Mirror Technology

The primary and secondary mirrors will be fabricated using Carbon Fibre (CFRP) technology, and taking advantage of the experience accumulated in the design and development of the FIRST (the Far Infrared and Submillimetre Telescope, an ESA cornerstone mission) telescope. The FIRST program has already achieved the manufacture of a 1.1 meter-diameter mirror with specifications similar to those required by Planck.

The baseline design for the Planck mirrors consists of an all-CFRP honeycomb sandwich structure designed to exhibit isotropic behavior and was chosen because it satisfies the requirements of low mass, high stiffness, high dimensional accuracy, and can be tailored to have a very low coefficient of thermal expansion. The sandwich concept consists of a thick (4-10 cm) honeycomb-like core, whose surfaces have been machined to the desired shape, and to which are bonded two thin (1-1.5 mm) skins. The core is fabricated by winding CFRP filaments around individual aluminum mandrels; these "cells" are integrated to create a large (machinable) honeycomb core panel. The face skins are manufactured by laying CFRP ``tows" (i.e. flat bundles of carbon filaments wetted with epoxy) directly on the surface of the pre-treated mould; the mould is made of a dimensionally and thermally stable material, which can be easily machined and polished to the surface accuracy and roughness levels required (e.g. monolithic graphite). The reflecting properties of the mirror surface are achieved by metalization of the concave side of the sandwich. Several techniques are currently available for this process; the present baseline is to use replication. In this technique a protective layer (of MgF₂ or SiO₂) and a $\sim$5000 Å layer of metal (e.g. aluminum) are evaporated onto the mould; a thin (10-100 $\mu$m) layer of epoxy is created between the evaporated metal and the finished CFRP sandwich. Once the epoxy has cured, the sandwich is lifted, stripping and carrying away the evaporated layers and the top protective coating with it. The existence of the latter allows the implementation of a cleaning procedure before launch; the ability to clean the mirror surfaces insures the required in-flight cleanliness level without the need to keep the payload in expensive high-quality clean rooms during the integration and test phases.

The 1.1 m reflector built by Dornier as a technology demonstrator for FIRST achieved an rms surface accuracy of $\sim$5.7 $\mu$m. The specification for the 3 meter-diameter main mirror of FIRST calls for a surface accuracy better than 5 $\mu$m. Thus, the specification of 10 $\mu$m set on the surface roughness of each of the Planckis comparatively much looser than those for FIRST and should be straightforward to achieve. This specification yields a WFE of $\sim$40 $\mu$m on the full telescope; inclusion of other sources of WFE (see Section 3.6.2) shows that the requirement of diffraction-limited performance at 800 $\mu$m can be met.

The mirror mounting structures will also be made of CFRP. The primary and secondary mirrors will be independently mounted via three attachment points in each case. The optical bench will be fabricated from a sandwich panel consisting of CFRP skins on an aluminum honeycomb. The all-CFRP design ensures that no significant dimensional changes due to temperature variations will occur during assembly, alignment and testing, and simplifies the testing and mounting scheme.