It is shown that optical surfaces traditionally ground in conventional glasses with high coefficients of thermal expansion may be polished by irradiation with a space- and time-controlled uniform CO(2) laser beam. Comparisons of a theoretical simulation model of the laser-driven heating process with the experimental results allow us to determine the conditions for successful and reliable use of this technique. The technique can be applied indiscriminately to preheated samples made of different glasses, with any topography, and, of any size in a limited range that depends only on the available laser power.
One of the new problems that has to be solved for segmented mirrors is related to periodic phasing, because for such mirrors to exhibit diffraction-limited performance the segments have to be positioned with an accuracy of a fraction of a wavelength. We describe the optical design of an instrument that measures the phasing errors (i.e., tip, tilt, and piston) between two segments under daylight conditions. Its design is based on a high-aperture white-light Michelson interferometer. It was developed at the Center for Sensors, Instruments and Systems Development (CD6) of the Technical University of Catalunya, Spain, and its final testing was carried out on the Gran Telescopio Canarias test workbench.
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