Map-making and photometric calibration

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 for the 2015 data release. They are described in A09 ref. These have common elements with the tools used for the 2013 release that are described in Planck-2013-VI and Planck-2013-VIII.

To build HFI maps, we use the destriping approximation, in which noise is assumed to decompose into two components : white noise plus low frequency drifts. Using the sky redundancy, the low frequency drifts are 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'. Detector's pointing are corrected for slow drifts and aberration (displacement on the sky indouced by the satellite's motion). This intermediate product is called HPR for healpix pixel ring. They have been constructed using the same map resolution as the final HFI products (corresponding to [math]N_{\mathrm side}[/math]=2048). This new dataset is used as input in the following steps.

Photometric calibration

Dipole calibration (100 to 353 GHz)

For the 2015 data release, the HFI CMB channels were calibrated using the orbital dipole modulation. This time-variable anisotropy results from the motion of the spacecraft in the solar system, which is precisely measured. Thus it provides an absolute calibrator for orbital CMB missions. Its measurement is now used to calibrate HFI data thanks to the improvements in the timne stability of the data brought by the ADC non-linearities corrections and a better control for the detectors time response.

For the 2013 data release, the calibrator for the CMB frequency was the solar dipole, as measured by the WMAP team.