| Abstract: | We discuss the methods employed to photometrically calibrate the data
acquired by the Low Frequency Instrument on Planck. Our calibration is based on
the Solar Dipole, caused by motion of the Solar System with respect to the CMB
rest frame, which provides a signal of a few mK with the same spectrum as the
CMB anisotrophies and is visible throughout the mission. In this data release
we rely on the characterization of the Solar Dipole as measured by WMAP. We
also present preliminary results on the study of the Orbital Dipole, caused by
the motion of the Planck spacecraft, which agree with the WMAP value of the
Solar System speed to 0.2%.
We compute the calibration constant for each radiometer roughly once per
hour, in order to keep track of changes in the detectors’ gain. Since
non-idealities in the optical response of the beams proved to be important, we
implemented a fast convolution algorithm which considers the full beam response
in estimating the signal generated by the dipole. Moreover, in order to further
reduce the impact of residual systematics due to sidelobes, we estimated time
variations in the calibration constant of the 30 GHz radiometers (the ones with
the most important sidelobes) using the signal of an internal reference load at
4 K instead of the CMB dipole.
We estimated the accuracy of the LFI calibration following two strategies:
(1) we have run a set of simulations to assess the impact of statistical errors
and systematic effects in the instrument and in the calibration procedure, and
(2) we have performed a number of internal consistency checks on the data.
Errors in the calibration of this Planck/LFI data release are expected to be
about 0.6% at 44 and 70 GHz, and 0.8% at 30 GHz. |
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