CONSERVING THE COHERENCE AND UNIFORMITY OF 3RD-GENERATION SYNCHROTRON-RADIATION BEAMS - THE CASE OF ID19, A LONG BEAMLINE AT THE ESRF

The lateral coherence length is of the order of 100 mu m at the 'long' (145 m) ID19 beamline of the ESRF, which is mainly devoted to imaging . Most of the optical elements located along the X-ray path can thus a ct as 'phase objects', and lead to spurious contrast and/or to coheren ce degradation, which shows up as an enhanced effective angular size o f the source. Both the spurious contrast and the coherence degradation are detrimental for the images (diffraction topographs, tomographs. p hase-contrast images) produced at this beamline. The problems identifi ed and the way they were solved during the commissioning of ID19 are r eported. More particularly, the role of the protection foils located i n the front end, the beryllium windows, the filters and the monochroma tor defects (scratches, dust, small vibrations) is discussed.