Continuous Tsys with the EVN

Since the first EVN session of 2002, EVN stations have been routinely making measurements of the total power levels (TPI) on the video converters (VCs) while the telescopes are on-source during scans. Combined with the system temperature (Tsys) measurements made at the beginning and/or end of scans, these TPI measurements can be used to estimate variations in the Tsys during the scan.

The software used to produce the EVN calibration files, for use with the AIPS task ANTAB, has been modified to retrieve this information from the log files, and give continuous amplitude calibration information.

Below is a plot of the Tsys measurements obtained in experiment N02L1, including the Tsys estimated from the TPI measurements (a postscript version is available here). This experiment observed at 18 cm using a bandwidth of 64 MHz and dual polarizations. There is one Tsys measurement every 15-30 seconds at those telescopes where TPI monitoring has been performed. In the plots the different colours/symbols refer to Tsys measurements on different VCs and the key on the right gives the corresponding AIPS IFs, where L refers to an LCP channel and R to an RCP channel.

A close-up of the first two hours of Tsys on one IF at Efflesberg can be seen here. The change in Tsys both across the scans and between sources is evident (this experiment observed two primary sources - one a bright phase reference and the other a weak target).

Note: Urumqi did not make TPI measurements. Onsala antenna was stowed from approximately 20:45 (hence the sudden rise in Tsys). IFs L7 and L8 at Jodrell Bank are probably affected by RFI. Other stations do not show evidence for strong RFI.