| Popis: |
The in situ testing of an unpolished off -axis parabolic mirror with a vertex radius of12.8 m, a diameter of 1.42 m, and an off -set of 1 m is described. Interferograms are shownthat were obtained during the fabrication process by means of a long- wavelength interferom-eter that uses a CW CO2 laser operating at a wavelength of 10.6 pm as the coherent light source. IntroductionToday, most of the mirrors of astronomical telescopes or tracking systems have a largediameter and a relatively short focal length with an aspheric surface configuration. There-fore, the sag, or the departure from the best fit reference sphere, approaches severaltens of micrometers or more. It is difficult to obtain the final surface figure by polish-ing from a surface that has been brought to a reference sphere by previous fine grinding.The best way to solve this problem is to reach the required shape by grinding with finergrades of abrasive until polishing is feasible. However, conventional interferometric,testing in the visible spectrum cannot be applied to the nonreflective ground surface. Thustesting becomes the most difficult task in fabrication.Long -wavelength interferometry,1'2 especially at the wavelength of 10.6 pm, has been usedsince Munnerlyn and Latta3 performed rough surface interferometry about a decade ago. |
