2009PASP..121..272R

We use observational data to show that 21 cm line profiles measured with the Green Bank Telescope (GBT) are subject to significant inaccuracies. These include errors in the calibrated gain and significant contribution from distant sidelobes. In addition, there are variations between the GBT and Leiden/Argentine/Bonn 21 cm line profile intensities, which probably occur because of the high main-beam efficiency of the GBT. Stokes profiles from the GBT contain inaccuracies that are related to the distant sidelobes. We illustrate these problems, define physically-motivated components for the sidelobes, and provide numerical results showing the inaccuracies. We provide a correction scheme for Stokes 21 cm line profiles that is fairly successful, and provide some rule-of-thumb comments concerning the accuracy of Stokes profiles.