ARCHEOPS: a balloon experiment for measuring the cosmic microwave background anisotropies Original Research Article

The ARCHEOPS experiment has measured the anisotropies of the cosmic microwave background at angular scales from 10′ up to 20°. For this, we use a 1.5 m telescope pointing at 49° from its vertical axis on board a stratospheric gondola. By spinning the gondola, a large fraction of the sky is covered when the Earth rotation makes the swept circle drift across the celestial sphere. This is only possible if the observations are done during the Arctic or Antarctic night when the Sun does not disturb the measurements. Another condition is to have very sensitive and fast detectors in the millimeter and submillimeter domain (high frequency radio domain of a few 100 GHz), where the 3 K radiation dominates the sky emission. This is achieved by cooling the bolometers with a dilution refrigerator, to a temperature of 0.1 K. The experiment has been launched four times by the CNES from the Swedish base of Esrange, near Kiruna. The two main flights ended in Russia with landings in Siberia, with the last one having a 19 h flight at float. The control of the experiment was done with satellite communication and the data were stored onboard. This experiment results from an international collaboration, including the USA (detectors and mirror), the UK (cold optics), Italy (gondola, pivot and star sensor) and France (cryogenics, electronics, instrument integration and operation). It is a precursor for the PLANCK satellite and the HFI instrument in many aspects: telescope, bolometers, cold optics, open cycle dilution refrigerator that provides the 0.1 K temperature, scanning strategy, and data processing.