Dragon monochromators are classically equipped with only one optical piece which is a spherical grating with constant ruling. Very high flux could be obtained with this kind of monochromator due to the presence of only one grating. The spherical shape permits to limit the slope error (in comparison to a toroidal one) and so to obtain very high resolution. The spherical grating introduces defocalisation during the scanning which is compensated by exit slit translation. It is possible to increase the spectral range covered by this kind of monochromator with two gratings that work at two different deviation angle. In this case the fixed exit is obtain with a plane mirror inserted in the beam when the grating with the lower deviation angle is used. This is the optical layout of the ELETTRA VUV Photoemission beamline.

Here are the main characteristics of a Dragon monochromator:

• High resolution and flux

• Movable exit slit for best focalisation solution

• One optical piece inside the monochromator

SB7 Beamline (SACO - LURE - Orsay - France)

VUV Photoemission beamline (ELETTRA - Trieste - Italy)