The VLT’s primary and tertiary mirrors

Detail of the optical system of one of the VLT's giant telescopes. At the centre of the 8.2-metre primary mirror (M1) is a black cylinder which supports a much smaller mirror: the tertiary mirror (M3). This flat mirror has the function of redirecting the collimated light to whichever instrument is in use. In the picture, the M3 is moved out of the way for observations with the instrument installed at the Cassegrain focal station, located below the M1 supporting cell.