Ultra-stable high resolution spectrographs for large telescopes

This short contribution presents the demanding requirements of scientific cases for ultra-high stability spectrography, from the study of subtle radial velocity changes induced by asteroseismology to the ones produced by extra-solar planetary companions. The analysis of physical conditions in cool interstellar clouds is presented as a typical application of ultra-high resolution spectrography. The main technical challenges associated with such instruments are outlined, including focal stations and light feeding methods. As a possible way to combine both modes in a single instrument, we describe a design case for the ultra-stable high resolution spectrograph that we are proposing for the Gemini south telescope, specified to detect radial velocity variations down to one meter per second and also to achieve near diffraction-limited spectral resolution approaching one million. This versatile instrument would occupy an insulated room in the pier of the telescope and would be fed initially by an optical fibre coming from the Cassegrain focal station.