  We describe and summarize the optical challenges for future instrumentation
for Extremely Large Telescopes (ELTs). Knowing the complex instrumental
requirements is crucial for the successful design of 30-60m aperture
telescopes. After all, the success of ELTs will heavily rely on its
instrumentation and this, in turn, will depend on the ability to produce large
and ultra-precise optical components like light-weight mirrors, aspheric
lenses, segmented filters, and large gratings. New materials and manufacturing
processes are currently under study, both at research institutes and in
industry. In the present paper, we report on its progress with particular
emphasize on volume-phase-holographic gratings, photochromic materials,
sintered silicon-carbide mirrors, ion-beam figuring, ultra-precision surfaces,
and free-form optics. All are promising technologies opening new degrees of
freedom to optical designers. New optronic-mechanical systems will enable
efficient use of the very large focal planes. We also provide exploratory
descriptions of "old" and "new" optical technologies together with suggestions
to instrument designers to overcome some of the challenges placed by ELT
instrumentation.
