Observational Astronomy

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Observational Astronomy

• Astronomicalinterferometers
• Part Deux

2
Radio interferometers

• Do not have many of the problems that optical
  counterparts have
• Wavelengths are longer (by a factor 103-106) and
  tolerances are larger
• Individual mirrors are similar to optical but
  baselines are larger
• Effects of atmosphere are not important
  (coherence length is larger than antennas and
  coherence times are minutes)
• Can calibrate phase by looking at a reference
  source nearby
• In some cases we can digitize the signal and do
  the correlation (fringing) of many baselines in
  the computer (the so-called unconnected
  interferometer)

3
Radio path difference

• Path difference is not known in advanced
• Signal is usually down-converted to lower
  frequencies using heterodyne principle
• Lower frequencies can be digitized at each
  antenna
• Various path difference can be tried in the
  correlator thus guessing the exact phase
• Correlation reduces noise as the noise signal is
  generally uncorrelated

4
Very Long Baseline Interferometry (VLBI)

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

5
Connected interferometers

• More complex to build (need fast analog or
  digital interconnect for all baselines)
• Solves the problem of knowing a priori the exact
  path difference (can be calibrated in real time)
• Example ALMA

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Radio telescopes (terminology)

• Primary mirror ? main dish
• Secondary mirror ? subreflector
• PSF ? far-field beam shape
• Noise ? antenna temperature
• Scattered light ? side lobes

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Radio detectors

• High-frequencies
• Amplifiers
• Bandpass filters
• Local oscillators
• Mixers
• Bolometers
• Low frequencies
• Amplifiers
• Bandpass filters
• ADC

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First observations with APEX/LABOCA
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ALMA science

• Imaging kinematics and chemical stratification in
  protoplanetary disks within 140pc
• Detecting CO emission lines in normal galaxies up
  to z3
• Imaging dust emission in evolving galaxies at z
  up to 10

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ALMA outline

• Fifty 12m antennas
• Two types of antennas
• Variable separation from 15m to 15 km
• Compact array (twelve 7mantennas)

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ALMA bands between the atmospheric H2O absorption
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ALMA fringes

• ALMA Correlator Specifications
• 64 antennas
• 8 frequency bands per antenna
• 4 Gsamples/sec per frequency band, 2 bits/sample
  correlated
• 1024 lead 1024 lag correlations per baseline
• 30 km maximum baseline delay range
• Full polarization capability plus autocorrelation
• Digital filter for bandwidths lt2 GHz
• Switch modes in less than 1.5 seconds
• This requires 4,194,304 multiply-and-add
  correlators at 4 GHz rate
• Total computation rate is 1.7 X 1016
  multiply-and-add operations/sec

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ALMA

• Changing array configuration

Compact size array
Intermediate size array
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ALMA

• Site development signal is mixed down to
  megaHerz range, digitized and sent via optical
  fibers to the correlator

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ALMA receivers

ALMA dewar
ALMA receiver cartridge
ALMA quasi-optics
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ALMA observing

• Submit a proposal
• If accepted workout observing strategy
• Service observing
• Get the data reduced by pipeline (images and
  calibrated uv arrays)
• Observing modes ALMA is an imaging instrument
  with spectral capabilities limited to narrow-band
  filters

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ALMA getting the Time

• Phase I Proposals are submitted using ALMA
  Observing Tool
• ALMA issues calls, provides documentation,
  proposal preparation and submission help, as
  well as coordinating refereeing process
• Regional Program Review Committee ranks
  proposals (HST Spitzer)
• Phase II Successful PIs submit observing program
  using the Observing Tool
• ALMA SC helps with observation planning and
  verifies observing schedule