The Archeops telescope is a two mirror, off-axis, tilted Gregorian telescope consisting of a parabolic primary (1.5 m main diameter) and an elliptical secondary. The telescope was designed to provide diffraction-limited performance when coupled to single mode horns producing beams with FWHM of 8 arcminutes or less at frequencies higher than 140 GHz.
For CMB anisotropy measurements, control of spectral leaks and beam sidelobe response is critical. Archeops channels have been specifically designed to maximize the sensitivity to the desired signal, while rejecting out-of-band or out-of-beam radiation. We have chosen to use the configuration developed for Planck HFI, using a triple horn configuration for each photometric pixel. In this scheme, radiation from the telescope is focussed into the entrance of a back-to-back horn pair (10 K stage). It creates a beam-waist where wavelength selective filters can be placed (1.6 K stage). Finally, the third horn (0.1 K stage) maintains beam control and focuses the radiation onto the bolometer placed at the exit aperture. A convenient aspect of this arrangement is that the various components can be placed on different temperature stages in order to create thermal breaks and to reduce the level of background power falling onto the bolometer and fridge.
Twenty two spider-web bolometers are placed on the 100 mK low temperature plate. There are 9 bolometers at 143 GHz, 7 at 217 GHz , 6 polarised bolometers at 353 GHz and two at 545 GHz, placed at different points in the focal plane. They observe the same sky pixel at different times, from 100 msec to a few minutes. The six 353 GHz channels are devoted to the measurement of galactic polarized emission. These are assembled in three pairs, with one single back-to-back horn and a polarizer splitter (the so-called OTM configuration) for each pair. The two bolometers of each pair measure the polarized intensity of the incoming signal in two orthogonal directions. Each pair makes a different polarising angle with respect to the scan axis to enable the full determination of the Stokes parameters. Archeops will provide the first measurement of polarization in this range of frequencies with a sensitivity adequate for measuring galactic dust polarised emission, a CMB foreground as yet unmeasured for the preparation of Planck-HFI.