Straylight analysis for the planck telescope

Abstract

PLANCK and FIRST will be launched from the European Space Port Kourou by an Ariane 5 in 2007 and spin-operated during 14 months at the L2 Lagrangian point. The aims of PLANCK are to obtain definitive images of the CMB fluctuations and to subtract the primordial signal to high accuracy from contaminating astrophysical source of emission. This can be achieved by a space telescope having a wide frequency coverage and excellent control of systematic errors (eg. stray light and thermal variations). The telescope is an off-axis aplanatic design consisting of two concave ellipsoidal mirrors with a 1.5-meter pupil, derived from radio frequency antenna, but with a very wide spectral domain ranging from far infrared (350 µm) up to millimeter wavelengths (10 mm). The short wavelength detectors (bolometers operating at 0.1 K) are located at the centre of the focal plane while the high wavelength ones (based on HEMT amplifier technology operating at 20 K) are located at the periphery. The Planck telescope operates at a temperature below 60 K. This level is achieved in a passive way, i.e. using a cryogenic radiator. Furthermore, this radiator must accommodate a set of coolers dedicated to the focal plane, cooling one of the experiments down to 0.1 K. The main performance of the Planck spacecraft is the result of the electromagnetic performance of its telescope combined with its capacity to reject parasitic signals characterised by the Straylight Induced Noise (SIN). In this case , three sources are studied and modelled, the internal straylight coming from the spacecraft itself, the galactic straylight coming from the sky, and the straylight induced by planets. This paper will describe the methods, tools and results obtained by Alcatel to assess this performance.