1 Introduction

The Galactic Emission Mapping (GEM) project (De Amici et al. 1994; Torres et al. 1996; Smoot 1999) is an on-going international collaboration, presently mapping the radio sky at decimetric wavelengths in order to provide a precise understanding of the spatial and spectral distribution of the synchrotron component of Galactic emission. In today's cosmological scenario Galactic foreground contamination plays a central role. Despite the unprecedented success that microwave astronomy achieved in the last decade (e.g. Smoot et al. 1992; Gundersen et al. 1995; Lim et al. 1996; Davies et al. 1996a), an unambiguous identification of the level of contamination from our own Galactic environment still awaits a more reliable treatment in the face of existing data (Lawson et al. 1987; Banday & Wolfendale 1991; Bennett et al. 1992,1996; Kogut et al. 1996a,1996b; Platania et al. 1998; Jones 1999; López-Corredoira 1999).

One often-neglected source of contamimation affecting the baseline determination of present-day surveys of the radio-continuum of the sky in decimeter wavelengths (Haslam et al. 1970,1974,1981; Berkhuijsen 1972; Reich 1982; Reich & Reich 1986) is the component of stray radiation emitted by the ground when coupled to the observational technique. These surveys were obtained with some of the largest single-dish radiotelescopes in the world as they scanned the sky over limited angular ranges either along the meridian circle or at constant elevation. In order to completely sample the accessible portions of the sky, however, low scanning speeds ( $3^\circ - 10^\circ$ min^-1) were required by the medium resolution of these large radio dishes. This requirement introduces striping in the maps as a result of 1/f noise enhancement along the scanning direction (Davies et al. 1996b; Dellabrouille 1998; Maino et al. 1999). In addition, scanning in azimuth can likewise produce horizontal (parallel to right ascension) striping due to an horizon dependent ground pick-up through the antenna sidelobes. In the GEM experiment we scan the sky from different sites at the constant elevation of $60\ifmmode^\circ\else\hbox{$^\circ$ }\fi$ with a portable 5.5-m dish rotating at 1 rpm. Thus a crucial element of our experiment is the reduction and proper accounting of the antenna sidelobe contamination by ground emission. Even though we make an effort to minimize and level out the ground emission signal by using fixed and co-rotating ground shields (see Fig. 1), the sensitivity goal for our low resolution sky measurements ( $S/N \sim 10$) demands a more comprehensive treatment of the role played by diffraction and spillover sidelobes. The importance of stray radiation corrections in survey experiments has already been made clear in the past as, for instance, in Hartmann et al. (1996 and references therein) when applied to the Leiden/Dwingeloo survey of HI in the Galaxy (Hartmann & Burton 1997).

Figure 1: Schematic representation of a ray-tracing diagram (dotted lines) for the double-shielded portable radiotelescope of the GEM project

In this article we first demonstrate the effective use of a fixed ground shield in levelling out the contamination from the ground (Sect. 2) for GEM observations at 1465MHz. We then set out to determine the extent of this contamination by comparing model predictions of the spillover and diffraction sidelobes that overlook the ground behind the shields (Tello et al. 1999, from now on Paper I) with differential measurements of the antenna temperature toward selected regions of the sky. In order to do so we will rely upon a complete radiometric description of the feed (Sect. 3) and a detailed study of its expected performance under different shielding configurations (Sect. 4). Then we will use the near sidelobe pattern (out to some $30\ifmmode^\circ\else\hbox{$^\circ$ }\fi$ from axis) of the radiotelescope to pin down the proper orientation of the feed pattern with respect to the optical axis of the secondary before finally subtracting the differential contributions of the atmosphere and the Galaxy (Sect. 5). The latter will be obtained from a template sky based on a preliminary GEM survey at 1465 MHz in the Southern sky. A summary of the article and its main conclusions are given in Sect. 6.