The distance from the source point is measured by projecting a fivefold slit into the detection plane. The distance between the slit and the detection plane d2 is precisely known from an interferometric measurement. The distance from the projection plane to the radiation source point at the location of the electron beam d1 can then be calculated from the distance of the projected slits at the detector plane. An accuracy of about 2 mm in the determination of the distance d = d1 + d2 to the radiation source point is reached. Typically, a detector to be calibrated is placed about 15 m from the radiation source point at the MLS, which gives a relative uncertainty of about 1.3•10-4 in the determination of the distance.
The vertical emission angle is normally chosen to be zero (measurement in the orbit plane) by adjusting the detector to maximum signal. A typical uncertainty is 5 μrad for a calibration at 15 m distance.
At BESSY II a typical calibration distance is 30 m. A relative uncertainty of about 0.7•10-4 in the determination of the distance and a typical uncertainty of 2 μrad for the vertical emission angle alignment is reached. The size a•b of a flux-defining aperture is normally a detector property and not a property of the primary source standard and therefore not included in this discussion.