Active Optics: IV. Set-up and Performance of the Optics of the ESO New Technology Telescope (NTT) in the Observatory

This paper is the fourth, and last, in the series on active optics as realized in the ESO New Technology Telescope (NTT). It gives a detailed description of the set-up and optimization of the optical system, based on the three levels (i.e. time frequencies) of active correction. The first results in direct imagery with the telescope optimized only near zenith (‘Astronomical First Light’) yielded immediately the best star images ever recorded in ground-based astronomy with a full-width-half-maximum of 0·33 arcsec on a CCD detector with pixels of 0·123 arcsec diameter.