A cryogenic high-stability optical bench

Space astrometry requires instruments that can operate with subnanometer stability at cryogenic temperatures. Cesic is a ceramic matrix composite characterized by high stiffness, high thermal and electrical conductivity, and low thermal expansion. We have shown that it can be manufactured quickly and relatively inexpensively for use in space applications.

The starting material for the composite is a lightweight, porous, relatively brittle carbon/carbon green (unsintered) body with blanks presently available in sizes up to 1100 × 1100 × 80mm. Upon machining and joining—required for meter-plus-class mirrors and structures—the green body is infiltrated under vacuum conditions with liquid silicon at temperatures above 1600°C. Capillary forces wick the silicon through the porous green body, where it reacts with the carbon matrix and fibers to form carbon fiber-reinforced SiC-Cesic. The density of the infiltrated Cesic composite is 2.70 ± 0.05g/cm³. The unique advantage of our infiltration and conversion process is that there is virtually no shrinkage of the green body in the resulting structure.

After controlled cooldown, the structure is carefully examined both visually and with nondestructive testing methods such as x-rays. It can then be micromachined with suitable diamond tools or by electro-discharge machining (EDM) to achieve the required surface figures and interface geometries. Unlike other SiC materials, Cesic can be subjected to EDM because of its electrical conductivity. Compared with grinding, this machining method is fast, relatively inexpensive, and yields a surface and location (e. g., screw holes and mounts) accuracy of about 10μm tolerance over a large area.

Figure 1 shows a complex monolithic optical bench demonstrator (819 × 360 × 394mm) fabricated using Cesic. The demonstrator is stiffened by inner lightweight cells, transverse ribs, and external skins, a design that optimizes both structural rigidity and mass.

The optical bench is equipped with a laser metrology line to measure its interarm distance stability (see Figure 2). The line consists of a MOUSE I (Metrologic Optical Unit for Space Environment generation I) system and a corner cube. It is mounted on the bench platform arms to measure its distance variation in the z direction.

The testing is done in a vacuum chamber. A double thermal cavity with multilayer insulation is mounted inside the vacuum chamber on insulating Permaglass cubes. The bench is also placed on Permaglass disks to insulate it from the chamber bench, as shown in Figure 2.

We conducted three test runs under identical conditions, that is, the same temperature sensor taken as a reference in the center of the bench, and the same temperature evolution range (from 18.9 to 19.9°C). For each run, we calculated the slope of the distance variation measurement in nanometers per degree Celsius by linear regression over the whole +1°C temperature range. Figure 3 compares the distance variation measured versus the bench temperature increase for runs 2 and 3. The resulting interarm Cesic bench homogeneity was equal to 21nm/°C.

Figure 4 illustrates the reproducibility of the measurements by comparing the distance variations for runs 1 and 3. The test reproducibility is characterized in the range of few nanometers.

The change in elongation measured between the arms of the bench demonstrator shows that 3.5% of the arm elongation is due to the coefficient of thermal expansion (CTE). This result is consistent with a CTE homogeneity of 3.4%, as measured on numerous samples in previous studies. These test results demonstrate that, at ambient temperature, a large and complex lightweight monolithic structure made of Cesic has the same CTE homogeneity as previously characterized samples. The material thus appears to be suitable for optical benches and instrument structures that require stability in the nanometer range. This conclusion is also valid for applications at cryogenic temperatures, where Cesic has a quasi-null CTE.

This demonstration project was carried out by ECM in cooperation with Alcatel Alenia Space in support of the ESA GAIA Project.