The Archeops experiment

Archeops is a balloon-borne experiment designed to fly in the stratosphere to avoid the atmospheric contamination. The telescope is two-mirror off-axis tilted Gregorian, with a primary mirror providing an effective aperture of 1.5 meters. Twenty-two bolometers measure the millimeter and submillimeter radiation in four frequency bands centered on 143 GHz (2.1 mm), 217 GHz (1.4 mm), 353 GHZ (850 3#3) and 545 GHz (550 3#3). These detectors are installed on the focal plane stage cooled at 0.1 K by helium dilution techniques. The instrumental setup and the calibration are detailed in Benoît et al. (2001).

Several recent balloon-borne experiments have concentrated on the small angular scales of the CMB, such as BOOMERANG (De Bernardis et al. 2000) and MAXIMA (Hanany et al. 2000). The main scientific goal of Archeops is rather to investigate a broad band of angular scales, in order to make the link between the COBE full-sky survey and the small-scales experiments of today and the near future. For this the aim is to map a large part of the sky (1#1 30 %) with unprecedented angular resolution (5'-10'). To achieve this goal, the scanning strategy is as follows: the gondola rotates at 2 rpm, allowing the beam to make large circles on the sky. These circles slowly shift with respect to the sky, because of the rotation of the Earth and the moving of the balloon on the Earth. Because of the high sensitivity needed, the experiment has to fly during the night. Since we want as long flights as possible in order to increase the total integration time, the polar night is the best. That is why the Esrange base in Kiruna (Swedish Lappland) has been chosen for launching Archeops. However, the first (technological) flight was done from Trapani (Sicily) in July 1999, and gave 4 hours of data. The first scientific flight was done from Kiruna in January 2001, and gave 8 hours of data. Longer flights are possible with normal atmospheric conditions, and expected for the winter 01/02.