1 Introduction

Observations of the Cosmic Microwave Background (CMB) temperature anisotropies have provided answers to fundamental questions in cosmology. The observational determination of the CMB angular power spectrum has already led to important insights on the structure and evolution of the universe. Most notable are the conclusions that the geometry of space is essentially flat (Miller et al. 1999; de Bernardis et al. 2000; Hanany et al. 2000) and that the measurements are consistent with the inflationary paradigm (Netterfield et al. 2002; Lee et al. 2001; Halverson et al. 2002; Sievers et al. 2002; Rubiño-Martin et al. 2002). Since the first detection of CMB anisotropy with COBE/DMR (Smoot et al. 1992), a host of experiments have measured the spectrum down to sub-degree scales, but measurements at large angular scales remain difficult, due to the large sky coverage required to access these modes. This difficulty will be overcome by the future full-sky space missions MAP and Planck.

This paper presents the first results from Archeops, an experiment designed to obtain large sky coverage in a single balloon flight. A detailed description of the instrument inflight performance will be given in Benoît et al. (2003b); here we provide only essential information. Archeops is a balloon-borne experiment with a 1.5 m off-axis Gregorian telescope and a bolometric array of 21 photometers operating at frequency bands centered at 143 GHz (8 bolometers), 217 GHz (6), 353 GHz (6=3polarized pairs) and 545 GHz (1). The focal plane is maintained at a temperature of $\sim$100 mK using a ³He-⁴He dilution cryostat. Observations are carried out by turning the payload at 2 rpm producing circular scans at a fixed elevation of $\sim$41 deg. Observations of a single night cover a large fraction of the sky as the circular scans drift across the sky due to the rotation of the Earth.