Planck early results: first assessment of the High Frequency Instrument
  in-flight performance
  The Planck High Frequency Instrument (HFI) is designed to measure the
temperature and polarization anisotropies of the Cosmic Microwave Background
and galactic foregrounds in six wide bands centered at 100, 143, 217, 353, 545
and 857 GHz at an angular resolution of 10' (100 GHz), 7' (143 GHz), and 5'
(217 GHz and higher). HFI has been operating flawlessly since launch on 14 May
2009. The bolometers cooled to 100 mK as planned. The settings of the readout
electronics, such as the bolometer bias current, that optimize HFI's noise
performance on orbit are nearly the same as the ones chosen during ground
testing. Observations of Mars, Jupiter, and Saturn verified both the optical
system and the time response of the detection chains. The optical beams are
close to predictions from physical optics modeling. The time response of the
detection chains is close to pre-launch measurements. The detectors suffer from
an unexpected high flux of cosmic rays related to low solar activity. Due to
the redundancy of Planck's observations strategy, the removal of a few percent
of data contaminated by glitches does not affect significantly the sensitivity.
The cosmic rays heat up significantly the bolometer plate and the modulation on
periods of days to months of the heat load creates a common drift of all
bolometer signals which do not affect the scientific capabilities. Only the
high energy cosmic rays showers induce inhomogeneous heating which is a
probable source of low frequency noise.
