PLANCK is the space mission of the European Space Agency devoted to measure of the anisotropies of the cosmic microwave background (CMB), the relic radiation left by the big bang.
The satellite will be launched in 2007 and it will carry state-of-the-art of microwave radiometers and bolometers arranged in two instruments, respectively the Low Frequency Instrument and the High Frequency Instrument, both coupled with a 1.5 m telescope and working in nine frequency channels between 30 and 857 GHz.
From the second Lagrangian point of the Sun-Earth system, the instruments will produce a survey that will cover the whole sky with unprecedented combination of sensitivity, angular resolution, and frequency coverage, and they will likely lead us to extract all the cosmological information encoded in the CMB temperature anisotropies.
The development strategy of PLANCK and the two instruments has been to set up a mission that inherently minimizes the systematic effects.
The optics, composed by an optimised telescope-feed array assembly, introduce unwanted systematic effects in the measurements like the so called external straylight due to the sidelobe pick-up.
A trade-off between angular resolution and external straylight has been carried out for LFI in order to reach the best optical performances preventing the Galactic contamination. The main product of the study has been the definition of the internal geometry of the flight model of the LFI feed horns and the characterization of the overall optical response of the instrument.
Thermal emission from all components of the spacecraft produces the so called internal straylight, that has been evaluated and controlled in the design phase.
In this paper we present the study carried out on the minimization of straylight contamination in PLANCK LFI.