Instrument and Scan-strategy

A complete description of Archeops can be found in [11] or on http://www.archeops.org, but the main characteristics of Archeops are:

• high angular resolution (10 arcminutes), due to a large primary mirror of 1.5 meters and to our horns, which guide the radiation to our bolometers;
• 30% sky coverage due to a scan-strategy consisting of making large circles on the sky. Associated with the first characteristic, this allows Archeops to measure the CMB power spectrum from angular scales of about 20 degrees (l=30) down to 10 arcminutes (l=800);
• bolometers cooled to 100 mK, by the same type of cryogenic system which will be used by Planck HFI, and does not rely on gravity;
• a frequency band, the 353 GHz band, polarized using OMT, is dedicated to the measurement of Galactic dust and point sources polarization.

The Archeops telescope is an off-axis gregorian telescope (see Figure 2), pointed at 41 degrees elevation, which defines the size of our circles on the sky at about 200 degrees.

Figure 2: Schematic drawing of the gondola: the off-axis gregorian telescope, the cryostat, and the bolometers with their feeding horns. The pivot on the top of the gondola spins the payload.

To make circle on the sky the gondola spins at constant elevation, and as the earth rotates, the center of the circle describes also a circle on the sky. This describes a typical ring or donut-like sky coverage (see Figure 3). For Archeops (41 degrees elevation and 68 degrees of latitude), the sky coverage is about 30% for a 24 hour flight.

Figure 3: Typical Archeops sky coverage : each line represents a circle, the time separation between the circles is 1 hour.

After the secondary mirror, the radiation goes through a polypropylene membrane (see Figure 4), and enters the horns, which define the part of the sky seen by each bolometer; they play a crucial role for the angular resolution of the experiment.

Figure 4: Optical path after the secondary mirror, from the membrane to the bolometers. The light passes through the horns and filters, to be received by the bolometers.

Figure 5: Spider web bolometer (left) with mesh size about 1 mm, the Archeops focal plane (right) filled with its 22 bolometers. This stage corresponds to the lower (100 mK) stage of Figure 4.

The detectors are spiderweb bolometers (Figure 5), which have a small calorific capacity so a fast response time, and are less sensitive to cosmic ray hits. These bolometers are cooled to 100 mK by a $^3$He-$^4$He dilution, which is produced in capillary tubes placed around the focal plane (see Figure 5). The focal plane is filled with 22 bolometers, with the following frequency band distribution :

• 8 detectors at 143 GHz, where the CMB is the dominant emission at large galactic latitude;
• 6 detectors at 217 GHz, where the CMB is still dominant but dust contamination is larger;
• 6 detectors at 353 GHz, where the dust and atmospheric emission are dominant; these 6 channels are polarized using OMT : 3 pairs of 2 detectors share the same horn, the signal being separated into 2 orthogonal polarized components;
• 2 detectors at 545 GHz, where dust and atmospheric emission are dominant.

This frequency distribution is used to distinguish the different contributions of astrophysical origin or from parasitic signals, such as atmospheric emission; this is illustrated in Figure 6.

Figure 6: Frequency bands of Archeops, with the spectrum of different astrophysical processes, estimated around a galactic latitude of 70 degrees, and a galactic longitude of 340 degrees (adapted from [12]). The different bands allow us to separate different emission sources, as they have different spectral shape.