A chopper acts as an optical switch within the entrance optics of the PACS instrument onboard the ESA Space Observatory HERSCHEL to discriminate the light of faint celestial sources from a large background flux, which is basically due to the thermal radiation of the only moderately cooled 3.5mtelescope (≈ 80K). This chopper primarily consists of a small electrically driven tilting mirror with a high position accuracy for the full range of elongations up to ±9◦ to be operated at 4K with a power dissipation of less than 4mW. At a chopping frequency of up to 10Hz in square wave modulation scheme a duty cycle of 80% is to be achieved. After a pre-development phase at MPIA, which resulted in an optimized and cold tested prototype, the industrial optimization and implementation of the PACS chopper started in 2000 in a cooperation with the company Zeiss (Oberkochen). We focus on the new design of the chopper and its critical components, including the recently developed double-stage flexural pivots as high reliable bearings of the chopping mirror axis. The monolithic CuBe2 layout has to resist vibration loads of up to 45g during the launch of the ARIANE 5 carrier and to guarantee for an accurate motion in a cryogenic environment during a lifetime of at least 3 years in space, which corresponds to 630 Mio cycles of the chopper. Other components like the cryomotor and the rotor will be considered with respect to the selection of optimal materials, the reliability of mechanical linkages and the compensation of thermal stresses within the whole multi-material chopper assembly. I. HERSCHEL SATELLITE In 2007 ESA will launch the HERSCHEL satellite (formerly known as FIRST) as another cornerstone mission of its ”Horizon 2000 long term programme” [1]. This multi-user device will be the biggest and most powerful infrared observatory of its time. With a wavelength coverage between 60 and 670μm it will be sensitive especially in the far-infrared and submillimeter region, thus exploring radiation in a very unknown range of the electromagnetic spectrum. Scientific objectives, which will be addressed by HERSCHEL, range from the exploration of the distant universe and the question how first stars and galaxies were born, across the formation of planetary systems up to the investigation of our own solar system. 3.5m primary mirror sun shield + collector cryostat + scientific instruments