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Paper: Phase Correction at OVRO Using 22 GHz Water Line Monitors
Volume: 217, Imaging at Radio Through Submillimeter Wavelengths
Page: 317
Authors: Woody, D.; Carpenter, J.; Scoville, N.
Abstract: The 22 GHz Water Line Monitors (WLMs) implemented on the OVRO millimeter interferometer array have been very successful in correcting the delay errors caused by water vapor fluctuations. The WLM on each telescope use a three channel radiometer to isolate the water vapor line emission from continuum emission and the phase correction works well even in the presence of clouds. A robust calibration procedure has been developed that successfully links the calibrator phase to the target source. The first critical step is the accurate normalization of the line brightness temperature scales of the WLMs using the source changes and the assumption that the average atmosphere over the array is planar and homogeneous. The second less precise step is determining the delay/K scale factor from the correlation between the line brightness and the astronomy phase. The same scale factor is used for all baselines and tracks. Good images are obtained during weather that was previously unusable and point source images have been produced with resolution close to the theoretical synthesized beam width. The phase correction is limited in the current implementation by the noise temperature of the WLM front ends to ~.1 mm delay error. A cooled front end is being developed that will reduce the noise temperature from ~200K to ~50K. This improvement will reduce the delay error to much less than our design goal of .05 mm.
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