urn:nasa:pds:context:telescope:eso-paranal.vlt_yepun_8m2_ut4 1.0 1.11.0.0 Product_Context 2018-10-09 1.0 Created from information from ESO Website: https://www.eso.org/public/teles-instr/paranal-observatory/ urn:nasa:pds:context:facility:observatory.eso-paranal telescope_to_facility 8.2 109.59604 -24.62704 2635.43 Unit Telescope 4 (Yepun) of the Very Large Telescope (VLT) at ESO's Paranal Observatory, Chile. The telescope is one of four 8.2-m Ritchey-Chretien Cassegrain telescopes with a 22-tonne Zerdour primary mirror of 14.4 m focal length, and a 1.1-m lightweight beryllium secondary mirror. The unit telescope has alt-azimuth mount. In such a mount the telescope tube moves around a horizontal axis called elevation axis. The two bearings which support the tube are mounted on a fork rotating around a vertical axis, called azimuth axis, thus allowing for pointing over the entire sky. The telescope tube itself consists of a steel structure supporting at the bottom the primary mirror (M1) in its cell, and at the top the M2 Unit by means of metallic beams called "spiders". The Very Large Telescope uses active optics.The optical quality of the image is continually monitored by an image analyser using a reference star and the contributions of the various optical aberrations (defocus, astigmatism, coma, etc.) are computed. To achieve the best optical quality, discrete correction commands are given to the primary mirror support system, controlling the shape of the thin flexible M1,and to the M2 Unit, which controls the position of the M2 mirror along 5 degrees of freedom, namely one for focusing,two for centering, and two for pointing. The M2 mirror itself is configured as a rigid lightweight convex hyperboloid. Information from the ESO telescope website: http://www.eso.org/sci/facilities/paranal/telescopes/ut.html