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Pixelisation

The Planck Surveyor has a maximum resolution of between five and thirty arcminutes, varying with frequency. A quick calculation shows that an all-sky map at the former resolution would have over ten million pixels. The sheer number of pixels is very high, but the way in which the sky is tiled is also somewhat problematic.

Thus far, three pixelisation schemes have been considered the analysis of Planck data. They are:

• HEALPix, which stands for Hierarchical Equal Area isoLatitude Pixelisation of the sphere. It was developed initially by Krzysztof Górski, but has grown to be a collaboration between several scientists.
• Equidistant Cylindrical Projection (ECP) is conceptually the simplest of the proposed pixelisation schemes, breaking the sky into lines of ``latitude'' and ``longitude''. Its simplicity is an advantage, as is the fact that it can be easily represented as an array; its main shortcoming is the small area (and hence low signal-to-noise) of the pixels near the poles.
• Igloo Pixelisation is related to the ECP scheme in that the borders of the pixels lie along lines of constant ``latitude'' and ``longitude''. However, as the name implies, the sky is tiled with roughly square cells which cover more azimuthal angle near the poles, and cover the requisite polar angle to all have exactly the same area (as is the case with HEALPix). However there is not as much software written for the igloo pixelisation as there is for HEALPix, and so HEALPix remains the default.