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 Increased collecting area Increased bandwidth Increased atmospheric coherence Increased observing time at 1mm band

Outline 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.

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 “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