Thin, lightweight mirrors are needed for future x-ray space telescopes in order to increase x-ray collecting area while maintaining a reduced mass and volume capable of being launched on existing rockets. However, it is very difficult to determine the undistorted shape of such thin mirrors because the mounting of the mirror during measurement causes distortion.
Traditional kinematic mounts have insufficient supports to control the distortion to measurable levels and prevent the mirror from vibrating during measurement. Over-constrained mounts (non-kinematic) result in an unknown force state causing mirror distortion that cannot be determined or analytically removed.
In order to measure flexible mirrors, it is necessary to over-constrain the mirror. Over-constraint causes unknown distortions to be applied to the mirror. Even if a kinematic constraint system can be used, necessary imperfections in the kinematic assumption can lead to an unknown force state capable of distorting the mirror. Previously, thicker, stiffer, and heavier mirrors were used to achieve low optical figure distortion. These mirrors could be measured to an acceptable level of precision using traditional kinematic mounts. As lighter weight precision optics have developed, systems such as the whiffle tree or hydraulic supports have been used to provide additional mounting supports while maintaining the kinematic assumption.
The purpose of this invention is to over-constrain a mirror for optical measurement without causing unacceptable or unknown distortions. The invention uses force gauges capable of measuring 1/10,000 of a Newton attached to nanoactuators to support a thin x-ray optic with known and controlled forces to allow for figure measurement and knowledge of the undeformed mirror figure. The mirror is hung from strings such that it is minimally distorted and in a known force state. However, the hanging mirror cannot be measured because it is both swinging and vibrating. In order to stabilize the mirror for measurement, nano-probes support the mirror, causing the mirror to be over-constrained.
A closed-loop software algorithm is used to control the forces applied by the nano-probes. These support forces are determined by finite element analysis (FEA) to have an acceptable and known effect on the mirror distortion. If necessary, distortions created by the nanoprobes can later be analytically removed based on the correlated FEA.
The mirror is hung from strings in the suspension metrology mount (SMM) fixture. Nano-probes mounted to the fixture in optimized locations extend to stabilize the mirror. The software closed-loop force control algorithm is run to adjust the forces to pre-determined levels. Once stabilized and in a known force configuration, the mirror is measured with an interferometer.
The novel feature of this invention is the ability to measure and control the forces supporting an object whose shape must be precisely known. The advantage of this invention over prior art in the field is that a flexible object may be overconstrained without causing unknown distortions. Also, imperfections in kinematic type mounts can be measured and controlled. Using this invention, mirrors have been measured and their figures have been correlated with the figures predicted by FEA.
This work was done by David Robinson, Maoling Hong, and Glenn Byron of Goddard Space Flight Center; Ryan McClelland of SGT, Inc.; and Kai-Wing Chan of the University of Maryland. GSC-16084-1