Map-making

Introduction

This page will give an overview of the map-making and photometric calibration procedures used by the HFI DPC to build detector and frequency maps. This processing is described in the the HFI DPC Paper and the the HFI DPC Calibration co-Paper.

We work under destriping approximation, where the noise is assumed to decompose into two components : white noise plus low frequency drifts. Using the sky redundancy, the low frequency component is modelled as one constant, or offset, per pointing period. To speed up the ulterior processing we first build intermediate products, by taking advantage of redundancies : we average signal and detector orientation on healpix pixels visited during each fixed pointing period,which we call hereafter 'ring'. This intermediate product is called HPR for healpix pixel ring.

Building of Maps

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Noise properties

Map noise properties can be evaluated using several methods.

Zodi correction

Zodiacal Emission is removed from the 353, 545 and 857 GHz channels. It is described in the HFI DPC Paper, but a synopsis of the procedure is as follows:

• During each survey, a large fraction of the sky has observations which all fall within a week of each other. That is, during a single survey, most pixels are observed during a short, well-defined period. The contribution from Zodiacal Emission to the total brightness seen, then, is well defined.
• We use the the COBE model of the Zodiacal Light to make predictions for this Zodiacal emission for those pixels observed over a span of one week or less, and use GRASP models of the beams to predict the emission from the Galaxy given our sidelobes.
• We fit the survey difference maps with these model templates to estimate the emissivity of each Zodi component and sidelobe at the Planck wavelengths.
• We reconstruct each ring of the the full mission using the combination of the COBE geometric model with the emissivities determined above and the sidelobe models.
• We remove the reconstruction above from each ring of data.
• We then make maps as described previously in this section.

Far Sidelobe Correction

The far sidelobe correction is described in the section above. Note that this correction is done only for the 857 and 545 GHz channels, as it is not seen at longer wavelengths.