Manufacturing, metrology, and assembly of the mirror system for the CDS-EM

Abstract

The Coronal Diagnostic Spectrometer (CDS) is one of the key-instruments on the ESA-cornerstone mission SOHO, scheduled for launch in 1995. It is designed to study the solar corona in the EUV. The engineering model (EM) of the Wolter-Schwarzschild-II entrance optics of the spectrometer has been manufactured and assembled at Carl Zeiss. The on-axis resolution of the system is specified to 2 arcsec half-energy-width which sets very stringent limits on the figuring and alignment tolerances. In connection with the rather large sagittal depths of the generating profiles, the strong curvature differences and the overall geometric dimensions very high demands had to be set on the fabrication-, metrology-and assembly-techniques, e.g. : Computer controlled polishing was used to finish the primary mirror; a contour-line-measuring device with 1.25 nm resolution and a dynamical range of several mm had to be developed; a dedicated assembly set-up allows for mirror-movements in the range of 5Onm, necessary for the alignment in connection with a sufficient stiffness during the glueing of the system.