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Two-channel six degrees of freedom grating-encoder for precision-positioning of sub-components in synthetic-aperture optics.

We investigate a novel two-channel grating encoder that can perform simultaneous measurements of six-degree-of-freedom (DOF) motions of two adjacent sub-components of synthetic-aperture optics such as pulse-compression gratings(PCGs) and telescope-primary mirrors. The grating encoder consists of a reading head and two separate gratings, which are attached to the back of the sub-components, respectively. The reading head is constructed such that there two identical optical probes can share the same optical components. The two probes are guided to hit each of the two gratings and can detect six-DOF motions simultaneously and independently. For each probe, the incident beam propagates through both a three-axes grating interferometry module and a three-axes diffraction integrated autocollimator-module, which detects translational and rotational movement, respectively. By combining the two modules it is possible to perform six-DOF measurement for a single point. The common-path configuration of the two probes enable identical responses to environmental variation, which ensures high accuracy.