Summary of Archeops campaign

This campaign started in November 15 2000, settling the instrument in Kiruna (see the journal). The winds were not favorable in December, except for the opening flight  in December 2nd that tested the telemetry and recovery operations in Russia. During December, ground based calibration allowed us to measure the beams of the experiment provided by the cold optics and the 1.5 meter diameter off-axis telescope. They are between 8 and 6 arcminutes, as expected. Polarisation was calibrated too, in early January .

First opportunity for January was taken on the 12th with a 400 000 m3 balloon and a perfect launch and ceiling at 3 hectoPascal (see the journal). Due to a technical problem on the experiment, the flight was stopped in Finland with only some technical tests on the experiment. The recovery was very quick and the experiment was ready to fly again after one week.

Later in january, wind conditions were unfavorable and the only possibility was to fly at lower elevation. The flight happened on Monday the 29th January 2001, for 7.5 hours at ceiling (see the journal). The experiment was sucessfully launched at 13h30 UT (14h30 Local time) with a 150000 m3 balloon. One hour later, the main window on the cryostat opened and the gondola started being rotated. The experiment could then be tuned and the observations of the sky could start. Scientific data began being collected when the balloon reached its ceiling at 16h00 UT at 31.5 km after 2h30 of ascent in the Arctic night. 21 bolometers observed the sky at frequencies of 143, 217, 353 and 545 GHz, with 6 of them being polarisation sensitive. The detectors were at nominal temperature: about 95 milliKelvin, i.e. 0.095 degrees above absolute zero (-273 degCelsius), obtained with a dilution refrigerator (cf. proposal archeops). First estimate from a quick look analysis gives good performance for all the photometric pixels: direct detection of the Galactic Plane crossing and the Cosmic Microwave Background Doppler dipole effect.

The flight ended before the Ural mountain at 23h30 UT (see here, here and here )  and the gondola touched ground after 40 minutes descent on a parachute. The gondola landed rather well considering the large number of high trees in this Russian area. It landed on the deep snow (1.5 meter), flat on its 6 crash pads. The flight chain which includes the parachutes and some CNES housekeeping boxes was stopped by some trees, so it did not drag the experiment. The mirrors of the telescope, the cryostat and its electronics, the onboard data recorder, the Fast Stellar Sensor, the front baffle and most of the aluminium structure are intact. The only damaged part of the gondola is one metal bar and the CNES telemetry. The gondola was carried by helicopter to a nearby path for further packing and should be back to Kiruna within a few days.

These 7 and a half hours of scientific data cover 20 percent of the total sky, of which a fair size is at high galactic latitudes for clean observations of CMB anisotropies. These data cover also a large part of the galaxie. Data analysis will start as soon as the onboard recorder will be back.

Summary of the Archeops experiment

The Cosmic Microwave Background Radiation was emitted by the Universe when it was 300,000 years old just after the Big Bang. Its spectrum is now precisely known, thanks to COBE, as a blackbody with a temperature of only 2.725 degrees above absolute zero. COBE also measured for the first time small temperature differences when observing different directions in the sky of the order of one part in 100,000, the equivalent of the altitude of a hill at the surface of the Earth. These so-called anisotropies trace the fluctuations of the density of matter that are needed to explain, by gravitational collapse, the large-scale structure of the Universe (galaxies, clusters, …) that we observe today. Because this ancient light has travelled through most of the observable Universe and has been “bent” by its gravitation, its pattern can also yield an indirect measurement of the density, age and curvature of the Universe. There have been many experiments, like TOCO, Boomerang, and Maxima that have measured these anisotropies.

The Archeops experiment aims at mapping the anisotropies from small to large scales at the same time. For this purpose, a beam of about 8 arcminutes (a fourth of the Moon’s apparent diameter) is swept through the sky by spinning a 1.5 m telescope pointing at 41 degree elevation around its vertical axis. 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 night and on a balloon when neither the Sun nor the atmosphere 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 hundred GigaHertz), where the 3 Kelvin radiation dominates the sky emission. This is achieved by cooling bolometers (that collect light over a broad frequency range) at a temperature of 0.1 Kelvin. A test flight of the experiment containing only 6 bolometers was done in Sicily
in 1999. The full experiment has just been flown in Kiruna
. It contained 21 bolometers with four frequency bands (143, 217, 353, 545 GHz) and covered 22 percent of the sky with a beam of 6 to 8 arcminutes. This experiment is unique not only for the large fraction of the sky covered, but also because, for the first time, polarisers were used for the analysis of the polarisation of the interstellar dust emission.

This experiment is a precursor for the Planck satellite in many respects (open cycle dilution refrigerator that provides the 0.1 K temperature, scanning strategy…) and prepares the scientific community working in Cosmology for the large Planck datasets.