Tracking ALMA System Temperature with Water Vapor Data at High Frequency

As the world-leading submillimeter telescope, the Atacama Large Millimeter/submillimeter Array observatory is now putting more focus on high-frequency observations at Band 7-10 (frequencies from 275 to 950 GHz). However, high-frequency observations often suffer from rapid variations in atmospheric opacity that directly affect the system temperature T _sys. Current observations perform discrete atmospheric calibrations (Atm-cals) every few minutes, with typically 10-20 occurring per hour for high frequency observation and each taking 30-40 s. In order to obtain more accurate flux measurements and reduce the number of atmospheric calibrations (Atm-cals), a new method to monitor T _sys continuously is proposed using existing data in the measurement set. In this work, we demonstrate the viability of using water vapor radiometer (WVR) data to track the T _sys continuously. We find a tight linear correlation between T _sys measured using the traditional method and T _sys extrapolated based on WVR data with scatter of 0.5%-3%. Although the exact form of the linear relation varies among different data sets and spectral windows, we can use a small number of discrete T _sys measurements to fit the linear relation and use this heuristic relationship to derive T _sys every 10 s. Furthermore, we successfully reproduce the observed correlation using atmospheric transmission at microwave modeling and demonstrate the viability of a more general method to directly derive the T _sys from the modeling. We apply the semi-continuous T _sys from heuristic fitting on a few data sets from Band 7 to Band 10 and compare the flux measured using these methods. We find the discrete and continuous T _sys methods give us consistent flux measurements with differences up to 5%. Furthermore, this method has significantly reduced the flux uncertainty due to T _sys variability for one data set, which has large precipitable water vapor fluctuation, from 10% to 0.7%.