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Once the Free Electron Laser became a reality, all the users were more than happy to use it, without complaining, or being worried, about the quality of the wavefront or, if the minimum spot was not that small. But, with the increase of the number of sources and, therefore, competition, this is no longer the case.
The need of higher and higher performance in terms of optics, is becoming paramount. Ultra precise optics are available and it is feasible to preserve the wavefront at level for which, experiments realized out of focus, may be possible, using the entire beam. Also, it is possible to have diffraction-limited spots in the experimental chamber. But, having good mirrors alone is not enough and it is important to figure out how good those mirrors and mirror systems must be.
The presentation will focus on three main topics; Mirror specifications (why one have to overcome the Marechal criterion to preserve the wavefront at the level needed for more advanced experiments); Beam performance at LCLS after initial upgrade, proving the need of specifying properly the optics; R&D efforts on various prototypes, measured in both the metrology laboratory and with Synchrtoron Radiation, to prepare the optics for the high repetition rate (or high heat load) FEL operation.
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