Tipping scans at the Very Large Array (VLA)

In radio astronomical observations it is sometimes desirable to measure the atmospheric opacity (τ) at the frequencies of observation. For example, if atmospheric attenuation corrections have to be made for sources observed at different elevations. The classical way to determine τ is by doing what are called “tipping” scans, where the antenna(s) are rapidly scanned from high to low (or vice versa) elevation at a fixed azimuth, and measurements of the sky emission are used to determine the value. Prior to the Expanded Very Large Array (EVLA) upgrade, there were a number of studies of techniques of determining τ from tipping scans and of the actual measured atmospheric opacity above the VLA site from such scans [e.g., 1 and references therein]. Since the EVLA upgrade, it has been possible to do tipping scans, but no CASA or AIPS task has been available to analyze such scans (to determine τ). We will present such a CASA task, and examine its performance on a number of tipping scans taken since 2020. These tipping scans were taken roughly weekly, covering the 18-50 GHz frequency range, and in a variety of conditions. We will discuss the use of the task and the differences in results when allowing different quantities to vary, under these different conditions. We find that the opacity around 22 GHz can reach 0.15 nepers in the summer, three times higher than its value in winter. The opacity around 43 GHz is typically higher than at 22 GHz, and less dependent on season. These results are consistent with [1]. We will finally present results comparing derived flux densities using the current opacity estimation technique [2] with our new results using tipping scans. [1] Butler, B., VLA Test Memo 232, 2002. [2] Marvil, J., EVLA Memo 143, 2010.