An 8.2-m VLT Unit Telescope with the FOIlS and CONICA instruments

The drawing shows an 8.2-m VLT Unit Telescope with the FOIlS and CONICA instruments attached at the Cassegrain and coude foci, respectively.
To reach one of the instruments, the light from a celestial object is first collected by the 8.2-m main mirror. From here, it is reflected towards the l.l-m secondary mirror (at the top of the telescope tube) and from there back towards the main mirror. Depending on which of the two instruments is used, a third mirror can be flipped into or out of the light beam.
For observations with FOIlS, the third mirror is pulled aside and the light beam passes directly from the secondary mirror through a central hole in the main mirror. An image is then formed at FOR.8 in the Cassegrain focus, below the main mirror. When CONICA is used, the third mirror is moved into the beam and deflects the light sideways. After six further reflections, it reaches the CONICA instrument at the coude focus, at the base of the telescope. The smallest of these mirrors (no. 8 in the sequence) is flexible and computer-controlled for the adaptive optics system. This optica.l system ensures that the light always reaches CONICA at its fixed position, irrespective of the direction in which the telescope is pointing. In this way, the observation of a particular object may continue while the telescope moves to compensate for the diurnal rotation of the Earth.