Ref :UM-CT7-11-001 Issue : 01 Date : 28/04/2011 Page 9 of 35 4.2 CT7 design protects telescope mirror 4.2.1 Contact of CT7 with telescope mirror As the instrument measures specular reflectivity, its angular position with respect to the normal to the mirror’s surface must be exact, and this can only be achieved by a three point isostatic contact. The actual contact is provided by three spherical touching feet made out of white Teflon. Teflon can in no way mark the mirror’s substrate, but it can mark the coating especially if rubbed.
The delayed start allows the operator to lay gently the CT7 on the mirror after starting the measurement and remove it after measurement without the need to touch it while it sits on the coating. Moreover, the small incidence angle of the measuring beams makes the result little sensitive to the measurement distance so that the instrument can stand on an optical tissue . This tissue (paper or cloth) must provide an opening for the beam (25 mm hole) and should be of equal thickness (+/‐0.05 mm) under the three feet. Black cavity Input beam Reflected beam Mirror Optical scheme of CT7 Taking a measurement with mirror protecting tissue. This document is OPO property.
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