The characterization of large aperture (> 2 meters), long focal length (> 10 meters) X-ray mirrors for X-ray astronomy with synchrotron radiation poses signi cant problems related to the available space at synchrotron radiation facilities. Intrafocal pencil beam characterization of part of the optics is advantageous if its results can be shown to have predictive capabilities with respect to the full system.

In this paper we present the routine characterization of silicon pore optics at the X-ray Pencil Beam Facility of the Physikalisch-Technische Bundesanstalt, located at the synchrotron radiation facility BESSY II (Berlin, Germany). In particular we show how measurements taken in the standard beamline con guration (detector at ve meters from the optics) can e ectively be used to predict the optical performance of the optics at their design focal length by comparing data taken on 20-meter focal length Silicon Pore Optics unit in the 20-meter beamline con guration (available only for a few weeks every year) with extrapolated 5-meter measurements.