Keck Planet Finder: Zerodur optical bench mechanical design

The Keck Planet Finder (KPF) is a fiber-fed, high-resolution, high-stability spectrometer in development for the W.M. Keck Observatory. To measure Doppler shifts to 0.5 m/s or better requires some of the optics be stable to 2 nm vertically and 2 nrad in pitch angle throughout a potentially one hour long observation. One traditional approach to this thermal stability problem is to build a metal bench and then control the spectrometer thermal environment to milli-Kelvin levels. An alternative approach used by KPF is to employ a Zerodur bench of extremely low coefficient of expansion (CTE), which relaxes the thermal stability required for the spectrometer assembly. Furthermore, Zerodur optics with integral mounts are used where possible, and are placed in contact with the bench through Zerodur shims. Springs are used to preload the optics and shims within pockets machined into the Zerodur bench. We will describe how this approach has been adapted for each optic (some of which are 450 mm high with a mass of 30 kg), and how the system meets our earthquake survival requirement of 0.92 g. This mounting scheme allows us to avoid using high-CTE metals or adhesives within the optic mounting system, and therefore fully exploit the high thermal stability of the Zerodur optical bench