Spatial scale of
  atmospheric coherence length larger than antenna
  for radio, smaller than telescope for optical,
  timescale for variation minutes (radio) vs.
  millisec (optical) can measure and calibrate
  phases in radio (by observing nearby source of
  known phase) but not in optical
• Type of detection beam combination
• Radio signal (amplitude phase) detected
  at antenna, digitized, then combined in
  correlator
• Optical light beams propagated to lab,
  forms interference pattern before being detected
• Signal processing Much easier to do complex
  signal processing at low frequencies

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Basics of radio interferometry
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Point Source and a Single Dish
f hour angle R dish radius l wavelength
f
q angular resolution
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A Simple Interferometer
f
Note improved resolution!
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Signal delays

• Problem signal arrives at different antennae at
  different times would yield no correlation
• Solution add a signal delay by sending signal
  from one antenna through one of delay lines
• Set of cables of various specific lengths, giving
  specific time delays
• Maximum cable length comparable to maximum
  baseline in interferometer, delay times in
  10-1000s nanosec

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correlation
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Downconversion

• Signal from source is often downconverted to
  lower frequency before correlation
• Easier to handle electronic signals at lower
  frequencies

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Cross-correlation

• Correlation reduces noise!
• (most of noise is uncorrelated)

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• Other important details
• Radio interferometers are used in radio astronomy
  for aperture synthesis imaging. This technique
  allows radio telescopes with resolution
  equivalent to very large effective apertures to
  be built using an array of widely spaced, smaller
  antennas. Many variations on the technique are
  possible, but all rely on collecting samples in
  the Fourier transform plane (u-v plane) of the
  image, taking advantage of the fact that the
  interference pattern (fringes) performs a similar
  mathematical operation to doing a Fourier
  transform. Each point in the u-v plane
  corresponds to a particular orientation and
  physical separation of the antennas (baselines)
  in the interferometer. Many samples in the u-v
  plane are required. These can be collected using
  an array of antennas (thus forming many
  interferometers at once, each with different
  baselines) or by motion of the interferometers
  relative to the source. Such motion is usually
  provided by the rotation of the earth ("earth
  rotation synthesis") or in the case of Space VLBI
  by the motion of an orbiting antenna. The results
  of such measurements can yield an image of the
  intensity of the radio emission at each frequency
  in the band (i.e. a 3-dimensional data-set, where
  one of the dimensions is frequency).
• If the array of antennas is compact,
  phase-synchronized local oscillators can be
  distributed to the antennas and the signals from
  the antennas can be directly connected to the
  correlator. If the antennas are widely separated,
  connecting them in real time is impractical. The
  technique of Very Long Baseline Interferometry
  (VLBI) was developed to overcome this problem --
  stable (atomic hydrogen) maser "clocks" are used
  for synchronization and the signals from the
  antennas are recorded on magnetic tape. Although
  this technique requires a more complex recording
  and correlator system, the antennas can be any
  distance apart, and very high resolution images
  (typically 10 milliarcsec) can be made.
• For Space VLBI the synchronization and the data
  are handled by a telemetry station where
  recording also takes place. The separation of the
  antennas is sufficient to achieve microarcsec
  resolutions. The time it takes to adequately fill
  the aperture depends on the number of antennas
  and their spacing in the array, as well as the
  size and quality of the image. The number of
  independent points in the image depends on the
  number of independent points in the synthesized
  aperture. A large, complex image will require
  more coverage of the u-v plane than a small
  image. Long observing time can partially
  compensate for a small number of antennas. If
  many antennas are used, then many spacings get
  filled in simultaneously and it takes less time
  to fill the aperture. In VLBI, where antennas can
  be separated by continents (or are in space) it
  is generally not possible to entirely fill the
  aperture. In these cases, images of the small,
  bright components of the radio sources are the
  objects of interest.

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Connected interferometers VLBI
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Most Radio telescopes that do cutting edge
research are interferometers need large
spacings to get decent resolution

• Connected radio interferometers
• cm-m VLA WRST Merlin AAT GMST -gt SKA
• mm OVRO/BIMA/PdB/Nobeyama -gt ALMA

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Very Long Baseline Interferometry (VLBI)

• Widely separated antennae not connected by cables
• Data recorded along with very accurate time
  signals correlated later

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VLBI Uncertainty in correct delay

• Delay tracks phase center Q0 (absolute RADEC)
• If you know absolute positions of antennae AND
  all atmospheric propogation effects, then you
  know correct delay to use, and therefore you know
  the location of phase center (absolute RADEC)
• If NOT (often the case for VLBI), you search for
  delay which gives maximum correlation, but know
  only relative positions Q

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VLBI

• If the position of the antennas is not known to
  sufficient accuracy or atmospheric effects are
  significant, fine adjustments to the delays must
  be made until interference fringes are detected.
  If the signal from antenna A is taken as the
  reference, inaccuracies in the delay will lead to
  errors in the phases of the signals from tapes B
  and C respectively. As a result of these errors
  the phase of the complex visibility is difficult
  to measure with a very long baseline
  interferometer.
• No phase ? no absolute positions, only relative
  positions over small region of sky

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Except...

