HERSCHEL PACS focal plane unit

The Photoconductor Array Camera and Spectrometer (PACS) is developed by an European consortium led by MPE, Germany. It is one of 3 cryogenic focal plane instruments of the Herschel Space Observatory, I of the 4 cornerstone missions within the ESA Horizon 2000 programme. The instrument will cover the wavelength regime from 60-210 mum to explore the cold universe. The input beam is distributed to 4 advanced IR-detectors - 2 Ge:Ga photoconductor arrays for spectroscopy and 2 bolometer detector arrays for photometry - via a complex and very compact optomechamical layout with approx. 50 passive and active optical mirrors and 4 precision mechanisms. The paper will give an overview about the final optomechanical and thermal design of the thermal mass dummy and the cryo qualification model of the PACS Focal Plane Unit (FPU). The manufacturing and coating techniques of the lightweight aluminum mirrors applied to fulfill the infrared performance requirements even under cryogenic conditions and the alignment plan and optical verification concept in the visible range is outlined. The advanced manufacturing and thermal treatment procedures for the all aluminum optical bench are described in detail. Special emphasis is given to the dedicated development and verification efforts of a sophisticated IR Black Paint with extremely high IR-absorption used for effective straylight suppression. The conceptual architecture of the 2 very temperature stable and homogenous calibration sources is reported.