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Astron. Astrophys. 318, L32-L34 (1997)

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3. Estimate of the D terms

In order to determine the D terms in amplitude (d) and phase ([FORMULA]), a polarization VLBI observation has been made. In fact the cross hand correlation (assuming negligible source polarization) is:


which becomes, expanded into a Taylor series:


Neglecting cubic and higher order terms, we get:


The source DA193 has been observed at 6 cm on 30 May 1995 for 12 hours by the 9 telescopes of: Effelsberg, Jodrell MK2, Medicina, Noto, Onsala, Shanghai, Simeiz, Urumqi and Westerbork. The data were correlated at the MPIfR (Bonn). No fringes were detected for the baselines with Urumqi. No data were recorded on tapes from Shanghai. The test was therefore performed on the remaining 7 telescopes.

The D terms were determined using both AIPS (Cotton 1993) and the software of the Brandeis group (Roberts et al.,1994). AIPS gives an estimate of D in each of the seven frequency channels, that is as a function of frequency. Table 1 gives average values for each telescope in amplitude and phase (column 1 and 3). In the table the average value for the Noto phase is not given, having large variations within the passband (discussed in detail in Paper IV, in preparation). In column 2 and 4 the values obtained with the Brandeis software are given. One may notice how large the D terms are for the European telescopes: Medicina and Onsala reaching values of 15% and 21%.


Table 1. Amplitude and phase of the D-Terms of the left hand circularly polarized feeds at 5 GHz

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© European Southern Observatory (ESO) 1997

Online publication: July 8, 1998