• By making use of closure phase and similar
  relations for sets of 3 or more telescopes
  (relations which hold independent of phase shifts
  caused by atmosphere or instruments), one can
  partly correct for errors and get more reliable
  maps and absolute positions
• Works best if large number of elements AND good
  u-v coverage

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Todays VLBI arrays

• There are several VLBI arrays located in Europe,
  the US and Japan.
• European VLBI Network (EVN)  -- most sensitive
  VLBI array a part-time array with the data
  being processed at the Joint Institute for VLBI
  in Europe (JIVE).
• US -- Very Long Baseline Array (VLBA) dedicated
  VLBI telescope
• Combined EVN VLBA known as Global VLBI. This
  provides the highest resolution, capable of
  imaging the sky with a level of detail measured
  in milliarcseconds.

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(No Transcript)
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Space VLBI

• First mission 1997-2003 VSOP (international)
• 8m dish in elliptical orbit, up to 3X earth
  diameter
• best resolution 90 marcsec (100x HST)
• ARISE (proposed 2008-13) 25m dish, 10 marcsec at
  86 GHz (AGN engines water masers around AGN)
• Moon?? (Roye 2000)

Multi-epoch imaging of the quasar 1928738  from
1997-2001 Seven 5 GHz images are shown above, the
first made in August 1997, the second in December
1997, and the last in September 2001. The
horizontal spacing between images is proportional
to the time between observations.   Image
courtesy D.W. Murphy (JPL)
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e-VLBI The future?

• Recently it has become possible to connect the
  VLBI radio telescopes in real-time.
• In Europe, 6 telescopes are now connected to JIVE
  with optical fibres at 1 Gigabit per second and
  the first astronomical experiments using this new
  technique (e-VLBI) have been successfully
  conducted.
• This speeds up and simplifies the observing
  process significantly.
• The data cannot be sent over normal internet
  connections as the data-rate in a VLBI
  observation is so high (far higher than the total
  global internet traffic.)

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How interferometers are used

• If you know where the antennae are, you can
  measure positions or make maps of astronomical
  sources, or determine locations of radio
  transmitters on ground or in space
• If you know where the sources are (e.g. distant,
  fixed quasars), positions of antennae can be
  accurately measured ? geodesy motions of earth

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VLBI helps define the Celestial Reference Frame

• The radio system (positions derived from
  radio VLBI observations to quasars) has replaced
  the traditional optical reference system based on
  star positions to define the International
  Celestial Reference Frame.
• The optical system which was used for the
  last 200 years had an average accuracy (of star
  positions) to about 0".01. The current average
  accuracy of quasar positions observed by
  radiosystems is about 0.1-0.2 milliarcseconds
  (50-100 times better).

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geodesy

• Determine positions of widely-spaced antennae to
  accuracy of 1 mm.
• In geodetic experiments the correlator output
  parameter of interest is the interferometer
  delay. When delay is known for several different
  radio sources at several different times, it is
  possible to accurately determine the coordinates
  of the antennas.
• Measure complexities of Earths Rotation (polar
  motion) precession, nutation, Irregular
  shifts of earths axis due to gravitational
  effects of sun and moon on equatorial bulge of
  earth
• Measure Tectonic motions of continental plates,
  continental rebound from ice ages (motion 1-10
  cm/yr)
• In 1970s first radio programs to monitor
  universal time and polar motion (USNO, NRL, NASA,
  National Geodetic Survey)

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SUMMARY Radio VLBI science results

• Definition of the celestial reference frame
• Motion of the Earth's tectonic plates
• Regional deformation and local uplift or
  subsidence.
• Variations in the Earth's orientation and length
  of day.
• Maintenance of the terrestrial reference frame
• Measurement of gravitational forces of the Sun
  and Moon on the Earth and the deep structure of
  the Earth
• Improvement of atmospheric models.
• Imaging high-energy particles being ejected from
  black holes at enormous velocities
• Measuring H20 masers in gas disks orbiting close
  to central black holes in active galaxies
• Imaging the surfaces of nearby stars at radio
  wavelengths

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Signals in phase
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Signals in phase