Andrew, What is the current plan for the Planck scan pattern? I see this quote from the 2005 Blue Book:

The spacecraft will spin at 1 rpm around an axis offset by 85 degrees from the telescope boresight, so that the observed sky patch will trace a large circle on the sky (Dupac and Tauber 2004). From L2, the spin axis can be continuously pointed in the anti-Sun direction, and the satellite itself used to shield the payload from solar illumination. This strategy minimises potentially confusing signals due to thermal fluctuations and straylight entering the detectors through the far sidelobes. It also enables aggressive use of passive radiation to cool the payload, a key feature in the overall thermal design of Planck.

As the spin axis follows the Sun, the circle observed by the instruments sweeps through the sky at a rate of 1 degree/day. The whole sky will be covered (by all feeds) in a little more than 6 months; this operation will be repeated twice, resulting in a mission lifetime of around 15 months.

This describes a terrible scan pattern that may ruin Planck's ability to measure the low-ell polarization signal that is essential for deterimining tau. And the claim of covering the whole sky is wrong since as described, a 5 degree radius about each ecliptic pole is left out. That's most of the sky, but not all.