The light has to follow a precise path once it has been reflected by the primary and secondary mirrors. So that it does, the telescope has a tertiary mirror, which ‘intercepts’ the light, deflecting it to the foci where instruments can be mounted.
In other words, it is the tertiary mirror's job to deflect the beam of light to the folded Cassegrain and Nasmyth foci.
Two problems came up when the GTC's tertiary mirror was being designed: first, how to deflect light directly from the secondary mirror to the Cassegrain focus if the tertiary mirror was in the way, and second, how to make sure the tertiary mirror deflected light to the 4 folded Cassegrain foci and the 2 Nasmyth platforms as and when needed.
The design adopted is a removable mirror, which rotates so it can deflect light to the different foci. It is a mirror that tilts automatically and that slides out of the way when not needed.