Motivations for an intermediate scale experiment

There is a part of the CMB power spectrum that lacks measurements: between the large angular scales measurements from COBE (around $\ell=20$) and the largest angular scales from BOOMERanG, MAXIMA and DASI (above $\ell\simeq 75$). This can be easily understood as when you are trying to measure large $\ell$, you concentrate on a small patch of the sky in order to reach high signal to noise ratio. This implies loosing all information on large scales. On the other hand, COBE covered the entire celestial sphere but with a poor 7 degrees resolution that limited the measurements to the low $\ell$. The intermediate part of the power spectrum, despite being difficult to measure, is of high interest.: first, this is the part of the power spectrum that is the most sensitive to the early Universe physics. Second, linking COBE measurements to high-$\ell$ measurements with one single experiment would ensure that no calibration problems are affecting the data.

Archeops is a balloon-borne experiment designed to measure the CMB temperature angular power spectrum from the large multi-degree scales ($\ell\simeq 30$) to the small sub-degree scales ( $\ell \simeq 800$). This is achieved via a large sky coverage (around 35%) along with a high angular resolution (10 arcminutes). Archeops is also very similar to Planck High Frequency Instrument and can therefore be considered as a real size testbed for Planck-HFI.