A pair of curved secondary mirrors and a pair of tertiary flat mirrors
are required to implement those focal sations. These smaller optics
are moved in and out of the lightpath by swing arms. The tertiary
mirrors rotate to direct the light to several central instrument
locations. (Prime focus cameras are not shown on the drawing below.
Images by ADS Italia and
Max-Planck-Institut für Astronomie (Heidelberg). The red "T-shaped" instrument in the center of
the telescope is an instrument which contains the
optics for combining and phasing the beams from the two telescopes.