ALMA Water Vapor Radiometers

In June 2007 Omnisys was awarded by European Southern Observatory (ESO) with the contract for detailed design and production of 58 Water Vapor Radiometers, one for each of the antennas at the ALMA telescope in Chile. The WVR is a support equipment used to compensate for the phase propagation delay due to differences in the amount of water vapor in the atmosphere above each antenna by measuring the energy on one of the spectral lines of water, 183.31GHz.

Since the measured signal is very weak, the measurement is calibrated by alternately measure on well defined hot or cold loads and the sky, so called Dicke-Switch radiometer. The overall accuracy of the WVR is 2K brightness temperature over the range from 50K to 370K To be able to reach this accuracy, the front end and parts of the back end is temperature controlled to better than 0.1K. Omnisys will build 58 radiometers. The expected lifetime is at least 15 years.

Watch the movie

Subsystems of the WVR are:

• Quasi optics, one flat mirror and three active mirrors, chopper wheel with mirrors for hot and cold loads.
• Front End, corrugated horn antenna, low noise (NF=7.5 dB) integrated schottky mixer and LNA (30 dB), local oscillator system based on a DDS controlled 15 GHz VCO and active multiplier chain.
• Back End, four channel filter bank with diode detectors and 50 dB of distributed stable amplification.
• Control and power, thermal control, motor control, computing and communication.

Radiometer key figures:

More information about the Water vapor radiometer in action can bye found here:

Correcting for the Effects of the Atmosphere by Richard Hills, ALMA Project Scientist.

Images

3D impression of the water vapor radiometer

WVR is being lifted to the antenna.

WVR installed inside the antenna.