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M. Rioja and H.S. Sanghera outlined the main goals of the current calibration experiments scheduled for session 3. It was expected that a CAL experiment would be scheduled in every session from now on. The experiments were to be correlated at Bonn as soon as the session finished. The main aims could be split into two main areas: quick feedback from the correlator and the accuracy of the apriori calibration at each antenna. This would allow a check on the performance of each of the EVN stations and to provide feedback to the technical friends before the next session started. Post-correlation analysis would attempt to check the calibration of each of the antennas. In particular a gain factor for the TSYS values would be computed.
Second order effects would also be investigated such as ``closure errors'' and polarisation impurities. Another aim of the project was to obtain a list of suitable sources for calibration purposes. The sources used in CAL1 and CAL2 had been obtained from Franco Mantovani. The observing mode would MkIIIa Mode A (single LCP polarisation). It was hoped to observe around 2 dozen sources over the next year.
M.A. Garrett asked that the sources used in the CAL experiment be advertised in the block schedule. In this way the PI's could also observe these sources as part of there own calibration runs. The final (gaussian) brightness distribution of the source should also be distributed to other PI's. In this way source structure could be taken care of - provided the source wasn't completlely resolved out. Provided the sources were not highly variable on the time scale of days a consistent calibration should be obtainable through the session.
T. Foley asked why the CAL experiment had not used the dual-polarisation mode. The feeling was that it was best to start off with the simple case first. P. Burgess felt that polarisation errors had to be taken more seriously and referred to the report by Dave Graham (see Appendix 4). T. Foley and J.D. Bregman suggested that the CAL experiment should use the 4MHz filters in future. M.A. Garrett felt that this was more of an engineering test and should be investigated with a seperate experiment designed for that specific purpose.
M.A. Garrett was concerned about the accuracy of each EVN stations gain curve (including Jodrell's!). There was no point in measuring TSYS to 1% if we then divided by a gain curve which was in error by 10%. M.A. Garrett described the standard proceedure which was used at Jodrell Bank to obtain gain curves for the telescopes. This involved nodding on and off source from low to high elevation. Assuming the source had a constant flux then any changes in the measured source flux (using a fixed K/Jy sensitivity) were due to changes in the gain of the antenna. The other EVN telescopes also used this method to derive normalised gain curves (except the Westerbork Array which was required to use a more complicated method). In most cases the resulting gain curves were presented as normalised polynomial fits. M.A. Garrett suggested that in principle the CAL experiment could provide independent gain curves for each of the EVN antennas by looking at the solutions produced by amplitude self-cal (before dividing by the gain curve). It would be interesting to compare the two curves.
M.A. Garrett felt that while the traditional method of measuring the gain curve might be reasonable below cm it must produce gross errors above cm where atmospheric attenuation would be highly variable. The VLBA had begun to think about these problems and we would do well to follow their progress carefully. There may be a need to provide accurate weather information in the future if we were to correct for these effects. P. Burgess also noted that at high frequency antenna pointing would also become critical. M.A. Garrett suggested that the CAL experiment could also monitor RFI at each of the antennas as a function of frequency and time. H.S. Sanghera requested that the previous suggestions be summarised in a separate appendix (see appendix 3)