2 Observations at 4.85 GHz

The observations reported here have been carried out between July 1994 and March 1999. Until August 1995 the old $\lambda$6-cm correlation receiver system was employed. This system had two feeds in the secondary focus of the 100-m telescope. The right-hand circular polarization outputs from each feed (obtained after the polarizers in the waveguides) were correlated via a 3-dB hybrid to yield a differential total-power signal of the two feeds. This double-beam ensured minimal atmospheric disturbance to the signal. Amplification in the first stage was achieved with cooled FET's. The main horn was connected to an IF-polarimeter to deliver the Stokes U and Q parameters for full linear polarization information. The system operated at a centre frequency of 4.75 GHz, with a bandwidth of 500 MHz. The receiver system temperature was $\sim$70 K on the sky (zenith, clear sky).

In August 1995 these receivers were replaced by two stable total-power systems, with HEMT amplifiers in the first stage. Here the differential signal is retrieved by subtracting the calibrated signals in the computer. Each of the two total-power receivers is connected to an IF-polarimeter. This system operates at 4.85 GHz, the bandwidth is 500 MHz. The receiver system temperature has been greatly improved to 30 K on the sky (zenith, clear sky).

The half-power beam width was 147$^{\prime\prime}$ for the old and 143$^{\prime\prime}$ for the new receiver system and the beam throw was 8$.\mkern-4mu^\prime$2 in both cases. The sources were observed by cross-scanning the telescope in right ascension and declination, with a scan length of 15$^\prime$. The scanning speed was 30'/min, and the total number of scans was adjusted to the expected flux density of each source. Sources with angular extents significantly exceeding the beam size or exhibiting significant confusion in the cross-scans were mapped in the double-beam mode and subsequently restored to the equivalent single-beam images using the restoration algorithm of Emerson et al. (1979). The scan separation was 1$^\prime$, and the map sizes adjusted such as to account for the source size and the beam separation. The total number of sources mapped this way is 6.

Telescope pointing, focussing and polarimeter adjustments were regularly checked by cross-scanning the point sources NGC7027, 3C48, 3C84, 3C138, 3C147, 3C196, 3C286 and 3C295. The latter two sources served also as flux density calibrators.