The VLT secondary mirror: a report

This paper relates the development undertaken to produce the first unit of the European ESO VLT Secondary Mirror in Beryllium. The specification will be presented with the historical evolution of the mirror substrate material choice. Beryllium material quality will be discussed regarding the long term stability and mirror design will be presented. An optical test set-up has been specifically designed and fabricated for the mirror performance verification. Blank fabrication and polishing is reported with the key step of external edge lip cutting to achieve coincidence between mechanical diameter and clear aperture as required for I.R astronomical observations. Mirror long term stability is assessed from data gained during fabrication.