Dear Colleagues,

We have discovered a problem in the 6-cm receiver at the North Liberty (NL) VLBA station from March 28, 1996 until December 3, 1998. The right-circularly polarized (RCP) channel apparently was subject to a subtle instability that caused its effective interferometer gain to be roughly 25% lower than the gain measured in single-dish mode. The single-polarization 6-cm default is LCP, so this affects only those programs that used dual polarization, roughly 2-3 per month. NOTE THAT NO OTHER BANDS OR POLARIZATIONS ARE AFFECTED!

We recommend that users with 6-cm VLBA data from the affected time period reduce the effective gain for RCP at NL, by modifying their a-priori gains file and/or by using their amplitude-check observations (if they made any). We remind observers that cross-checks of antenna calibration are ALWAYS desirable, by use of observations of strong point sources. Users SHOULD NOT rely on amplitude self-calibration to correct the systematic gain offset we report here, since this may bias the overall flux scale.

We apologize for any inconvenience that may have been caused. For further information, please contact Jim Ulvestad at julvesta@nrao.edu.