Transcript MyWVR_Ohio.ppt

Water Vapor In The Atmosphere:
An Examination For CARMA
Phase Correction
Y.-S. Shiao, L. W. Looney and
L. E. Snyder
Department of Astronomy
University of Illinois at Urbana-Champaign
CARMA
BIMA (6.1m x 9)
OVRO (10.4m x 6)
SZA (3.5m x 8)
CARMA Improvements
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Increased collecting area
Increased bandwidth
Increased atmospheric coherence
Increased observing time at 1mm band
Outline
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Motivation
Water Vapor Line Model
Water Vapor Radiometers
Power Pattern of Antenna
Issues in Practice
Future Work
Atmospheric Fluctuation of Baselines
Motivation
• In interferometry, correcting fluctuations of
atmospheric phases is improving sensitivity of
telescopes.
• With improved sensitivity one increases
observablilty of weak molecular lines
• Water vapor plays a key role on the refraction
index in the atmosphere (e.g. Waters 1976).
• Observing the water vapor provides precious
information of phases on the paths toward
astronomical sources.
Water Vapor Line Model
• Computational tool: ATM Code (developed by
University of Maryland)
• Multi-layered model
• Optically thin in each layer
• Each layer is big enough to cover main beams so that
antenna temperature is equal to water line brightness.
• 22GHz and 183GHz water lines are those in
centimeter and millimeter regimes.
Tb 
 T (1  e
s
layers
 s
)e

 s
lo wer _ la yers
Simulated Water Vapor Spectrum
Water Vapor Radiometer
• Basically add another receiver on every antenna,
either in 22GHz or 183GHz.
• Instead of interferometry, water line data will be
obtained without correlators i.e. single dish
observation.
Problems to Date
• Available data shows incoherence between water
line data and phase deviation
• Less than 10% of astronomical data are improved
by water line data, but 50% of time the coherence
is decreased!
• Only short time scale data can be used.
So we need to consider more…
Field Regions of an Antenna
R1  0.62 D 3 / 
R2  2 D 2 / 
D  10m
  1cm
R1  200m
R2  20km
“Antenna Theory Analysis and Design” by Constantine A. Balanis
Field Pattern of Parabolic Antenna
Spatial Power Strength Distribution
Practical Issues
• Atmospheric fluctuation only involves slabs of
water vapor and oxygen.
• Line shape function is not very well known.
• Power pattern is not used in the model calculation
in near field.
• Calculation in far field has not considered air mass
with different antenna elevation.
Future Work
• Use a more realistic model of the atmosphere
• Verify stability of water vapor radiometers.
• Collecting more observations to better understand
atmospheric fluctuations.
• Improve phase correction scheme!
• Increase sensitivity
• Observe astronomical sources in long baseline and
high frequency regime