Current Status of Amplitude Calibration in the EVN. --------------------------------------------------- The following table shows the median absolute amplitude error for EVN stations in 2006. These results are derived from the pipeline amplitude self-calibration results of all EVN experiments recorded in 2005 which have been pipelined to date. The number in brackets after each entry is the number of experiments that were used to determine the median error for that entry. Station 6cm 18cm 21cm X-band 6GHz UHF Cm 0.12(2) 0.19(2) 0.06(9) Ef 0.07(8) 0.02(7) 0.15(2) 0.03(3) 0.03(10) 0.24(3) Hh 0.05(4) 0.04(4) 0.04(2) 0.11(1) Jb 0.16(8) 0.08(7) 0.07(8) Mc 0.06(8) 0.07(7) 0.06(3) 0.03(11) Mh Nt 0.05(8) 0.07(7) 0.07(3) 0.05(7) On 0.07(7) 0.04(7) 0.18(2) 0.09(2) 0.08(8) 0.68(1) Sh 0.05(5) 0.11(2) Tr 0.07(9) 0.06(7) 0.12(7) 0.51(3) Ur 0.04(8) 0.38(5) Wb 0.07(9) 0.07(7) 0.27(2) 0.07(3) 0.16(10) 0.22(3) Ar 0.04(2) --------------------------------------------------------------------- VLBA 0.05(1) 0.13(2) 0.07(1) The blank entries are where insufficient data were available. The numbers above are the median absolute error in the antenna gain amplitude (as calculated from pipeline amplitude self calibration). A value above 0.1 indicates a significant error which should be investigated. In addition to the absolute errors summarised here, the EVN pipeline provides details on every experiment processed at JIVE including the sign and time variability of the errors. ASIDE: Note that this is not the same statistic as was quoted in previous reports (the median absolute error in SEFD) and will be systematically smaller than the values quoted in previous reports. The reason for this change is that the numbers above are simply the median of the absolute amplitude gains reported by the pipeline and so make comparison between the numbers quoted here and those presented on the pipeline pages more intuitive. To convert the numbers above to the same scale as used in previous reports: add 1, square, subtract 1. Points of note: Results are similar to those reported previously, but some improvement is noticed especially at 6 GHz. Cm still does not provide Tsys monitoring. Tr calibration has improved greatly in the last year (partially due to the repair of a software error in the ANTABFS script which affected VLBA racks). C band calibration is generally reasonable. Jb-MkII developed a severe problem in 2005 which continued into session 1 of 2006. However, results of session 2 2006 were considerably better. The previous problems were probably due to a faulty determination of the gain curve. X-band calibration seems to be good across the network. 6 GHz (used for Methanol and OH line observations) has shown considerable improvement in the last year though there is still room for improvement. Calibration at 18 cm is quite variable with occasional experiments having quite large errors. RFI is probably the major source of error. Calibration at Ur is frequently poor at this frequency. However, most 18cm experiments give good results at most stations. 21 cm calibration in 2006 was poor. However, only 2 experiments have been analysed and these were at non-standard frequencies (red-shifted HI) where RFI is severe. These 2 experiments also used Arecibo which typically results in a lot of low-elevation observing for the European antennas. Problems at UHF are due to the extreme RFI in that band (some stations are unable to take any calibration at all). ANTAB Scripts -------------------------------------- Stations continue to produce their own ANTAB files using the 'antabfs' scripts. In early 2006 it was discovered that 'antabfs' was not handling continuous tpi for VLBA racks properly (especially when dual sidebands were being used) and this was introducing an error (typically of a few percent) to the Tsys for VLBA racks. A new version of the code which corrects this was used for experiments in 2006 with an apparent small improvement in the overall calibration at Nt and Tr. UVFLG -------------------------------------- Stations continue to provide off-source monitoring data which allow data taken when the antenna was off-source to be flagged. Westerbork have also introduced the capability to monitor the off-source status of the array and to write output to the FS logs. Efforts are continuing to integrate this fully into the existing EVN flagging system, though the 10 second granularity of the Wb telescope control system reduces the effectiveness of this scheme for that station. Cormac Reynolds.