FIRST payload module and accommodation of instruments.

FIRST (Far Infrared and Sub-millimetre Telescope) is one of the satellites of the next ESA scientific mission. FIRST/Planck, which will be launched in 2007 to the 2(nd) Lagrangian libration point L-2. It will be a multi-user observatory, watching the universe in the infrared and sub-millimetre wavelength range from 60 to 670 mu m. The payload Module (PLM) of FIRST will accommodate 3 instruments built by large scientific consortia: SPIRE (Spectral and Photometric Imaging REceiver), PACS (Photo-conductor Array Camera & Spectrometer), and HIFI (Heterodyne Instrument for FIrst). All of them have detectors or mixers operating in the range 0.3 K-2 K. It must also support a 3.5 m diameter telescope. The design of the payload module is inspired from the one of ISO (Infrared Space Observatory). It is based on a large superfluid helium (HeII) Dewar (25601 at less than or equal to 1.7 K), with cooling of the instruments either directly on the tank, or on the helium vent line (4 K and 15 K). The last cooling stage to 0.3 K is performed inside the instruments using a recyclable He-3-sorption cooler. The paper will describe the latest design status of the cryostat, and its interfaces to the instruments and the telescope.