![Oh1 newprimslits 35 vignette Oh1 newprimslits 35 vignette](/files/live/sites/www/files/UsersAndScience/Experiments/EMD/ID28/BeamlineLayout/OH1/oh1_newprimslits_35_vignette.jpg)
They are located at 27 m from the source point, and constitute the first beam defining element. They are set such that a large part of the central undulator radiation cone is transmitted. Typical opening values are 1.6 mm (horizontal) x 0.6 mm (vertical).