2011

Titanium Alloy Strong Back for IXO Mirror Segments

Potential applications include glancing-incidence mirrors and optics.

A titanium-alloy mirror-holding fixture called a strong back allows the temporary and permanent bonding of a 50° D263 glass x-ray mirror (IXO here stands for International X-ray Observatory). The strong back is used to hold and position a mirror segment so that mounting tabs may be bonded to the mirror with ultra-low distortion of the optical surface. Ti-15%Mo alloy was the material of choice for the strong back and tabs because the coefficient of thermal expansion closely matches that of the D263 glass and the material is relatively easy to machine.

This invention has the ability to transfer bonded mounting points from a temporary location on the strong back to a permanent location on the strong back with minimal distortion. Secondly, it converts a single mirror segment into a rigid body with an acceptable amount of distortion of the mirror, and then maneuvers that rigid body into optical alignment such that the mirror segment can be bonded into a housing simulator or mirror module. Key problems are that the mirrors are 0.4-mm thick and have a very low coefficient of thermal expansion (CTE). Because the mirrors are so thin, they are very flexible and are easily distorted. When permanently bonding the mirror, the goal is to achieve a less than 1-micron distortion. Temperature deviations in the lab, which have been measured to be around 1 °C, have caused significant distortions in the mirror segment.

The Ti-15%Mo is with a CTE of 7.2 microinches/in./°C (7.2 mm/m/°C). It is 200 mm in height, 15 mm thick, has an azimuthal span of 51.5° and an internal radius of 242.50 mm. Mounting of the x-ray mirror consists of suspending the mirror segment in mid-air using two parallel strings and positioning a strong back to a location behind the mirror such that the mirror can be aligned and temporarily bonded to the strong back. Once this is accomplished, the surface map of the mirror is re-measured. Tabs, which are used to support the mirror at the edges, are then located precisely to the edge of the mirror and then fastened to the edge of the strong back with fasteners. Epoxy is then injected through portals in the mirror tabs and allowed to cure. Once curing of the epoxy is complete, the temporary bonds at the back of the mirror are disconnected. The mirror surface is then remeasured with an interferometer and the results are compared to prior measurements.

Novel features include the near match between the D263 glass and the Ti-15%Mo. The mirror tabs allow epoxy to be injected in front of the mirror or behind the mirror. The strong back acts not only as a mirror segment transfer mechanism but also as the medium to mount the mirror into a permanent housing.

This work was done by Glenn P. Byron and Chan Kai-Wing of Goddard Space Flight Center. GSC-15850-1