National Radio
Astronomy Observatory

4.3.3.2 Determining opacity corrections for VLA data

For VLA data, zenith opacity can be measured at the frequency and during the time observations are made using a VLA tipping scan in the observe file. Historical tipping data are available at:

Choose a year, and click Go to get a list of all tipping scans that have been made for that year.

If a tipping scan was made for your observation, then select the appropriate file. Go to the bottom of the page and click on the button that says Press here to continue.. The results of the tipping scan will be displayed. Go to the section called ’Overall Fit Summary’ to find the fit quality and the fitted zenith opacity in percent. If the zenith opacity is reported as 6%, then the actual zenith optical depth value is opacity=0.060 for gaincal and other calibration tasks.

If there were no tipping scans made for your observation, then look for others made in the same band around the same time and weather conditions. If nothing is available here, then at K and Q bands you might consider using an average value (e.g. 6% in reasonable weather). See the VLA memo

for more on the atmospheric optical depth correction at the VLA, including plots of the seasonal variations.

More information about CASA may be found at the CASA web page

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