WALTHAM, Mass., Aug 22. -- Thermo Electron Corp. said it plans to take its subsidiary Spectra-Physics private with a cash offer of $20 a share, the last move in a restructuring announced early last year. Thermo Electron, which already owns 80 percent of the lasers and optical equipment builder, will pay a 46 percent premium over the unit's closing price of $13.69 on Monday. In Tuesday's trading, shares rose to $20.29 on the Nasdaq. The company has spent the last three years buying back nearly two dozen publicly traded subsidiaries and selling a number of other businesses in an effort to simplify its structure.